Chevrolet Silverado 1500 / 2500 / 3500:

2002 Silverado A/c clutch won't engage

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# 1  2/3/2012 7:14 AM

Eagle70ss
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Registered: 2/3/2012
Posts: 6

2002 Silverado A/c clutch won't engage

So here's the story. My compressor sounded a little noisy so I decided to do an a/c system evacuation. I also drained the compressor oil and replaced it with new oil. Put a new orifice tube in there as well. Pumped down to 30 vacuum.

I went to recharge the system and I noticed the a/c clutch will not engage. So I jumped the low pressure switch connector on the accumulator and the clutch still didn't engage. Next I also jumped the high pressure switch connector and still no clutch.

The only way I can get the clutch to engage now is to jumper the relay pins at the fuse box. When I manually engage the clutch at the relay pins, the clutch engages fine and the air inside the cab is blowing cold.

However as soon as I remove the jumper wire, the clutch shuts off.

Where should I start looking next?

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# 2  2/3/2012 1:14 PM

TechHelp2
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Registered: 11/24/2009
Posts: 1299

Re: 2002 Silverado A/c clutch won't engage

Did you try swapping the A/C compressor clutch relay with a known good one?

The A/C compressor is turned on by the PCM supplying a path to ground through the A/C compressor relay. Power is supplied by two fuses, the IGN fuse 10 AMP and the A/C fuse 10 AMP.

Try looking at these suggestions first. It could still be a faulty PCM or A/C control module.

I have given you a schematic for the A/C compressor circuit below.

Let us know how you make out when you can.

https://www.realworldautomotive.com/forums/uploads/thumbs/174_989355381.gif


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# 3  2/4/2012 2:06 AM

Eagle70ss
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Registered: 2/3/2012
Posts: 6

Re: 2002 Silverado A/c clutch won't engage

Yes, I swapped the A/C relay with the horn relay (horn works) and still no clutch.

Both the A/C fuse(10) and the IGN E(10)  fuses are good.

The HVAC controls on the dash work as normal. The A/C button lights up, the recirculation button lights up as normal and all the heater and blend door controls work.

So when I activate the clutch manually, I jumper wire pin 30 and 87 on the relay.So I should have 12v at pin 30 & pin 86 if the truck is running. So all I'm missing is the ground from the PCM right?

So the PCM needs a good signal from the low and high pressure switches, plus HVAC control module signal (is that my dash controls?) to complete the ground and activate the a/c relay right? What else does the PCM need to provide that ground?



I'm stumped?

Last edited by Eagle70ss (2/4/2012 2:19 AM)

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# 4  2/4/2012 2:22 AM

TechHelp2
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Registered: 11/24/2009
Posts: 1299

Re: 2002 Silverado A/c clutch won't engage

Terminal 85 is a controlled ground inside the PCM, when 85 is grounded it will close the terminals inside the relay completing the circuit to 87 which should be hot to the A/C clutch.

86 is hot and 30 are hot, when 85 is grounded you should have voltage at 87.

Try to manually ground terminal 86 and see if you can get the compressor clutch to engage, if it engages then you probably have a PCM that can't ground the circuit and would need to be replaced.


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# 5  2/10/2012 4:06 AM

Eagle70ss
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Registered: 2/3/2012
Posts: 6

Re: 2002 Silverado A/c clutch won't engage

So I put pin 85 to ground while relay was still in and it pulls the clutch in each time I put 85 to frame. All hvac fuses are good. I wasn't able to get a reading on the high switch for voltage because I don't have a lift and the compressor is in a dumb place. I miss my ol' 95 chevy where the compressor is right on top.

However, I pulled the connector off of the high switch (engine off) and I put my meter across the two connector pins and I get a constant .3 ohms across the pins. This mean I have good continuity?

Then, I do the same for the low pressure switch connector and at first I get no constant reading. So I move further down the wires and I pierce into each wire with the meter leads. I get a stable reading of something like 20 Meg ohms? Should the resistance be this high at the low pressure connector? Should I get about the same resistance as the high switch connector?

Thanks

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# 6  2/10/2012 4:53 AM

TechHelp2
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Registered: 11/24/2009
Posts: 1299

Re: 2002 Silverado A/c clutch won't engage

If you grounded pin 85 and the clutch worked then that would be the problem and not the pressure switches.

Pin 85 is ground from the PCM, if you don't have a ground with the controls activated then the ground problem is in the wire to pin 85 being open somewhere or the PCM itself.

There is a procedure that is mentioned on how to reset the A/C module memory, this activates the compressor clutch. Sometimes this works.

Disconnect the battery for about a minute then reconnect the battery cable. This won't hurt to try, let me know if it works.


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# 7  2/11/2012 2:25 AM

Eagle70ss
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Registered: 2/3/2012
Posts: 6

Re: 2002 Silverado A/c clutch won't engage

TechHelp2 wrote:

If you grounded pin 85 and the clutch worked then that would be the problem and not the pressure switches.

Pin 85 is ground from the PCM, if you don't have a ground with the controls activated then the ground problem is in the wire to pin 85 being open somewhere or the PCM itself.

There is a procedure that is mentioned on how to reset the A/C module memory, this activates the compressor clutch. Sometimes this works.

Disconnect the battery for about a minute then reconnect the battery cable. This won't hurt to try, let me know if it works.

Nope. Removed the battery cables for an hour and then hooked it back up.Clutch still won't engage when a/c is switched on.

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# 8  2/11/2012 3:26 AM

TechHelp2
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Registered: 11/24/2009
Posts: 1299

Re: 2002 Silverado A/c clutch won't engage

Like I said, sometimes it works.

It looks like the problem is in the ground wire (85) or the PCM.

From what the schematic looks like the ground comes from the PCM, you said when you ground the 85 terminal everything works normal.

You will probably need to replace the PCM from what you are telling me.

Sorry for the bad news.


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# 9  2/13/2012 5:36 AM

Eagle70ss
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Registered: 2/3/2012
Posts: 6

Re: 2002 Silverado A/c clutch won't engage

TechHelp2 wrote:

Like I said, sometimes it works.

It looks like the problem is in the ground wire (85) or the PCM.

From what the schematic looks like the ground comes from the PCM, you said when you ground the 85 terminal everything works normal.

You will probably need to replace the PCM from what you are telling me.

Sorry for the bad news.

Yeah, that would have been great if a simple reset would have fixed it. I would really try anything at this point. I appreciate all your help on this problem. I have learned a ton about these systems. The funny thing is that all of this stuff worked before I took the compressor out and put in a new oil. The compressor was just a little noisy. It seems too convenient that the PCM malfunctions exactly the same time I dropped the compressor. What could I have changed when I took the compressor out? What other readings (other than high and low pressure switch) does the PCM monitor before providing that ground to the relay? Does it monitor the clutch coil resistance, outside air temp, or other conditions inside the a/c system? Maybe I didn't pump the system down for long enough?


For instance, I see the high & low pressure switch inputs on the schematic, but what makes the HVAC high pressure recirculation switch activate in this part? Is that only for Max AC setting on the controls?

https://www.realworldautomotive.com/forums/uploads/thumbs/2598_snap1.jpg


Thanks again for your help

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# 10  2/13/2012 2:46 PM

TechHelp2
Administrator
Registered: 11/24/2009
Posts: 1299

Re: 2002 Silverado A/c clutch won't engage

If you can get the truck to someone who has a scanner to see if they can tell you if there are any trouble codes stored in the computer. They can also check if the PCM is giving the ground signal and if other sensors are working properly. If you don't then I have given you some other things to look at to see if we didn't overlook anything.

If you don't have anyone then go to your local Auto Zone auto parts store and they will be able to scan the computer for any codes and they usually do it for free. 

Is it cold where you are? Is you’re A/C system automatic or manual?

The list below is for an automatic system with the use of a factory scanner.

If the PCM doesn't receive a signal from the A/C control module the clutch will not engage.

The A/C compressor clutch will not engage if temperature outside is below 38 degrees.

The A/C compressor relay output is disabled if engine coolant temperature is above 121°C (250°F).

The engine coolant indicator will illuminate at this temperature.

These actions will enable the A/C compressor to operate.

If the HVAC control module does not respond to requests, the module is inoperative.

This action ensures that the powertrain control module (PCM) is getting an A/C request from the HVAC control module.

If the A/C request LED does not illuminate the HVAC control module is not grounding the A/C request signal circuit.

Since the A/C system pressure has been verified in Step 4 this step will verify the integrity of the A/C request signal circuits, if the A/C Request Signal parameter is Yes then there is a concern with the A/C high pressure switch.

Since the A/C system pressure has been verified in Step 4 this step will verify the integrity of the A/C low pressure signal circuit and the ground circuit of the A/C low pressure switch, if the A/C Compressor Cycling Switch parameter is Normal then there is a concern with the A/C low pressure switch.

If the test lamp remains illuminated the A/C compressor clutch relay control circuit is shorted to ground.

If the fused jumper opens during this step the A/C compressor clutch supply voltage circuit is short to ground.

Ensure that both A/C request signal circuits of the A/C high pressure switch are tested for listed conditions.

Take a look at these suggestions and let me know how you make out.


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