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2000 Chevrolet Silverado 1500:
A/C Compressor Not Engaging

Alex Steele
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I'm experiencing a problem with my A/C compressor. It's never engaging when I hit the button to turn it on. I stopped at a car shop yesterday and they told me it's charged with Freon and that they think it is an electrical problem.


Well, since I'm an electrician not a mechanic I figured I might be able to troubleshoot the problem. All fuses seemed to be ok by sight and checked good with my meter. The larger relay in the electrical box had continuity between two diagonal poles.


Sorry for the length of the question, just trying to be informative. Thanks a lot, and I forgot to tell you it's a 2000 model Chevrolet Silverado 1500 with the 5.3-liter V8.




Trust me, there's never too much information in a reader's question, unless there's 12 questions in a reader's question. Note: a U.S. built 2000 model year truck uses R134a refrigerant, not Freon (R12).


OK, you're on the right track. If the system is fully charged (pressure correct), but the A/C compressor clutch does not engage, the fault is electrical. You're experience as an electrician directed you to first check the basics (fuses), and now it's time to trace the circuit and determine what we lost (power or ground) and where we lost it.


If you test for 12 volts at the A/C relay terminals (engine compartment fuse block, left side fender) you should detect voltage at two of the four terminals which are marked (30) and (86). That's power from the A/C fuse and Ignition fuse E, respectively. Now take a jumper wire and connect it between terminal (30) and terminal (87). With the ignition on this should send the 12 volts from the A/C fuse directly to the A/C compressor. You'll hear and see the compressor clutch engage if everything on that side of the circuit is good.


If not, there's a break in the power wire form the relay to the clutch, a break in the clutch ground circuit, or the clutch itself is no good. If it works fine, it's going to require more advanced testing on the control side of the circuit. The relay could be bad, or it's not getting the needed activation ground at terminal (85) which comes from the Powertrain Control Module.


The powertrain control module needs to receive a bunch of correct data from switches (three pressure switches under the hood), sensors and the HVAC control module before it will engage the A/C compressor. The best diagnostic route for a technician would be to check all these inputs with a scan tool before taking the next step.


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Chevrolet | Compressor | Refrigerant | Relay/Fuse
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22 Jun 2019, 10:46
You don't have enough pressure to trip the low pressure switch you may have to jump out the low pressure switch to get the compressor to turn on.
21 Jun 2019, 16:02
My 2000 GMC Sonoma A/C will not take freon.
01 Sep 2018, 09:04
RE: Michael, the question is, is the A/C cold? What is the Temperature coming out of the vents? I am only asking this because the compressor is suppose to cycle on and off. If it didn't work or stayed on all the time then there is a problem.

There is a service bulletin out Bulletin No.: 11-01-39-002

Date: May 05, 2011

Subject: Air Conditioning (A/C) Blows Warm Air Intermittently. This condition may be caused by the A/C compressor clutch cycling out early when the low pressure side reaches approximately 380 kPa (55 psi).

The fix is to bring the vehicle to the dealer to have the latest software installed into the vehicle.

I hope this helps.
31 Aug 2018, 19:13
I have had my gauges hooked up and my scan tool everything checks out that's the first thing I done im a 8 year mechanic but I can't seem to get this one I feel the pcm is losing its ground in turn cycling the compressor
31 Aug 2018, 18:55
RE:Michael, It is susposed to cycle on and off. This is so the evaporator won't freeze.

If you think you have a problem put a set of professional gauges on the A/C system to make sure it is charged with the proper amount of R134a freon. It the pressures are low you may just need to add a few ounces of R134a.

If you are having an electrical issue a scan tool can help diagnosis the problem by looking at the electrical componients of the A/C system to see if they are working properly.
31 Aug 2018, 18:42
So when your checking the wire at the pcm it's ground with compressor on but when it Cycles it goes to 14 volts that mean it's not a bad pcm I have checked everything the pressure is perfect I changed the pressue switch and the temperature sensor on front of the truck could it be the climate control unit it Cycles the compressor every 8 Seconds
31 Aug 2018, 07:25
RE: Michael, Power is coming in on relay terminal 85 goes through the relay out of terminal 86 to the PCM. You will have voltage on the wire until the PCM grounds the circuit, when the circuit is grounded it is pulled to 0 volts showing the circuit is complete and working properly.
30 Aug 2018, 20:31
I'm experiencing similar issues with a 2011 Chevy Silverado AC clutch is cycling the green with white Tracer wire coming from the PCM is ground while the compressor is on but 14 volts on the compressor is off
30 Aug 2018, 20:30
I'm experiencing similar issues with a 2011 Chevy Silverado AC clutch is cycling the screen with white Tracer wire coming from the PCM is ground while the compressor is on but 14 volts on the compressor is off
19 Aug 2018, 07:08
RE:Max, 30 is the wire that has 12 volts to it from the A/C 10 amp fuse, take a jumper wire and touch it to the dark green wire (87) and the compressor clutch should engage.

Yes you can jump the wires while the engine is running.
18 Aug 2018, 21:28
Which ones are terminals 87 and 30?

Also should you jump them with engine running? Thanks
25 Jun 2018, 14:31
RE:Craig, Jumping across 30 and 87 and it blows the fuse I would check the continuity of the compressor clutch coil. Measure the resistance of the coil making sure it is not shorted to ground. It should measure about 4 ohms. Also check the dark green wire 87 to ground making sure it is not shorted to ground.

25 Jun 2018, 13:56
1999 Silverado. I am having some issues I haven't found on here yet. Compressor won't engage. I checked voltage at 30 and 86. Good. Ground on 85. I jumped across from 30 to 87 and it blows the 10 amp A/C fuse right above the relay socket. Is the compressor bad?
John Campbell
12 Jun 2018, 09:30
Thanks for your help
12 Jun 2018, 09:20
RE: John Campbell, Yes it is possible it could be a bad ground, poor connection as of corrosion or loose wire connector on either the relay side or the PCM side. It could also be the PCM starting to fail.
John Campbell
12 Jun 2018, 09:05
My 2006 Silverado Z71, 5.3L has A/C acting much the same as "Terry" from 05/June/2018. The difference is the 85 relay terminal is "pulsing" between green indicating a good ground and anywhere from 0.1 to 0.4 V. Would this indicate a bad ground wire coming from the PCM or maybe a bad PCM?
12 Jun 2018, 08:59
RE: Terry, That is the cheapest way to repair your A/C problem at this time. The PCM is expensive and this will by pass the problem until you decide to replace the PCM. Good Job!
12 Jun 2018, 08:38
RE: Ken, The compressor is supposed to cycle on and off. This is proper operation to ensure that the evaporator does not freeze up.

If you use the over the counter R134a with the gage on it they are almost always not accurate. Using a professional set of gages or having the A/C system checked by a professional certified A/C technician would be the way to go.

The pressures may be too high because you may have too much R134a in the system.
11 Jun 2018, 15:37
My 1998 GMC Sierra 4x4 5.7 engine is having an air conditioning pain in my hind end. The pump engages then shuts off engages, shuts off. Last year it was doing the same thing so I figured it needed charging. Put on the can that has a gauge on it hooked up the hose. Pushed the trigger it would not take anything in. Dial was going from down to almost 0 then almost up to red zone while pump was cycling. It never did take any charge and low pressure side and high pressure side were both warm. Any help would be wonderful, thank you.
10 Jun 2018, 18:24
Ok, 1st we tried to engage the compressor clutch with the scan tool, to no avail. 2nd we located the wire from the relay to the PCM and it had continuity. so we assumed the PCM is bad. At this point I have connected a wire from number 85 at relay to a switch and to ground. For now my air conditioner runs.
05 Jun 2018, 18:59
Ok, thanks for all your help, Im going to check the ground for continuity if it is ok then going to take to shop as you suggested, again thanks, I have learned a lot .
05 Jun 2018, 18:47
RE: Terry, The other option you can do is to take the truck to a dealership where they can run a diagnostic check with there factory scanner to see if there are any trouble codes and see the proper operation of the pressure switches and computer components.

It would wind up being cheaper in the long run because they can pin point the problem for the hour service charge and stops you from spending money on parts that may not work. You can decide to do the repair with them or take the information they give you for the repair and do the job yourself to help save on the cost of the total repair.

05 Jun 2018, 18:24
RE: Terry, The terminal 85 is the ground coming from the PCM. It relies on the inputs from the low pressure and high pressure switches and control module. I would feel more comfortable knowing the operating pressures before I tell you to replace the PCM.

You can jump out the low pressure switch and the high pressure switch (one at a time) to see if the compressor engages with the relay plugged in. If it engages you may just need to add a can of R134a. If you don't get clutch engagement from doing this then you may need a PCM. The PCM supplies the ground for the relay terminal 85 as I mentions above.

If you can start the truck and turn on the A/C while you jump out the relay again (terminals 87 and 30) to activate the A/C compressor. With the A/C running what are the pressures? The low side should be above 28 to activate the low pressure switch and under 350 on the high side.

Check the wire terminal 85 on the relay to make sure the wire is not broken. Check the continuity of the wire with an ohm meter (terminal 85 at the relay to the other end at the PCM terminal number 43 dark green and white tracer) on the C2 connector of the computer.

With the relay installed half way, add a good ground wire to terminal 85 to see if it will engage the compressor clutch. If it does and the A/C system works properly and all of these switches check out and the pressures are good you will probably need to replace the PCM.
05 Jun 2018, 17:35
Ok went back and checked the relay, discovered I have no ground at 85. So where do I go from here?
05 Jun 2018, 15:44
ok, let me go back and check to make sure I have power where it should be and also re check the ground, I will let you know what I find, and thanks.
05 Jun 2018, 15:39
RE: Terry, If you have power on 86 and 85 is a confirmed good ground from the PCM and you jumped 30 to 87 and the compressor clutch engages then it is possible the relay is faulty.

To check the relay, put 12 volts on 86 and ground 85 then use an ohm meter and check the continuity at 30 and 87. If the resistance is high or open then the contacts at 30 and 87 must be bad.

I know you said you tried a different relay but this doesn't make sense. If you have power on 86 and the ground is good at 85 and you jumped the terminals 30 and 87 and the clutch works it all points to the relay or wires being faulty on 86 and 85. Is it possible to try a different relay?

Check the wires on terminals 86 and 85 for any problems if switching the relay doesn't work.
05 Jun 2018, 15:11
Thats the way I did it, I pulled the relay out just enough to get a small wire to it and grounded the wire and nothing happened.
05 Jun 2018, 14:03
If you can ground the terminal number 85 while the relay is still plugged in is what I ment. Pry the relay up just a bit so you can see the terminals and it is still plugged in. Then groundthe 85 terminal. You should hear the relay click and the compressor clutch should engage.
05 Jun 2018, 13:56
When I ground terminal 85, no the compresser clutch does not engage, yet when I jump 30 and 87 it does, im at a loss..Thanks for your time and help.
05 Jun 2018, 13:35
When you jump terminals 87 and 30 you are just doing the job the relay does. I don't see any issues for doing this for a short time.

Terminal 85 on the relay is the ground coming from the PCM. If you can take a jumper wire connected to a ground and attach it to terminal 85, does the clutch engage? If it engages then the ground from the PCM is faulty and it could mean an open wire or bad connection at the PCM or a bad PCM itself.

The ground wire on the PCM is on plug C2 wire 43 (dark green white tracer).

The Power train Control Module is located in the Left Front of the Engine Compartment to the Left of the Fan Shroud.

04 Jun 2018, 16:10
I replaced the climate control module today , not cheep, and the air still does not work. I have voltage on terminal 86, and when I i jump 30 and 87 the compression clutch engages,, is it safe to run the compressor like this for a short time? thanks. By the way, where is the pcm located and how do I check it?
31 May 2018, 16:45
RE:Danny, The first thing to check is to see what the pressures are when the compressor shuts off. Low side must be over 28 psi and the high side must be under 350psi.

If the pressures are good see if you have voltage at the compressor clutch when the compressor shuts off. If you have voltage and a good ground it could be a bad compressor coil. The reading should be between 3 to 5 ohms at the terminals of the compressor coil.
31 May 2018, 16:18
RE:Terry, On the A/C relay terminal number 86 should have 12 volts, terminal 85 is the ground from the PCM. With the relay still in place try to ground terminal 85, if the compressor engages it could possibly be a bad PCM. If you don't have 12 volts then check fuse IGN E 10 amp.

Go to the low pressure switch and see if you have 12 volts on the dark blue wire, if you have voltage then jump the dark blue wire with the black wire and see if the compressor clutch applies. If you don't have 12 volts on the dark blue wire then it is possible the control module is bad.
31 May 2018, 11:54
I have a 2002 Suburban 5.3l a/c compressor cut off intermittenly and I can turn a/c off and back on again and it will stay on for a while and cut off again. I already replaced all three pressure switchs , relay a/c control panel and a/c clutch is good. Please help!
31 May 2018, 11:34
Some people have told me I need to replace the climate control module. I don't mind doing this if I knew this was the problem, Im just not that sure it is. I hate to spend $140 if thats not the problem.
31 May 2018, 11:31
I have power to the relay. when I jump 30 and 87 the compression clutch pulls in. I have replace the relay and the same results.
30 May 2018, 18:06
RE: Terry, You would first have to find out the pressure readings on the high and low side. The low side needs to be above 28 psi and the high side needs to be below 350 psi. A quick way to check this out is to take the low pressure switch connector and jump the two wires together; if the compressor engages it could just be low on R134a and would need a little added to the system.

A scan tool can help with diagnostics because the scan tool can read codes related to the A/C system and the pressure at the sensors themselves. It can also tell you if the compressor clutch is commanded on which can tell you that the computer is functioning properly which would tell you to look for a wiring problem or compressor clutch coil.

The clutch coil could fail under extreme heat so if you have current at the compressor clutch coil you can check the resistance of the coil to see if it is out of range. The ohm reading should be about 4 ohms. If it is out of range then replace the coil.

Check the wire harness for any broken wires or corroded wire terminals or connectors. Also check the ground wire to the compressor coil.

30 May 2018, 06:59
2003 GMC 2500hd Sierra, when you start the truck the air will work, but if you shut off the air at the switch and turn it back on it will not come back on, Im stumped need help. thanks
25 May 2018, 11:56
RE:Wille B, Sometimes when you put the R134a into the system it may not be enough pressure to trigger the low pressure switch. Take the connector off the low pressure switch and jump the two wires on the connector together and see if the compressor clutch engages. If it does engage continue to charge the system to the factory specs. If it doesn’t engage check to see if you have good continuity on the clutch coil (resistance check). It should be around 4 ohms. You may have a bad compressor clutch coil or a bad ground at the compressor.

Check the ground wire at the compressor by adding your own ground and see if the clutch engages. If you have 12 volts at the compressor clutch and a good ground it may be a bad clutch coil.
willie b
21 May 2018, 17:59
Hello guys, Just purchased a 2003 Avalanche 5.3 and the ac didn't work. I have bought a new compressor, charged the system checked the fuses and relay Im getting about 5 volts to the low pressure switch and a good ground. Im getting 12v, around 5v and a good ground to the high side and Im getting 12v but NO ground to my clutch. Any suggestions?
12 Mar 2018, 09:15
RE: Eddie, The plug on the back of the compressor is used for a high pressure safety switch. When the compressor reaches dangerous high pressure then it will disengage the compressor clutch.

If your compressor didn't have this before then don't worry about our problem is else where. As long as you have the ignition on check for power at the compressor clutch by jumping the A/C compressor relay at terminals (30) and terminal (87). If you have power check to make sure you have a full charge of R134a, if you don't check the A/C fuse and ignition fuse E.

Other problems could be the low pressure or high pressure switches, PCM or broken or corroded wires and connections.

If you know someone who has a scan tool it can help diagnosis this problem a lot easier
11 Mar 2018, 22:40
I have a 2001 suburban. I purchased a used compressor and when I took out the original, the orinal only had one plug in the front. The one I bought used has one plug in the front same as my original but it has an additional one (purple one) on the back? I installed it and the compressor does not engage?? What is the plug at the back for? Is that why it’s not engaging? What’s a cheap fix?
20 Jan 2018, 15:52
Good info was helpful.
22 Jul 2017, 09:55
RE: Kevin, Monitor the voltage at the compressor clutch coil, when the clutch disengages and doesn't come back on is the voltage still there? Check the ground wire on the compressor plug. When the clutch disengages and power is still there at the terminal wiggle the wire harness to the compressor and see if it goes back on when wiggling. If it does come back on while wiggling repair harness or connector at the compressor.

Check your pressures again and monitor when the compressor cuts out. If high side pressure is too high it will cut out the compressor function until the high side pressure gets into factory specs. If it gets too low (below 25 psi) it will also cut out the clutch operation.

Also check the A/C clutch relay for proper operation. Check the A/C compressor fuse (10 amp) is battery voltage and has power all the time. The IGN fuse (10 amp) is powered on by the ignition switch. When the clutch cuts out; check the voltage on both of these fuses. If you lose the voltage at the IGN fuse you could have a faulty ignition switch.

There is a dark green/white tracer that goes to the A/C relay, this is the PCM (power control module) it is the control wire for the A/C relay. If the PCM is faulty you will lose the ground to the relay and you will have no compressor function. If all other inputs to the PCM are there for proper A/C function then you would then have to replace the PCM for this problem.

Check the resistance to the clutch coil, if too much resistance is present then the coil could cause a malfunction causing clutch engagement failure. You may need to replace the clutch coil if found faulty. Resistance of the coil is 3 to 5 (ohms). Less than 2 or greater than 5 is unacceptable.

Try these few suggestions I hope they help.
20 Jul 2017, 20:28
I own a 2005 gmc sierra 1500 with a 5.3. my issue is the AC. compressor kicks on when selected. AC works for 20 minutes then the compressor shuts off and the vent still blows. I have purchased the following.... new control unit. new proffesionally installed R134a, lower ac switch at the canister. I'm clueless. and everyone I talk to has no idea. no forums talk about it. also, I turn the truck off when the ac goes out, then turn the truck back on and the ac works again. only for another 20 minutes again.
04 May 2017, 05:55
RE:Travis, It sounds like you may need a new control module. Module switches usually are non serviceable.

If the knob does turn and it just won't switch from the rear to the front then you may need a door motor or have an electrical issue.

More information is needed to diagnosis the problem.
03 May 2017, 16:46
I have an 01 Chevy Silverado and the AC vent knob won't turn from the dash to the front vents. How can I fix it?
10 Apr 2017, 18:54
RE:Adam, Check the resistance of the clutch coil it should be between 2 to 5 ohms, anything above or below you should replace the coil.

Also check the diode between the wires on the coil plug. It should have continuity one way and not the other. If the diode is faulty a voltage spike could be causing this to burn out the relay contacts and/or other electronic components.

Also check the grounds in the clutch and relay circuit.

10 Apr 2017, 13:17
I have a 2006 GMC Sierra 5.3l. My A/C Compressor Control rely keeps going out. I will replace it, and the compressor kicks on and works great for a few on/off cycles, then the AC CCR goes out again. Any ideas?
20 Mar 2017, 20:51
RE:aautotowing, I would keep the drier that came with the truck and then go to a good auto parts store that can make custom A/C hoses. They can make a new hose that can fit the updated application. You would have to show them what you did so they can get the correct configuration and measurements to make the correct hose. They have universal connectors and the hose can be cut to the correct length for a perfect fit.
20 Mar 2017, 19:15
I have a good question for someone, my truck is a 1995 C3500HD one ton. Ok I pulled the motor out and put a rebuilt 5.7 Vortec 350 in it. The motor I took out was a diesel ok I am trying to use the same ac unit that was on the 5.7 however the drier for the 5.7 will not fit the fitting coming out the firewall. If I get the drier that fits the line coming out the firewall will it still let me screw the line from the compressor to it or what must I do to change it up to be used with everything that came off the 5.7. I have everything expect the drier hooked up.
30 Sep 2016, 05:21
RE:Bill McWaters, Which fan are you talking about, the blower fan or the radiator fan?

You can check if you have voltage at the fan and also check to see if there is a good ground.

For the compressor you can check the A/C compressor relay in the fuse panel. These are the easy checks. Also check for voltage and ground at the compressor clutch.

Bill McWaters
28 Sep 2016, 10:23
I have a 2000 Silverado 1500. My problem is for no apparent reason my air condition quite working along with the fan motor. Checked all the fuses and replaced the "heater/ac climate control switch panel. Is there anything else I can check before going to a repair shop.
05 Sep 2016, 11:47
RE:Kyle, Check the electrical connector at the compressor making sure the wires are not broken, loose connectors or have corroded connectors. If those suggestions check out OK then you may have a bad compressor clutch coil.
04 Sep 2016, 13:37
My ac compressor wont come on until I tap on it.... what is the problem?
01 Sep 2016, 07:03
RE" Joseph Gee, I don't understand your question, does it not accept the gas and not get the pressures to where they allow the compressor to turn on? Why did you change the orifice and flush out the system? If you have a conversion kit for the system to accept R134a then check the adapters on the pressures ports to make sure they are screwed on all the way.
joseph gee
31 Aug 2016, 17:48
I have a GMC 1500 and it won't recharge, I have changed the orifice tube and accumulator it still won't charge. It has been changed from R 12 to R134a and flushed out. No leak at compressor and the clutch works.
23 Aug 2016, 07:45
RE:Chris W, Try to jump the low pressure switch harness with a jumper wire. This will check some of the electronics like the A/C compressor relay, control head and PCM. If the compressor clutch engages then it may just need a little more R134a to trigger the low pressure switch or if the pressures are in the range of factory specs then it could just be a faulty low pressure switch. The factory specs for the low pressure side are 26 to 30 PSI to allow the pressure switch to turn on the compressor. If it is lower then that then add a few more ounces of R132a until the compressor engages. The running low side pressure should be about 30 PSI on a cool non humid day. The higher the humidity and temperature the higher the low side pressure can be.

If this doesn't work then check your compressor relay. You would need a scan tool to check the proper functions of the PCM and the control head.

Some times it is worth the diagnostic fee from a mechanic then you can still do the repairs yourself.
chris W
22 Aug 2016, 23:15
My 06 1500 WT V6 compressor went out a while ago. I've checked the refrigerant pressures and those are ok, no signs of leakage. Pressure switches have continuity. I have jumped the relay for a short time and the compressor runs fine and pumped out cool air immediately. All other fuses involved are fine. Now I am stumped at where the issue could be. I removed the HVAC controller and inspected the connectors and wiring... they look fine, the leads on the wiring harness had voltage. Is it just a faulty controller? One of the LEDs for the temp control died several years ago and I have read that those arent the most reliable pieces of equipment on the trucks. Perhaps the ECM? Should I just bite the bullet and have a shop try and diagnose it? I hate paying people to do repairs.
07 Aug 2016, 06:39
RE:Scott, Check the end of the high side hose making sure the valve inside the hose is being pressed in so the pressure can be read. The center core inside the hose could be damaged or missing or the high side connector could be bad.

If the low side is normal then there should be a high side reading.

Let me know your results we can diagnosis the problem further if needed.
06 Aug 2016, 17:38
SO my readings are showing normal on the low end and my high end does not move. any thoughts?
27 Jul 2016, 20:29
RE:Scott, If you push the contacts together weather the system is empty or full the compressor will still go on as long as you have 12 volts to the relay.

Check the low side pressure to see if it has enough pressure to trigger the low pressure switch on the accumulator.

The low pressure switch makes a connection when the pressure is with in range (24 to 29 PSI) telling the computer to ground the path to the relay to activate the compressor relay.
27 Jul 2016, 15:05
Re:TechHelp2 we can engage the clutch through the relay by pressing the connection together so it does work. when we complete the connection on the Relay.
27 Jul 2016, 14:55
RE: Scott, The first step is to see if you lost any refrigerant (R134a). Install a set of gauges and see what the pressure on the low side is at. If it is below the threshold of 25 to 29 PSI then you won't have enough pressure in the system to activate the low side pressure switch. The low side switch is like the on and off switch for the compressor clutch.

Try to jump the wires on the low pressure switch on the accumulator and see if the compressor clutch engages, if it does then add more R134a to the system until the system is at normal pressures. Then check for a leak in the system to find out where it is escaping.

Also check the wiring at the compressor to make sure you have a good connection at the plug at the compressor coil and also check the wire harness to make sure there are no open wires or shorts in the compressor circuit of the wire harness. Try to wiggle the harness to see if the compressor will go on and off.

Try these suggestions, we can diagnosis the problem further if needed.
27 Jul 2016, 14:06
So I replaced my compressor with a brand new one. I've installed it and it started working then it just stopped. The clutch won't engage unless you pull the cap off the relay and hold the contacts together so it engages the Compressor clutch. I am just not sure why it is all of the sudden not working.
17 Jul 2016, 19:12
RE:CKAc13, It sounds like the electrical connection at the coil on the compressor is faulty or the coil is faulty itself. Check electrical connections first, if all is ok then change the coil on the compressor.
17 Jul 2016, 10:37
Hello guys, I seem to be having similar problems to a lot of you. My air conditioning on my 2005 Chevy Silverado HD 2500 will just stop working. I checked the fuses in the relays and everything and then found out that if I just gently tap on the compressor with the prybar it starts right up immediately. So it seems like everything along the way is working up until he gets to the compressor. And then a gentle tap started right up. Does this mean the compressor is bad, Oregon is that not enough power getting to the compressor?
27 Jun 2016, 16:55
RE:Alex, It sounds like the compressor had a leak and let out the R134a and oil most likely from the front seal of the compressor, a common problem. Then when you put in the R134a the compressor seized.

You should be proud of yourself with all the work you did. Good Job!
27 Jun 2016, 12:46
UPDATE : 2003 Chevy Suburban 8.1L w/Rear AC

Thanks for your advice and help! I did take it in to a professional and had everything checked out. They hooked it up to the "evacuator" and let it run for about 30 minutes. The machine removed everything and passed a vacuum leak test. That was the first bit of great news I have had in awhile. They then pumped it with 3 lbs. of R134A with dye. No leaks and it is blowing cold.

So I drove off and at first I only had the front AC on - ice cold. When I turned on the back it initially was just slightly cool but got progressively colder as the drive went on. Normal? I will keep monitoring to make sure the back is working like I would expect. I also check the lines in the back to make sure they are getting cold.

Just for a reference they charged me $129 (included the R134a) + tax. I found a few places that were slightly cheaper but this place got me in right away without having to leave it - something I couldn't afford to do. Took about an hour.

Question : Does the way the compressor failed make sense? Wouldn't engage presumably because of low pressure and then when freon is added, the clutch rattles and locks up. You would think the clutch would have locked up before it got low. Or is this perhaps 2 things happening independent of each other? Just curious and wanting this to make sense in my head. Happy that (for now) everything seems to be back in order but hoping to be able to interpret the symptoms if they come along again.
26 Jun 2016, 21:06
RE: Alex, You can have a licensed professional do the rest for you and if there are any other issues the technician would be able to spot the problem and tell you. The worst thing to do is to buy parts and install them and they don't work.

You did say you had a problem with the front system so it would be a good idea to have the mechanic look at the entire system.

The key word here is licensed professional. The EPA does not want amateurs venting R132a into the atmosphere. The proper equipment is required in all states.

The cost should be around (don't hold me to this) but I think it is around 80 to 120 dollars. This is depending on where you go.

You already spent a good amount of money so to be safe let a professional mechanic do the rest of the job for you. For the price of a mechanic doing it for you is cheaper that finding out you missed something and caused more damage and it costs you even more money then you wanted to spend.
26 Jun 2016, 16:40
2003 Chevy Suburban 8.1L w/Rear AC

The information in this article has been invaluable to my troubleshooting. Thanks for all that contributed!

My problem started last summer when only the rear AC would get cold - the front blew warm. So for last summer, I let the rear AC cool the vehicle. I had read that the blend actuator door could be the culprit but I never got around to it.

Fast forward to this summer.... Now the AC blows warm front and back. After reading this article and ALL the posts I am much more educated. I observed the clutch was not activating no matter what I did.

I started by jumpering the relay (87 & 30?). The clutch clicked on and off although it didn't sound right but it would engage immediately. If I gained anything from all the posts it was - CHECK YOUR PRESSURES. So I went to AutoZone and borrowed a set of manifold gauges. The pressure was almost non-existent but there was still some there. So I figured at this point I found the reason for the clutch to not activate - low pressure. I bought some freon that had Stop Leak in it figuring it was a slow leak since it lasted all last summer.

Almost immediately after hooking up the can the compressor engaged. That was about as good as it got. It was making a horrible rattling now and then LOCKED up completely. The clutch started glowing, possibly even caught fire and had smoke coming out. Compressor fried!

So I removed the compressor and receiver drier. I cleaned out the lines as best I could with a solution specific to that. I pulled out the orifice tube and I was expecting it to be "black death" but it was actually pretty clean. Picked up a new compressor, drier, orifice tube, and seals. I put it all back together being really nervous that I got things right (didn't expose the drier to any outside air, filled the compressor with the right amount of oil, ensured I used all the supplied seals, etc). So it is all back together.

The next logical step is to charge it. I am hesitant to say the least. Did I do this right? Did I miss a step? Am I going to waste a whole bunch of money on freon?

My questions are:
1. Should I have it professionally vacuumed or can I do this myself with a rental?
2. What is a reasonable price for a professional to evacuate and recharge the system?
3. Am I safe in assuming that if the front orifice tube is clean, the rear expansion will be clean too?
4. What else should I do before doing the recharge?

Sorry for the long post but just making sure I get it right!
22 Jun 2016, 08:48
Watch the low side pressure gauge when it cycles on and off to see when the compressor turns off. If it turns off at 30 or lower then you just may need a little more R134a. I mean a little like maybe 1/4 a can.

The compressor should turn off at about 24 psi to 29 psi (depending on calibration of cycling switch). It should also turn on at about 35 psi to 39 psi.

Get back to me with your results we can diagnosis the problem further if needed.
22 Jun 2016, 08:32
Hi I have a 2001 Tahoe 5.3 the ac was always nice and cold . Now when I put the truck in drive the compressor short cycles every 2 -3 Sec. If you put it into park it's fine only in drive. The truck only has 60,000 miles on it. I vacuumed it down and recharged it, the low side is at 35-38 the high is 175. Its driving me crazy I changed the cycle switch and the low pressure switch. If I drive around its great as soon as I stop the compressor cuts on and off. Thank you for your help.
15 Jun 2016, 21:59
Thanks for that insight. I did just what you said to no avail. Wiggled the wiring harness running across the top of the engine and from the ambient temp sensor up toward the engine bay. I also unplugged and plugged the high and low pressure switch. I let the truck sit for a couple hours, in the 94 degree sun. Got in started her up and the AC worked. Makes me not want to wash the truck anymore. LOL.
15 Jun 2016, 19:18
RE: Cory, Next time you go through the car wash or have the problem with the compressor not working inspect and wiggle the wire harness. There is a service bulletin on wire harness issues and a wiggle test is the first thing GM wants you to do.

It sounds like when it gets wet the wires or the connectors somewhere in the harness or compressor are being affected.

Try this and get back to me with your results. We can diagnosis the problem further if needed.
15 Jun 2016, 17:12
Got an odd one here. My 06 Chevy 2500HD Duramax AC quits when I go thru a carwash. It will come on after a day or sometimes several miles of highway driving. I've replaced the high and low pressure switches and the ambient temperature switch up on the core support. This 95 degree Texas heat makes window AC suck. Any help would be greatly appreciated.
14 Jun 2016, 13:40
I have a 2002 Chevy S-10 Blazer with the same problem. The system is full of R-134 refrigerant, but the clutch won't engage in the A/C compressor. I removed the clutch fuse from the fuse panel and jumped it across with a paper clip. The clutch engaged, and the cold air flowed.(I am sure this isn't recommended) but I ground off two posts the same size as the contact posts on the clutch fuse. I then ran two wires from my home made posts up under the dash. I drilled a hole and mounted a toggle switch. I also put a fuse in the wire in case it shorted or overheated. Now when I want A/C, I just turn on the factory switches like I normally did, and I flip the toggle switch! I have A/C! One thing you must remember is the wires running to the toggle switch are always HOT! Their fore you can't forget to turn the toggle switch off when you park or shut the A/C off. I am "NOT" advocating dangerous repairs that could burn up your vehicle. I am showing you what I did to save money on a vehicle that I have already poured a fortune in and it just keeps on breaking somewhere new each time! I am penny poor from this P.O.S.! I can't put hundreds more into trying to save it from the crusher. So it is a cheap fix with some risks. But for me it was worth it.
14 Jun 2016, 08:11
RE:Rb, Take your truck to another mechanic and get a second opinion. The low side pressure needs to be monitored to see when the compressor engages and disengages. The mechanic you are going to is missing something, maybe the second mechanic can spot the problem.

14 Jun 2016, 07:27
They are telling me the unit has the proper amount of refrigerant in the system. They have checked it with the scan tool and it's not throwing out any codes. I drove it yesterday towing and while driving down the highway it does decent. Get in stop and go traffic it starts to shut off. Even seemed at one point when you started to climb a hill it would kick in. I personally don't think it puts out as cold of air as it did before the compressor went out and I had it replaced. I have thought about adding a little more to see what it does but I don't want to mess with it and loose any and all Ac Freon.
13 Jun 2016, 16:07
RE: Larry, Make sure you have more than 40 PSI on the low side or the low pressure switch won't tell the PCM to engage the compressor clutch.

Having a certified mechanic look at the gauges and also look at the system with a scan tool will help diagnosis the problem.

There are too many issues that can happen to make a compressor clutch not to engage. A scan tool and professional gauges can help narrow down the possibilities.

I also need a lot more information on your problem like what engine is in the truck, is it automatic or manual A/C, are all the fuses good, is the compressor clutch relay working the list can go on.

The more information you can give me the better I can help you
11 Jun 2016, 20:34
I have a 2003 Silverado also and I can't get the A/C clutch to engage. I checked the low & high side and still won't come on.
11 Jun 2016, 13:43
RE: Mark, As I told Rb check your low side pressure when the clutch engages and disengages. If the clutch disengages notice where the low side pressure is at. The compressor will cut out at about 24 to 29 PSI and will turn back on when the pressure gets to about 35 to 39 PSI.

You may just need to add just a little R134a. But if the pressure goes above the 40 PSI mark and the compressor does not engage then check out the low pressure switch on the accumulator. Give it a tap it may be sticking or just needs to be replaced.

Try these suggestions and get back to me with your results, we can diagnosis the problem further if needed.
11 Jun 2016, 12:54
Have the same issue as RB above. Compressor would cut off every now and then at idle. Problem has gotten progressively worse, not will not come on at all. System has refrigerant.
Anybody have an ideal what the problem is?
10 Jun 2016, 23:12
RE:Rb, The low pressure switch helps control the operation of the a/c clutch by sending a signal to the computer to ground the a/c relay to engage the clutch.

The outside ambient temperature and relative humidity of the outside air can affect the operation of the a/c system. If the air outside is hot and humid it will keep the low side pressure higher than normal keeping the compressor engaged. If the temperature is cool and less humid the low side pressure could be kept below the cutoff pressure of about 25 to 29 PSI keeping the compressor disengaged. This is because the a/c pressures are lower on cool less humid days.

Observe the low pressure when the compressor cycles on and off. The pressure to turn on the compressor is about 35 to 40 PSI. The compressor is turned off when the low side pressure goes below about 25 to 29 PSI (depending upon the calibration of the switch). If the low side pressure stays below about 25 to 29 PSI the compressor may not engage.

If the pressure on the low side stays below the cutoff threshold then you may need to add a little more R134a to get the pressure up. I'm talking about 1/4 to maybe 1/2 a pound, no more than that.

Try these suggestions and get back to me with your results, we can diagnosis the problem further if needed.

10 Jun 2016, 20:33
I have a 2004 3500 Silverado LT Duramax. It has new compressor dryer accumulator etc. new blend doors, new control unit in dash. Ac works fine while driving. But seems to shut off sometimes when idling. There is no rhyme or reason. Professional gauges used and levels are fine. Fan clutch works as I tow with this truck and you can definitely here it come on. Shop is at a loss. Computers say all is working right and sending the proper signals. Wiring at compressor is good no issues there. Where do we go from here. All of us are at a loss. Any help would be great

03 Jun 2016, 22:45
RE: Don, The plug wire connectors could be worn and not making a good connection. Wiggle the plug at the compressor and see if the compressor turns on and off. If it does it could be the connectors or the terminals on the clutch coil itself. Replace the connectors if needed.

Using an OHM meter on the coil terminals and moving them around can determine if the coil windings are good and can maintain a connection for a complete circuit. If this test fails then replace the compressor clutch coil.
03 Jun 2016, 15:33
Had my 2500 avalanche (2003) to the dealer for ac not working. It will work fine for sometime then it won't they checked pressure and refrigerant levels and all was good. The modules were putting all the data needed for it to work but not engaging the clutch. They un hooked the plug at the compressor and checked voltage and good and when they plugged back in it engaged. Their theory is it might be a loose plug but on my way home it stopped working again and when I checked the clutch was not engaged.Could the electronics in the clutch assembly be bad or any other ideas.
01 Jun 2016, 19:24
RE: Larry Baca, It is possible but I would check the pressures first. If the pressure on the low side drops below the threshold set by the manufacturer of about 25 to 29 psi the compressor will turn off. Then when the pressure goes above that threshold it will engage again.

It could just need a can of R134a.
Larry Baca
01 Jun 2016, 17:38
My AC pump shuts off after running for awhile. If I let it sit a few hours then try again, it runs again for a while then shuts down. Could this be the low pressure switch?
27 May 2016, 05:31
RE: Mario, The low pressure switch is located on the A/C accumulator.
26 May 2016, 21:07
Where is the low press switch located on a 2011 Chevy Silverado. 5.3 v8. 1500 4wd?
27 Dec 2015, 12:01
Thanks TechHelp2!
27 Dec 2015, 05:24
RE: Robert, When the compressor cycles on and off fast it is a sign that the system is low on Freon. Check the pressures of the A\C system. You may just have to add some R134a to the system to get it back to normal pressures and have normal performance. You may have to check for a leak if this problem happens again.
26 Dec 2015, 10:18
I forgot to add it is a 2002 Chevy 2500HD 6.0 engine
26 Dec 2015, 10:17
I have a Chevy 2500HD with a 6.0
My A/C is not working properly. The Heater will blow hot when you turn the temperature to hot, but in cold it is just regular air.
So I changed the motor that opens & closes the air mixture. That did not solve the problem. After I bought the new part & installed & removed it again & was able to open & close the flap manually.
Then I opened the hood up on the truck & noticed the compressor cycling off & on.
What should I check next?
15 Dec 2015, 17:22
RE: Wm. Ryan, First you need to put a set of gauges on to see if the A\C system is fully charged with refrigerant. You may have a small leak where there is not enough refrigerant in the system to have the correct pressures to trigger the pressure switches to engage the compressor clutch. When you jump out the low pressure switch you are checking the electrical part of the system, it seems to be operating properly. Check the pressures first. Then check for a leak. You can use a dye to put in the system and check for a leak with a UV light.
wm. ryan
15 Dec 2015, 16:43
I had a new A\C Delco air conditioning compressor and drier installed in a 2002 Chevy 2500 HD 4x4 gas 6.0. It was in Oct. I went back to the shop they said there may be a leak. The weather has been cold. I don't see the clutch engage. If I jump the low pressure plug it will engage. The shop said it will not always engage, but if I turn on the defroster the A\C will not engage. Temperature has been about 30 deg. Can you help?
Bob Wire
10 Jun 2015, 18:54
Thank you so very much. With around 300k I will just parts replace cheap enough items. I was so excited when I received this I went out with a flashlight to see if I could see the location. I could not but I will find it and swap it out. Here's a weird thing. I was on a job today and it was near 100 degrees out. I know starting it up at idle it blows cold so I decided to let it run for 15 minutes to cool the cab down for my 45 min. ride home. Well low and behold it worked all the way home? Then when I went to shut it down to go into the bank and came out it was done. Thank you
07 Jun 2015, 13:20
There is a pressure switch on the power steering system that opens the compressor circuit if the pressure of the power steering system goes past the limit of the switch. If the switch is faulty it could be causing your problem. A simple way to check it is to unplug the connector to the switch and see if the problem disappears. If it does it will indicate the switch is faulty. These sounds like it could be the area of your problem since you said the compressor turns off on an upgrade in the road or accelerate.

If you can get someone to put a good set of A/C gauges on your van they can check the pressures of the A/C system. You can still have cold A/C but if the system is low on refrigerant it will shut off the compressor until the predetermined value set by the low pressure switch is met then it will turn the compressor back on.

The same goes for the high pressure switch, if the pressure gets too high then the high pressure switch will open and it will turn off the compressor until the predetermined value of the switch is met then it will close and complete the circuit turning on the compressor.
Bob Wire
06 Jun 2015, 19:31
I have an '02 Chevy van. My A/C works great when I start it at idle. As soon as I drive and accelerate it cuts out and never works again, until it sits. then I start over. I just drove out for 15 minutes with ice cold A/C. As soon as I hit a grade and accelerated it cut out and didn't work again. I am trying to find out the location of the WOT sensor. Unless someone else knows what it is?
04 Apr 2015, 16:39
Check your A/C fuse and also your ING fuse 10 AMP in the under hood fuse block.

The powertrain control module needs to receive a bunch of correct data from switches (three pressure switches under the hood), sensors before it will engage the A/C compressor. The best diagnostic route for you would be to check all these inputs with a scan tool before taking the next step.
stephen catino
03 Apr 2015, 11:46
I have no power to low pressure switch on 2002 blazer
27 Mar 2015, 07:58
RE: Horace Anderson, If you are not using professional gauges then I would not trust the gage. The compressor cycles fast because it is still below the required amount of R134a Freon. The low side pressure should stay at 30 psi for the clutch to stay engaged at an outside temperature of about 70 degrees. The pressure will be higher or lower depending upon the outside temp and humidity. Have the system checked by a professional with the proper gauges for the accurate amount of Freon in the system.
27 Mar 2015, 07:50
I sent you a electrical schematic for the A/C clutch circuit.
Herbie automotive
26 Mar 2015, 20:31
When fixing the aircondition, i bridged the fuse box to get the compressor on, but cannot get the compressor on by the a/c switch on dashboard of a 1999 chevy silverado 1500, 4.3L What might the problem be?
Horace Anderson
25 Mar 2015, 21:10
My a/c is blowing warm air. First I found the a/c belt was missing replaced the belt and added some refrigerant and the clutch started to engage only clicking on and off real fast still no cold air. The gauge says its charged but the clutch will not stay engage just clicking on and off real fast. What should I check. Thanks
15 Oct 2014, 19:58
Re: Barbara, Going to Sears is not going to a professional. Go to a repair shop or dealer that is ASE certified to check out the A/C system. Any mechanic who tells you an expensive repair like that without putting a scanner on the vehicle or even putting on A/C gauges is not the mechanic to go with. Giving you a full report pressure and electrical wise is what you want to hear before you make any decisions.
15 Oct 2014, 19:52
Re: Marcus, I would try a control module brand new from the dealer before looking for problems. I personally went through this, buying aftermarket parts for A/C issues is not a good idea. I bought three aftermarket blower modules and they all blew in a week. I bought the blower module from the dealer and I haven't had a problem since.
15 Oct 2014, 19:44
To Dale Kiser,

I would check the pressures of the A/C system to see if it has enough R134a in it. If pressures are within factory specs then have a qualified mechanic or dealer scan the truck for any trouble codes related to the A/C system. The mechanic can use the scanner to check the inputs and outputs of the system to help locate the area of the problem.
15 Oct 2014, 19:37
To Shawn Berlyak, The compressor may be the reason the belt broke in the first place. Before you replace the belt check to see if the compressor is seized.
15 Oct 2014, 19:35
To Scott, The only true way to check the A/C clutch system is to have a dealer check the A/C system when the problem happens with a dealer scan tool. The mechanic can check the inputs and outputs of pressure switches and modules to determine where the problem area is. It could be a sticking pressure switch, intermittent compressor clutch coil, A/C compressor relay the list can go on. That is why a scanner is needed to narrow down the possibility's.
15 Oct 2014, 19:21
To Mike in Nashville,

Tapping the dash and having the clutch go on would tell me a loose connection at the control head, blower module or plug on both. Using a scanner to check out the compressor clutch circuit is the only way to do this job properly. The scanner at the dealer can check all the inputs and outputs for the pressure switches, controller's and modules. The mechanic can tell what is operating properly by looking at the data and he can locate the area of the problem.
15 Oct 2014, 18:54
Using a scanner can help diagnosis pressure switches and power and ground to the clutch controlled by the PCM. If you can ground the other side of the compressor and it won't work then the compressor clutch may be failing. If it does work then check the ground for the clutch circuit.
27 Sep 2014, 06:02
25 Sep 2014, 20:39
Help...I have a 2005 2500 HD with a 6.0 gas. The other day my AC quit about 20 minutes from work heading home. did not come back on. It did it again this afternoon. I hooked up my gauges and there is plenty of Freon. pressure was equal about 120 psi if I remember right. left it running and checked the connector at the compressor and I have 12 volts there. stumped with this one
22 Sep 2014, 16:13
Replace the ac belt
28 Aug 2014, 12:34
2006 Silverado 1500 2wd 5.3.
3 separate times this summer my ac quits working when I take off from a stop sign etc. Blower just blows hot air. If I turn my truck off and restart it works fine ?
Any thoughts ?
Shawn Berlyak
26 Aug 2014, 17:37
my a/c belt broke while a/c was running will that damage my compressor
24 Aug 2014, 14:31
I checked voltage going in and out of the low pressure switch; 12v at relay pins 87 and 30. Cannot locate the vehicle control module to check ground there.
Dale Kiser
01 Aug 2014, 14:58
I have a 2000 Chev 2500
My air conditioning hasn't worked correctly Ever since it was new until this year. you could turn the unit on and it would work for about 10 minutes, then it would quit cooling, but the unit would still blow out air. After about 20 - 25 minutes it would again blow out cool air, but would only last about 5 minutes. It kept repeating the cycle. have had it to the shop they can't find out what is wrong with it.It did have a leak they said they fixed it put every thing NEW on it but the Compressor do you think that's what it need. Thanks
28 Jul 2014, 14:59
The electrical connection on the back of the a/c compressor broke on my 2001 Chevy suburban 2500 8.1 liter. Do I need to take out the whole compressor to fix?
28 Jul 2014, 11:58
I have an '05 Chevy PU 1500. The A/C fan works intermittently and sometimes when it does run it stays on after I cut the truck off. I have tried a new/rebuilt control module and it didn't correct the issue. I do get a slight amount of air while driving down the road, and very cold when the fan is working. It did not work all weekend, stared truck this morning and the fan came on. Started truck 4 hours later and it's not working. HELP! It's hot in Florida.
21 Jul 2014, 08:41
Mechanics are trying to sell me a new compressor for $1000. Ouch! Help guys. I need to find a reputable mechanic in Southeastern Michigan. Thanks.
Mike in nashville
20 Jul 2014, 20:35
After tapping top of dash thee a c will sometimes run all day and sometimes I can tap all day and nothing but hot air. It has correct charge in it when engaged and working I had a certified tech check with gauges. I have been told the way it's acting and working directly after tapping top of dash on top were controls are, sometimes it could be a bad relay. Ok we're is it ? It's driving me crazy because it is my company's van and so many ahead of me it will be weeks. I do A/C service on light commercial and residential so I'm really busy right now and need it working because it makes me angry at a customers house when I just came out of the attic that was 130 or so degrees I want my truck or really. My office. To cool me down. Man this sucks so bad and this van only has 40000 miles on it. Come on! Help me, it is a 2013 model 2500 cargo van.
Barbara Brown
15 Jul 2014, 13:33
I went to Sears for air condition service on my 2000 Silverado Pickup truck. They said I may need a new compressor for $1000 or more . No mention of electrical service. I'm taking this article with me when I return. Thanks.
Barb Brown
14 Jul 2014, 09:53
RE: Jake
10 Jul 2014

Have the compressor checked for a leak at the front seal and also have someone look at the pressures on the low and high side. The high side pressure could be too high and blow the safety valve on the back of the compressor causing the R134a to be blown out.
14 Jul 2014, 09:46
RE: Chip Gore
03 Jul 2014

Have the system checked with a professional set of gauges for the correct pressures, you need at least 30psi on the low side for the compressor to be activated. If pressures are good then check for current at the compressor clutch and also check for a good ground. If you don't have current unplug the compressor and ground one terminal on the clutch coil and take a jumper wire from the battery positive and attach it to the other terminal on the clutch coil and see if the clutch engages. This will check the coil to see if it good or bad.
14 Jul 2014, 09:36
RE: Roger
29 Jun 2014

Check to see if the system still has refrigerant (R134a)the system could be low and would have to be recharged. There are many inputs to PCM to have the clutch engage. If you don't have the pressure inputs accurate then the clutch won't engage. Put a set of A/C gauges on the system to check this problem out.
14 Jul 2014, 09:31
RE: jared
28 Jun 2014

Inspect the HVAC actuator harness for contact with the instrument panel support brace. It is behind the glove box to the left. There is a service bulletin on this. Also have the A/C system scanned for any trouble codes that may be related to any faulty actuators.
14 Jul 2014, 09:24
RE: Green
24 Jun 2014

Check all of your mounting bolts on the compressor just to be sure. The A/C puts a strain on the belt system and can cause the belt to vibrate more than usual. I don't know what year, make and model you have but get back to us so we can check for any service bulletins that may be available for your vehicle.
14 Jul 2014, 09:11
RE: Richard Young
21 Jun 2014

It sounds like you would need a compressor if it tries to stall the engine. But first have the pressures checked in the A/C system to see what is exactly going on.
14 Jul 2014, 09:07
RE: Edward Bodine
09 Jun 2014

Check the charge of the system with a set of A/C gauges you may be low on refrigerant. If the system is low you don't have enough pressure on the low side to activate the low pressure switch which controls the engagement of the compressor clutch.
14 Jul 2014, 09:01
RE: Brian
22 May 2014

The icing up could be a blockage in the line sometimes caused by moisture or contamination in the system. You will probably have to evacuate the system and recharge the system to get the moisture out. Sometimes the accumulator will have to be replaced, that is where the desiccant bag is (absorbs moisture) sometimes it burst and clogs the orifice.
14 Jul 2014, 08:55
RE: Mike
18 May 2014

Check your pressures again, the low pressure cut out switch is set to cut out the compressor clutch at 28 to 30 psi. If you are low on refrigerant the low side will go below that threshold and cut out the compressor clutch.
12 Jul 2014, 18:56
I have an 01 Silverado 5.3L. I have the right amount of refrigerant in the system. It gets cold at idle but when I accelerate to 2000 rpms the compressor cuts off. I only have a low pressure gauge and the reading at idle and ambient temp of 90 degrees is 45 psi. When I rev the engine, the pressure drops quickly to 20-25psi low side and the compressor cuts off. I am at a loss as to what the issue is. Can you suggest where I should troubleshoot?
10 Jul 2014, 20:47
31 May 2011, 14:01

The symptoms you are giving are indications that there may be moisture in the system and also low refrigerant. Check the system with A/C gauges to verify the low level. You may have to evacuate the system to try to get all of the moisture out and then recharge the system.
10 Jul 2014, 01:54
I have a 2002 chevy duramax. My ac does not blow cold at an idol but when I start driving the are turn cool. It seems that the ac stays on flow through and not max. The clutch on the compressor spins but never clicks and stops spinning. The other day I opened the hood and freon was being slung around from the compressor area. Is my compressor bad or what may be the problem?
09 Jul 2014, 12:59
Get it to a shop to be properly evacuated and recharged to the specific amount of refrigerant. This is almost always the first step in accurately diagnosing an A/C problem.
Trey Rodgers
08 Jul 2014, 11:55
I got a 2000 Chevy 1500 the ac blows but not cold so I put ac pro in it and it was holding pressure but it wouldn't blow out cold I still had some left n the bottle of ac pro so I decided to put the rest in it but when I did I notice that the pressure had went down quit a lot but not all the way I used the rest of bottle and seemed to get a little bit cooler but not cold enough to ride with my windows up I just bought the truck 2 months ago
Chip Gore
03 Jul 2014, 11:15
Hey guys. I have a 2002 2500hd with a 6.0 . I started having a/c problems about a month ago. It would stop blowing cold after a period of time. If I shut the vehicle off and restarted it, it would resume working. It then became more intermittent and now the clutch will not engage. I had it sucked down and recharged to oem specs. No result. I then changed the high/low pressure switch. still nothing. i have power to the relay under the hood. PLEASE HELP!
29 Jun 2014, 13:15
Hi, I have a '03 Silverado. The A/C stopped working a couple days ago. The clutch on the compressor clicks when I jump at the relay but does not engage to run compressor. I have checked that compressor will turn by hand so it is not locked up. Is it possible the clutch coil is still good but clutch disc itself is faulty somehow?
Thank you any help you could give me on this.
28 Jun 2014, 13:59
My a/c only blows on the floor and defrost what could be my problem I have a 2001 chevy silverado 2500hd 8.1 ltr
24 Jun 2014, 04:39
My a/c compressor makes a vibration sound when on and accelerate but not all the time also the a/c tensioner seems to bounce a little more than you would think it should. I'm still getting cold air really good and can use it but every so often you will hear that vibration sound when accelerating while on. I already replaced the tensioner and serpentine belt. It did help but it still happens. Can it be the bearings or the a/c clutch. Please help trying to fix it before it gets worser
Richard Young
21 Jun 2014, 17:18
I have an 01 V8 Silverado. My problem is that the compressor stalls the engine when I engage it by jumping the switch. I know that it is low on freon but for me to add freon to the system I jump the safety switch. When I do this compressor comes on briefly and wants to stall the engine so I remove the jumper wire to keep from stalling engine.
Edward Bodine
09 Jun 2014, 20:04
When I first bought my '02 Chevy 2500 HD my A/C would freeze up after about 2 hours of running. Now the only way I can get it to come on is to use a paper clip across two of the four leads on the fuse or relay in the electrical panel under the hood. If anyone could tell me anything about it would be so much appreciated.
22 May 2014, 20:39
I have a 2002 Chevy 2500 HD Duramax. The AC clutch will run and the line ices up, then AC clutch stops. Any ideas?
18 May 2014, 22:26
2000 Chev 6.0. Checked pressure, switches, orifice, all OK but intermittent clutch operation with 12 volts applied. Sometime the clutch will cycle on and off with the applied voltage, and sometimes not, i.e. 12 volts and clutch not on. Checked ground, was good, check coil, 4 ohms, even when not working. What could cause the clutch operation when voltage is supplied?
16 Oct 2013, 12:44
Not sure which relay you're testing, but a 4-pin relay must have 12 volts at 2 of its pins. One is the activation circuit, the other is the load circuit.

Typically the relay is turned on with a ground at the activation circuit.

Re-check the voltage with the ignition on, and check all related fuses. If you're working with the "Compressor Control Relay" it should have voltage from the 10 amp A/C Fuse and the 10 amp IGN E Fuse.
10 Oct 2013, 05:56
I am sorry. I did not give the model. It is a 2000 Silverado with a 5.3. The HVAC relay pins are what I checked I found no presence of voltage but I can cross pins diagonally and the HVAC system or another relay on the driver side engages and sounds similar to a fuel pump pressurizing and then shuts off but the ac clutch does not come on. The pressure on the freon is in the safe zone.
09 Oct 2013, 21:05
I have checked to see if voltage is present on two of the four pins and it is not. I jumped the pins diagonally but the compressor clutch will not engage. Please advise.
04 Sep 2013, 14:46

Once again, you must have the system serviced to be sure it's fully charged before going any further with diagnosis.
04 Sep 2013, 14:44

You didn't give the specific model, and I don't see circuits 30 and 87 anywhere.

You know the clutch is good, so first check the relay. If you have the same relay in the fuse block for another component which works, swap them and see what happens.

Otherwise, be sure system is fully charged, then all HVAC module inputs, power, ground and communication circuits must be tested and confirmed.

04 Sep 2013, 11:55
I have a 2000 Chevy Silverado ,with a 4.8 liter v8 ,I have never had no problems with the AC until recently ,and it was working great and then all of a sudden stopped ,the AC clutch stop engaging ,it'll work sometimes and sometimes wont what could my problem be
17 Aug 2013, 07:55
while truck is running with ac on, i jump start circuit 30 and 87. Clutch comes on, take wire off clutch stops. what now?????
29 Jul 2013, 13:57

You need a charging station to be sure the system is fully charged, and check for appropriate system pressures and temperatures. Once all that's confirmed, you look for an intermittent electrical failure.

Gotta start there, or you're gonna waste a lot of time.
29 Jul 2013, 13:53
Diesel tech,

Double check your schematic, but if you have 12 volts to the clutch (and the clutch has a good ground), but it does not engage -- you need a clutch and coil. Just be sure you can rotate the compressor by hand. Sometimes a seized compressor will burn up the clutch and coil.
24 Jul 2013, 11:38
I have a 2005 silverado with 5.3 v8 my air blows cold air for about 15 to 20 mins. then it turns warm. when working the compressure spins but when not blowing cold air it is not spinning. One place told me the compressure is losing pressure on the high side and someone else told me it is an electrical issue please help
Diesel tech
24 Jul 2013, 01:26
Ok I evacuated and recharged my system today, air blew awesome and clutch engaged like it should. I shut the truck off for an hour or so and when I started it back up the clutch wasn't engaging. I have voltage from relay to ECM to compressor with the switch on, I even manually applied 12 volts directly to clutch. I think it's the clutch, but it's only a year old compressor. Any help is awesome.
19 Jul 2013, 19:13
Too many possibilities to list. When it's not engaging, check for power at the compressor clutch coil. If there is power, but the clutch doesn't engage, it could be an intermittent coil.

After that I'd start by verifying system pressures. There could be an obstruction in the system sending pressures out of whack, which results in the PCM shutting down the compressor.

Again, lots of other choices, and it may very well be worth the diagnostic fee. Typically it's 1 hour labor (about $100) at Chevy service.
Matt M
10 Jul 2013, 11:34
i'm having similar issues, how do I go about checking sensor/PCM/input side wires?

Very hesitant to wasting money on a tech to tell me a wire is lose.

2001 1500 Silverado LS Extended
Checked refrigerant, verified relay, compressor turning off after 5-10 minutes of driving.
23 Jun 2013, 13:39
Little confused by the story. All I can say is that if you do have 12 volts and a good ground at the 2 terminals at the compressor clutch, the clutch coil is bad and needs to be replaced. You can also disconnect the clutch coil and check resistance. If it's infinite, that confirms and open in the clutch coil.
17 Jun 2013, 17:54
checking in for response
17 Jun 2013, 13:52
I have 2000 Silverado, new system put in two years ago. Worked for a week, back to shop, new control module. Worked for several months, back to shop, scabbed a wire from control output dark green to 87 on relay, worked for 5 months. Shop gives up. Last week pulling boat to lake, ac switch off, blower on suddenly freezing cold air for 2 hour trip. Now, I have jumped 30 to 87 relay pins, getting 12v and ground at compressor still won't engage. Are there any other direct inputs to compressor that I can jump?
Really just need to be able kick clutch on manually to make next move. I have checked scan codes nothing there. Thanks for your help.
07 May 2013, 07:22
You've done a good job of checking the basics, but it's tough to guess at this point.

The powertrain control module is what actually controls compressor "On" by supplying an activation ground at the compressor relay.

Stuff like the "on" signal from the AC control head, circuit from the PCM to the relay, along with other inputs and circuits have to be tested.
Anthony Smith
30 Apr 2013, 08:33
I have a 2000 silverado lt z71 4x4 the compressor clutch will not engage. when i jump the terminals on the relay it comes on and cools like a champ. freon is good. jumped pressure switches with no change. is there a way to test the switch control in the dash? i read that one of the light blue wires is for the ac controls and traced it back to the relay under the hood. could the ac button itself be bad and how do i check it without just buying one and putting it in. its a little to expensive to just throw parts at it. also i swapped the relay with no change
12 Nov 2012, 19:24
Can't be sure without seeing the exact high and low side gauge readings. It could still be low on refrigerant, or there could be a system blockage causing the compressor to shut down due to high pressure.

You're just guessing with a can of refrigerant.
18 Oct 2012, 13:53
Hello I have a 2004 silverado ss dual climate control is not blowing any cold air so i went to buy some refrigerant started to charge my unit but gauges went up so quickly. noticed the compressor was trying to engage and would spin for about 3 seconds and stop then it would try again and it would stop. when compressor tries to engage the gauges level would go down and then as soon it would stop it would go up.
do you think my compressor is bad now or any ideas of what it might be. i checked all fuses and relay already
02 Sep 2012, 11:16
Yes, it does sound like a bad compressor. You should get it to the dealership or a quality A/C shop. Often times just replacing the compressor does not fix the problem because the system has been contaminated.
31 Aug 2012, 10:31
I have a 1999 Silverado Z71 my A/C has not given any trouble, until recently. I started down the road and the A/C was blowing very cold as usual, but the compressor started making an awful sound. I pulled over and verified it was the compressor, turned off my A/C, and the noise stopped. I tried to turn on my A/C again and the compressor would not engage. I tried the A/C some days later and the compressor came on with the awful noise, and cooled fine until I turned it off, and now the compressor will not engage. Do you think the compressor is bad? Could it be something else wrong? Thanks!
12 Aug 2012, 15:52
Spin all components by hand before installing the new tensioner and belt. Including A/C, alternator, PS pump, etc.

Be sure nothing is seized, loose, or making abnormal bearing noise.

At the A/C compressor, be sure both the outer pulley and the center clutch hub turn freely.
bobby phillips
09 Aug 2012, 19:21
my ac belt tensioner broke it was only 16 months old do you know why, can i turn the ac by hand to see if it is free, before i install the new belt tensioner freon is good and also the fuses are good. i saw the amp meter move just before the ac stoped
13 Jul 2012, 14:42
By the book, requires removal of the compressor and the use of special tools to remove the bearing/pulley assembly and the clutch coil.

A Chevy dealership tech may have a trick to do it on the car. But in your case, I suggest replacing the compressor (assuming the re-manufactured unit comes with the clutch and coil).
09 Jul 2012, 08:49
I forgot to mention, my truck has the 2nd design AC system for the model year.
07 Jul 2012, 16:57
I have a 2000 Silverado 1500 with the 5.3L. I have done quite a bit of testing, and determined that the clutch coil is bad. Once it gets hot, it won't pull in the clutch when given 12V. It still flows about 3.6A, but it won't pull in. Is it possible to replace the clutch coil with the compressor still installed on the truck? I'd like to not take off the compressor, because if I do that then I will just replace it since it is 12 years old now.
04 Jul 2012, 14:51
Hey Jacob,

Pretty sure on your model, when you press the A/C button it adds or removes the line through the snowflake designating the A/C (compressor) being on or off.

If the line remains always on while pressing the button, it could be the control head itself causing the problem.

But first, you need the correct scan tool to check for HVAC trouble codes. The one they use at AutoZone won't do it.
28 Jun 2012, 11:08
I have an 03 Silverado 2500 with dual climate control. When you push the AC button the little snowflake that comes up has a line thru it. Jumped posts 30 and 87 and the compressor engages. Its full of refrigerant and I tried switching relays. Thanks in advance.
24 Jun 2012, 14:28
Hey Dave,

That could be a lot of things. You really have to test the system while the problem is occurring.

If the compressor is not engaging, you should begin by testing for power at the compressor clutch coil.

If it has both power and ground, the coil is bad. If you're missing either, the circuits must be traced back to locate the cause of the open circuit (example: bad relay on the power side, or pressure switch on the ground side).

And, as always, be sure the system is fully charged.

18 Jun 2012, 16:06
The a/c on my 2001 Sierra C3 works fine going down the highway, but after a while, when it's hot out, the a/c stops blowing cold, especially, if I slow down due to traffic.

I checked when this happened and the clutch to the compressor is not engaged.

Once it quits, it won't come back on until the engine has been off overnight. The next day it will run just fine again for an hour or more before it quits.

The refrigerant level was checked and is fine. The engine temperature and voltage are ok.
10 Jun 2012, 14:44
Hey Steve,

You didn't give me all the details on the truck. But on a 2000 1/2-ton applying 12 volts to the "power" wire at the compressor should engage the clutch.

The entire compressor isn't necessarily bad, but the clutch/clutch coil isn't working. Also must be sure that the ground wire at the clutch has a good ground. 12 volts does nothing unless it has a path back to the battery!
06 Jun 2012, 20:14
I ran 12 volts direct to the compressor and the clutch does not engage. Does that mean the compressor is bad?
26 Mar 2012, 21:33
Not quite sure where you're coming from on the low-pressure switch. Need to look at the schematic of the specific vehicle to determine what voltage should be where.

As always -- and more than always -- gotta check for the correct refrigerant charge. Low on 134a will stop compressor engagement due to the low pressure signal from the low-pressure switch.
rey herrera
26 Mar 2012, 20:00
I have ground fault at low pressure switch no 12v just grounded out. I disconnected clutch and line in back of compressor and still have fault all 12v at fuse box under hood are good. I even pulled the AC switch on the interior to see if that was it, and it wasn't. Please help it's hot!
25 Mar 2012, 13:55
Brant & Amy,

As usual, must be sure the system is fully charged. Then begin the electronic diagnosis.

No experience? Better off having it diagnosed by a tech.
19 Mar 2012, 21:26
I am having a similar problem with my 2007 Chevy Silverado 1500 classic. first my resistor went out and my blower fan was broken. Got all that fixed and replaced, and my A/C compressor won't kick in. It was before all other parts were replaced. What should I do?
18 Mar 2012, 07:17
My truck ac will blow air but only randomly will the air blow cold?
08 Mar 2012, 19:30
Yes...I agree I am lost too. to save a huge explanation I will wait until the issue rises again and make comments then. Insufficient data to diagnose categorically at present and the rest is a bunch of surmising.
08 Mar 2012, 18:53
You lost me?

You said "Problem is resolved...it was just gas".

My translation is it was low on refrigerant (R134a).
08 Mar 2012, 18:39
Refrigerant Dye is in the system. No Leaks. Discussed with mechanic today and he says definitely had the correct pressures prior to the dealer looking at it. Maybe we have a sticky pressure switch or something more sinister like a bad temp sensor somewhere.
I would love to get my hands on a Hummer H2 2007 wiring diagram if anyone has one.
Thanks again
08 Mar 2012, 17:48
Glad you got it! But remember, if it was low on refrigerant, you have a leak -- and it will happen again.

Did the guy who checked it out see a visible leak? If not, he should have added refrigerant dye to make it easier to find next time around.
08 Mar 2012, 14:40
Thanks for the Imput all, Problem is resolved. I took the vehicle to someone still servicing H3s (ours a H2) they found that it was just gas. I need to discuss this with our usual mechanic as he assured me the gas pressures were good.
Still not certain why bridging the Pressure sensor with the 2 resistors did not produce results but hey.
I must say, I do agree with the comments by Olivia. Great content and useful info. Well done
07 Mar 2012, 14:37
Gotta have the schematic, HVAC service manual information, and be familiar with the system -- or at least knowledge of automotive electrical troubleshooting.

First you have to confirm the circuit between the control module and compressor relay is good (along with any switches in series with the circuit).

Then confirm all power and ground circuits to the control module are there.

And then, confirm all the necessary inputs are being received. Ex: pressure switch(s)/sensor(s), evaporator temperature, on input from the control head, etc.

As always, let's be sure the system is fully charged.
01 Mar 2012, 22:33
I have a Hummer H2 2007 Model, (stretch) I have an issue with the Cabin (Front) standard AC not cooling.
We have pulled the Relay and confirmed we have volts at pin 30 and the compressor clutch engages when looped to 87. We have Volts at Pin 86 But do not have the Ground at Pin 85. This comes from the EMC module.
I would like to know how I can determine the number of sensors in the system, where I would find them and how I can determine if they or good or bad.
We think the pressure sensor (Front of vehicle drivers side, close the evaporator core) is good, we used 2 x 47kOhm resistors from the 5v and ground to create a voltage divider and fed this to the Sensor output wiring, but still no Joy.
The vehicle is in Australia and understandably NO one wants to look at it.
Any help you can provide is much appreciated.
21 Sep 2011, 19:54
Hey Mike,

Read all of the above. It can be many different failures, but always start by checking the refrigerant charge.
19 Sep 2011, 10:34
2000 Silverado...all the AC lights came on and it blows warm. Any idea what may be wrong?
01 Sep 2011, 09:53
Eliminating a low refrigerant charge was a good first step, but after that there are still a large number of possible failures which will prevent the A/C compressor from engaging.

I like to start at the main objective, and check for good power and ground at the compressor clutch coil. If power/ground are good (but no engagement) you need a new coil or compressor.

After that you should test the "command circuits", usually consisting of a circuit from the powertrain control module to the compressor clutch relay (sometimes with the pressure switch in-between), and of course testing the relay, fuses, grounds etc.

All looks good up to this point; next step is checking all the inputs to the PCM needed to enable compressor engagement (evaporator temperature sensor, "On signal" from the A/C controller, etc.).

And the list goes on, and on.

You really need complete service information, including schematics and some experience, to diagnose correctly the first time.

If you would like to give it a try, and learn a little in the process, subscribe to ALLDATAdiy.com.
29 Aug 2011, 21:21
I am having a problem with my ac on a 2000 1500 Silverado. The ac (manual) quit working, I had the refrigerant checked, it was a bit low on R134a so I had some added and the pressures seem to be fine. When this problem first started the ac would work for a few minutes when the truck was first started, if I turned the ac off for ten minutes or so then back on, it would work for a few minutes and then start blowing ambient air again. Now it will only work for five minutes when the truck is first started. It appeared to be the low pressure switch, replaced that and it still has the same problem. A few times the ac compressor has started and cold air would blow when the ac button was turned off. I thought the problem could be a bad temp control unit, but this has not seemed to happen recently. Could it be a problem with the high pressure switch, the clutch, or the temp control unit? When the compressor stops, the low side tube goes from being very cold to hot. Thanks for your help.
09 Aug 2011, 13:29
The control head is a good possibility, and if you have a known good unit for testing -- great!

But to be sure I would perform a detailed analysis of the related circuits before buying any parts.
08 Aug 2011, 23:32
nothing was spilled at all on it i was thinking maybe cause i just bought the truck and they guy said that the ac has been out for a while that the switches in the head are just going bad because i replaced like i said everything under the hood i know that it is odd that when the blower is of the compressor will work but with it on it wont i think I'm going to switch out the unit out of my GMC it looks to be the same and see if it works or not so i can go from there 4 shops today also told me it prob that head unit
08 Aug 2011, 19:20
That's a tough one. Typically the A/C light should be off with the blower off (along with the compressor). This may indicated a problem at the control head.

Was anything spilled on the control head, or buttons? That can cause a lot of erroneous problems.
07 Aug 2011, 21:54
i have a ? for my '97 Silverado i had just put all new ac system from compressor condenser hoses new orifice , switches o rings the whole thing it was running fine for few day and now the compressor only kicks on when i have the blower fan set to off the ac light will come on also when i have it set to off but as soon as i change the fan setting to anything but off the compressor cuts off and the ac light goes off also the system has no leaks and is fully charged could it be i need new buttons in my dash or is there something with the blower
01 Aug 2011, 08:47

Thanks so much for the help.

I should have caught that quicker. Oh well, I learned a few things along they way, that's the most important thing.

31 Jul 2011, 16:54
This one's for you, Ryan.

Kind of unsure what you meant by "adding coolant"?

But with the compressor cycling but not cooling, it needs to be tested for proper refrigerant charge and system pressures.
31 Jul 2011, 11:45
That would indicate an open in the DK GRN wire between terminal B of the cycling switch and terminal B of the high pressure cut out switch. There is a splice (S159) in that circuit.

Yes, you can jump terminal B to terminal B to see if that fixes the problem. If it works, you can do a quality job of connecting and routing the jumper wire as a fix, as opposed to finding the actual open in the harness. There's nothing wrong with that.
30 Jul 2011, 11:56
So I don't have 12v from the low pressure cutout switch to the high pressure switch.

I do have 12v into and out of the low pressure switch.

Can I simply cut out the existing green wire and just replace it with a new one?
29 Jul 2011, 21:56
Not getting the command "On" ground at the relay can have a lot of root causes.

Definitely let us know the outcome.

29 Jul 2011, 21:29
Yeah, I may end up there.

I found a good schematic diagram. I think I can do a bit more checking, now that I have it in black and white...

Still feel like the issue is at/near the relay.

Will post the solution...eventually.

Thanks for the help.
29 Jul 2011, 21:25
I have a 2000 Silverado. The A/C hasn't worked in a few years. I just changed the belt and the pulley. The belt pulley was frozen. I added coolant until the gauge said full and no cool air. The clutch catches for about five seconds, but then lets go and tries again every ten or fifteen seconds. It will not stay engaged to give cool air. What's next?
29 Jul 2011, 20:21
Hey Todd,

Like I said, it get's more complicated. Before replacing electronic components you need to completely test all related circuits with a high impedance digital volt/ohm meter.

This requires experience and training. There may be excessive resistance (poor connection) in any of the input, output, power or ground circuits causing you grief.

This may be a good time to bring the vehicle to a Chevy service department. They will typically charge about a $100 diagnostic fee. And your problem may be something they've run across before -- meaning a quick fix.

Explain the problem, and what you've tried thus far.
29 Jul 2011, 16:26
New HVAC control unit in dash
2 new pressure switches
new relay
replacement (salvage) PCM computer
clutch & compressor working great (when jumped)
Blows cold when engaged
Will not engage when jumped at either pressure switch

Still will not engage w/o jumping at the relay.

28 Jul 2011, 22:38
Yes, about 13.5 volts at both relay terminals.

So I'll investigate the grounding signal on the relay terminal tomorrow. I think I already check it out of curiosity today, but I'll look again.

Already looking into a used PCM unit at local salvage yards....

Thank you very much.
28 Jul 2011, 21:49
Did you check for 12 volts at 2 terminals at the relay?

The larger terminals are the load circuit (sending power to the compressor clutch from the "A/C Comp" fuse).

The smaller terminals are the activation circuit (to turn on the relay). One should have 12 volts from the "IGNE" fuse, and the other receives a path to ground from the VCM when the compressor is commanded on.

But again, if there is no ground from the VCM, and all the basics have checked good -- it gets more complicated.
28 Jul 2011, 12:46
system will not engage when jumping either pressure switch.

I have 80lbs static, 36lbs when running.

I actuated the new relay w the battery charger, it's fine.

I can't get any variation in readings on the relay pin sockets when system switched on -vs- switched off.

I can manually jump the relay sockets diagonally AND horizontally and engage the clutch, so the amperage is there.

How do I check for on/off signal at the relay?

27 Jul 2011, 16:19
Hi Todd,

There are only 2 pressure switches (clutch cycling and high pressure cut out). They are both normally closed. You might want to double check by jumping the 2 terminals at each switch 1 at a time. If the compressor engages (engine running/AC On) with the cycling switch jumped it could be a bad switch or a low charge (even though it blows somewhat cold with the relay jumped). If it works with the high pressure cut out jumped, it's probably a bad switch.

I'm assuming you tried a new relay. You should have 12 volts at 2 of the 4 relay terminals -- the "load" circuit from the "A/C Comp" fuse, and the "activation" circuit from the "IGNE" fuse.

If everything checks good it's gonna get more technical and involve testing the power, ground, input data and output command circuits at the VCM (Vehicle Control Module).
27 Jul 2011, 15:25
CORRECTION "1996 5.7 Silverado"

My AC is working fine, but I can only get it turned on by jumping at the relay. I have already installed a new HVAC control unit. I believe the high and low pressure sensors are fine. Is there a third sensor on this truck?

27 Jul 2011, 15:24
1994 5.7 Silverado

My AC is working fine, but I can only get it turned on by jumping at the relay. I have already installed a new HVAC control unit. I believe the high and low pressure sensors are fine. Is there a third sensor on this truck?

27 Jul 2011, 11:09
First step is confirming if the compressor clutch is engaged, or not, while the warm air is coming out.
19 Jul 2011, 13:56
I have a 2003 silverado. The air will work for a few minutes then we get a musty smell and it blows out warm air.
We check the freon and it is fine. What else should we do?
18 Jul 2011, 08:08
Compressor only engaging at higher RPM doesn't really make sense. All I can suggest is evacuate and recharge the system with the correct amount of refrigerant, be sure coolant fans are functional, and get it to a technician if there's still a problem.

Often you need an accurate assessment of system pressures by a professional for a correct diagnosis.
17 Jul 2011, 00:37
well here is my question. i have a 2000 Chevy Silverado 5.3 and the ac compressor will not kick in unless i run the RPM up and than it kicks in and than about a minute later it kicks off. Now i hooked a gauge to it and it was way over in the danger spot on the gauge " and i mean slammed all the way over to danger side " so i let some of the freon out and now it in the good on the gauge. but the compressor still only kicks in when i run the RPM up and it cools but when i just let it idle it doesn't kick on at all. any ideal why and what i can do to fix this problem? any help would be appreciated thank you....
06 Jul 2011, 22:22
Thanks! I will make sure I get that checked!
06 Jul 2011, 20:49

You're losing me, buddy?

We left off where the A/C worked great and you were having an engine overheat condition. That's why we were discussing condenser fan operation.

As far as "cool only at blower speed 5" is concerned, check to see if the compressor is conclusively engaging at speed 5 only. And not at all on 4, 3, 2 or 1.
06 Jul 2011, 20:40
Hey Humberto,

Seeing it worked great after charging, and now the compressor is slow coming on, my first thought would be a somewhat large refrigerant leak which needs to be found and fixed.

You may just be losing refrigerant fast, and the low pressure is preventing normal compressor engagement.

06 Jul 2011, 14:12
I have another update/instance with my a/c. My fan settings are 1 - 5, with 5 being the highest. If I ever turn my fan to a lower setting, my a/c quits cooling. As long as it's on 5 it will cool to the point of freezing your butt off!!!

Is this the definitive answer that it's the a/c condenser fan?
06 Jul 2011, 13:44
Hi I was reading all the comments above. My problem is that my compressor wont engage. I recharge with the correct refrigerant and everything. Compressor kicked in perfectly. It worked fine for like a week. Now when I hit the A/C button the compressor won't kick in directly. It takes about 5-7 minutes sometimes and some days it just takes a minute or 2. Could this be an electrical problem? Thanks for the help!
29 Jun 2011, 10:09
There's a Technical Service Bulletin (Bulletin No.: 04-01-39-007) involving replacement of the control assembly for loss of mode or temperature control.

But in that TSB there's a message.

Important: Prior to replacing the HVAC control head, review Corporate Bulletin Number 01-01-39-007.

This TSB instructs the repair of a poor electrical connection that can cause very similar problems.

Above all, be sure the evaporator is cool. This will confirm it's an air direction problem, and not an internal refrigerant pressure issue.
28 Jun 2011, 20:59
The ac for my 2000 Chevy Silverado 4.8 blows only hot air. I fully charged the system with R134 freon, but that didn't work. I was told to replace the ac control panel, which I did. The air continues to come out hot. The compressor seems to be working fine. Is there anything else that I could do, to fix my problem???
27 Jun 2011, 19:53
The fan (s) are mounted at the engine side of the radiator. First see if one or both are running with the A/C on.
27 Jun 2011, 09:56
Thank you guys! Appreciate all the help.

Ok, so the A/C condenser fan. Where is it located and how easy is it to replace?
19 Jun 2011, 14:18

Yes, coolant fan (s) operation, and coolant level are the first thing to check in that situation.

Typically a stuck thermostat will be a more drastic overheat condition.
17 Jun 2011, 21:39
I think that it's your A/C condenser fan.
17 Jun 2011, 13:18
An update on my a/c. I got a couple can of the R134A and was going to charge my a/c. I hooked up the first can, punctured the hole and you could hear a quick sucking noise, and all the freon was gone! My a/c system sucked that first can dry in a matter of seconds. So, I hooked up the second can. It didn't take long for it to get sucked empty, either. So, I went and bought a can of Leak Stop, R134A and Dye marker all in one can. I also bought a can of oil and two more cans of R134A. I put everything in except the last can of R134A. The a/c unit is working great. I can only think that there was a small leak that caused me to lose all my freon.

Now, another problem has arose since the a/c has been fixed. As I drive down the interstate, after a while my truck starts running hot. It doesn't get to the red, but it gets close. My last two numbers on my temp gauge is 210 and 265. There is a mark in the middle that I would say is 235. It gets up to that mark and stops. I can turn the a/c off and let the fan run and it will drop back down to around 190. When I turn my a/c back on it doesn't take long to go back up to 235.

My question is, can the thermostat have anything to do with this problem?
12 Jun 2011, 13:03
Could be a lot of things just like we've mentioned.

By jumping the relay you've proved that the compressor clutch and it's circuits are good, but you still have to figure out why the relay is not being activated.

Again, it could be as simple as a low refrigerant charge. Therefore the low pressure switch is preventing activation of the relay.
07 Jun 2011, 16:36
so what do i do or what could it be when i jump the relay with a jumper wire and the compressor works fine my truck is a 1999 silverado 5.3
01 Jun 2011, 10:22

Delco and Delphi are both big-time names in A/C parts. You're usually best-off replacing a compressor with the same brand that's coming out.

The air direction is electronically controlled. Stuck in the defrost position can be caused by a bad mode door motor. But don't replace any parts before having the problem accurately diagnosed.
01 Jun 2011, 10:12

There can be a lot of possible causes to an intermittent A/C compressor engagement. First off, you should check for an adequate refrigerant (R134a) charge.

If there's not enough refrigerant (due to a leak), the system pressure will be low. When pressure gets low enough the pressure switch will prevent compressor engagement to protect the system.

You may be at a pressure border-line -- so sometimes it works, and sometimes it doesn't.

Again, it could also be a number of electronic issues causing the problem. But get it to a qualified shop to test system pressure, and take it from there.
31 May 2011, 19:18
I have a 2000 Chevy silverado 5.3L. What is the best OEM replacement A/C compressor, AC Delco or Delphi?

Also my selector switch from DEF. to face vents and floor vents etc no longer works, only blows air through the defrost vents near the front of the dash at the windshield, where is it and what is that part called? I need to replace that as well?

31 May 2011, 14:01
My air conditioning hasn't worked correctly for the past two years and this being the third summer, it doesn't work at all. For the past two years you could turn the unit on and it would work for about 10 minutes, then it would quit cooling, but the unit would still blow out air. After about 20 - 25 minutes it would again blow out cool air, but would only last about 5 minutes. It kept repeating the cycle. Now, it won't come on at all. I can turn all the knobs on the coldest settings and push the a/c button and nothing happens. The compressor doesn't kick in, the motor doesn't idle down like the compressor came on, it just blows hot air through the vents. I checked all the fuses and not a one was blown, all were good. HELP!
14 May 2011, 13:50
When the air conditioning compressor engages you'll hear a "click". That's the sound of the compressor clutch engaging. You can confirm this by looking at the front of the compressor. The clutch plate (round thing in the front) will begin rotating with the engine when engaged, and stop when disengaged.

If you have a refrigerant leak (empty system), the compressor will normally engage as you charge the system and pressure builds.

But keep in mind, the correct procedure involves first finding and fixing the refrigerant leak, before evacuating and charging the system with a recovery-equipped charging station.
10 May 2011, 20:36
1999 model Chevrolet Silverado 1500 4wd 5.3-liter V8. A/C blowing warm. I attempted to try and charge with R134A. I ended up disconnecting because I kept hearing a clicking noise like something was trying to engage not sure don't know how else to describe it. What should I expect to hear when you say engaging. Being a woman I don't know how else to explain but it clicks on and off on and off. Thanks

12 Jul 2010, 17:07
The A/C compressor clutch will only make noise if it's slipping (not fully engaging) or the clutch bearing is bad.

If the bearing is bad it will make noise all the time -- not just while the compressor is engaged. If the clutch is slipping, you can visibly see that it's not engaging properly with the A/C on.

If the bearing is OK, the clutch is holding tight, and the compressor and its brackets are secure -- you most likely have a bad compressor which needs to be replaced.

If that's the case, you should remove the orifice tube and inspect for debris. If there's a black substance (Black Death) or metal fragments, the proper repair is replacement of the entire system. That includes the compressor, condenser/receiver dryer, hoses and evaporator. The metal lines can be flushed with an approved solvent.

It's expensive, but the only way to do it right, otherwise the left-over contaminants may trash any new components you install.
12 Jul 2010, 14:54
I have a 2000 Silverado 5.3 and last year I accidental over charged the system, know there's a "grinding" like noise coming from the AC compressor? do I need to replace the entire unit or can the clutch be replaced?
13 May 2010, 06:30
Again, most common reason for an A/C compressor not engaging is a lack of refrigerant (check that first).

You should have 12 volts going in and out of the low-side pressure switch (if there is sufficient pressure).

After that you're gonna need a schematic to accurately trace down the break in the circuit.

Also, be sure the compressor turns by hand -- not locked up.
11 May 2010, 23:38
I have no power at the a/c clutch. I do have power at the condenser and low and high switches. I jumped the relay as described and still nothing. Please help. Thanks, Wes
15 Nov 2009, 19:07
There are a couple of ways to test an A/C compressor clutch.

-- Resistance check of the clutch itself taken at the two connector terminals.
-- Check for power and ground leading to the clutch connector terminals.
-- Apply 12 volts and a good ground to the clutch to see if it engages.
-- Check for a clutch-apply command from the powertrain control module with a scan tool.

But remember, the majority of the times when the clutch does not engage, the problem is something other than the clutch itself.

Often there's a refrigerant leak; therefore a low pressure switch (sensor) will prevent the clutch from engaging to protect the system.
13 Nov 2009, 22:12
How do you know if the clutch on the ac compressor is bad?


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