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2005 Chevrolet Silverado:
Service 4WD Light Comes On Weekly

Alex Steele
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I have a 2005 Chevrolet Silverado and my SERVICE 4WD light comes on about once a week. When the light is on, the truck won't engage into 4-wheel drive. When I turn my truck off and start it again, the 4-wheel drive will work until the next time the light comes on. If I turn off the truck with the 4-wheel drive engaged, when I turn the truck back on, it is still engaged.


I have replaced the switch on the dash. GM has replaced the electronic control module and updated a program. It seems to happen most often when I hit bumps, especially wash boarded gravel roads. It seems like it could be a bad connection or ground.




The VIN tells me you're driving a half-ton with the 5.3-liter V-8. When the Service 4WD indicator lights up it's telling us there's a problem related to the Transfer Case Shift Control Module, the appropriate Diagnostic Trouble Code has been stored in the control module's memory, and it's gonna shutdown 4x4 operation to prevent additional damage.


When the failure is intermittent, it's like it never happened every time you restart the engine (cycle the ignition), until the fault reoccurs. Then the light comes back on, and you're out of business once again. According to your records the transfer case shift control switch and the control module have been replaced, topped off with a re-flash of the module's software. But, not a mention about the diagnostic trouble code which was stored in the control module's memory bank -- nothing more important than that.


If it was DTC C0374, chances are there's a bad connection in the wiring harness between connector C151 (under the hood) and the transfer case. This is part of the circuit between the front and rear propshaft speed sensors (mounted on the transfer case) and the transfer case control module.


Chevy technical service bulletin 05-04-21-003C, dated April 15, 2008, discusses a water intrusion condition affecting the harness and gives part numbers for modified harness assemblies and connectors (part numbers vary by vehicle option codes). This problem has been setting DTC C0374 and causing erratic 4x4 system operation on certain Silverado, and additional GM built full-size and mid-size trucks and SUVs ranging from 2003 to 2007.

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28 Jan 2016, 05:46
RE:Marshall B, On the oil pressure problem try to get a manual reading with a oil pressure gauge to check the actual oil pressure of the engine. This means remove the oil pressure switch and put a gauge in its place to get an oil pressure reading. If the reading on the gauge reads normal then change the oil pressure switch on the engine. Get back to me with your results, we can diagnosis the problem further if needed.
28 Jan 2016, 05:36
RE:Marshall B, On the four wheel drive problem, Check the pigtail wiring on the transfer case. The wiring is about 88 inches long under the hood to the transfer case. As per Service Bulletin No:05-04-21-003C there are two part numbers that can be used. Use either harness P/N 15832722 or 15224663 depending on vehicle equipment. If you can scan the system for a trouble code that can help further in the diagnosis.
Marshall B.
27 Jan 2016, 15:18
I have a 2005 GMC Sierra(slt) 5.3L Z71. The last 2 or 3 months, my oil pressure Guage has been shooting up to 60 when I take off or accelerate, it shoots up there to 60. But when I let off the gas it goes back down to normal, and my truck oil pressure has always stuck around 40 to 42. I can't think of anything that would cause this. I was wondering if I should put some kind of oil Flush additives or something like that. ??????
Marshall B.
27 Jan 2016, 14:56
I have a 2005 GMC Sierra (slt) Z71, And my "SERVICE 4WD" display has been coming on and off over the last 6 months. And the first 4 months my 4wd worked just fine, it'd change to all the settings=AUTO4WD, 4hi, 4low, & 2hi just fine. But I noticed that when I did change it to any of the 4wd settings it would make a loud CLUNK sound even changing back to 2hi it would make it too. And the service 4wd light won't stay on all the time, It usually comes on when I hit a bump. I live in the country & I live on dirt road & I'm always hitting big puddles of water, I don't know if I might have shortened something out. I work on my own vehicles & I'm not to good with 4WD systems. I need sum help, Please
23 Dec 2015, 09:37
It would not have any codes on it? My understanding was not unlike yours, I thought it had all the codes on it and the light would come on when it recognized the code was interrupted? I was told by them it could be the TPS (Throttle position sensor) I unplugged this while the car was running at idle it did not shut down nor did the check engine light come on.
15 Jul 2015, 14:49
RE: Efrain,

On the full-size pickup and full-size utility models, replace the 2.2 m (88 in) pigtail harness that runs from the C151 connector under the hood to the transfer case. Use either harness P/N 15832722 or 15224663 depending on vehicle equipment.

This is from Service Bulletin No.: 05-04-21-003C Listed on the Alldata information
14 Jul 2015, 04:42
does anyone know the part number to the speed sensor wiring harness? im going to just replace mine since I know for a fact I was in deep waters during a couple of storms here in southwest texas.
18 May 2015, 09:46
Hi Rob, Don't replace anything until you have the wire harness to the transfer case speed sensors inspected. There is a service bulletin on this condition where the wire harness to the speed sensor wears through and can make the transfer case act erratically. Some people say they have an intermittent illumination of the Service 4WD light. Upon inspection the technician may find DTC C0374 set. The customers may also say they have an intermittent erratic operation of the 4WD. You may notice the problem may occur on rough road or when wet conditions are present.
18 May 2015, 07:05
2005 Sierra Z71 with 5.3 V8
A couple of weeks ago the "Service 4WD" message came on and went off a couple of times. About 2 days ago I was driving and noticed that the selector switch was in "Auto 4WD", I selected 2Hi and went on; about a mile up the road my transmission kicked into neutral but the gear selector was still in drive and 4wd selector was still in 2Hi. I pulled over, turned the engine off, started back up, put gear selector into drive and it was like it was still in neutral. I pushed 4Hi, it shifted hard, and started moving. Once I got back on the road I pushed 2Hi and it shifted regularly and I drove to the house.

This has happened a few times in the last couple of days. Looking at replacing the switch and encodoer motor to see if that helps.

Any other suggestions other than taking it to the dealer for diagnostic?
06 May 2015, 11:56
RE: Sergio,

Chances are there's a bad connection in the wiring harness between connector C151 (under the hood) and the transfer case. This is part of the circuit between the front and rear propshaft speed sensors (mounted on the transfer case) and the transfer case control module.

Chevy technical service bulletin 05-04-21-003C, dated April 15, 2008, discusses a water intrusion condition affecting the harness and gives part numbers for modified harness assemblies and connectors (part numbers vary by vehicle option codes). This problem has been setting DTC C0374 and causing erratic 4x4 system operation on certain Silverado, and additional GM built full-size and mid-size trucks and SUVs ranging from 2003 to 2007.

Have someone put a scanner on the truck and see if a code is stored in the computer memory. It is possible that code CO374 is stored, if not please let us know what the code that is stored so we can help you diagnosis the problem further.
05 May 2015, 13:52
I have the same issue with error"service 4wheel drive" combined with the selector light going randomly to neutral this is where it differs from the rest of symptoms there was a error notification"reduce engine Power" after it wouldn't. Shift into drive or reverse but engine would rev just fine the morning I was able to somewhat engage 4hi that when "reduce engine powers" came on .what do you think it can be thanks for your time
. Joe78
15 Feb 2015, 11:11
RRE: Pierre, If it was DTC C0374, chances are there's a bad connection in the wiring harness between connector C151 (under the hood) and the transfer case. This is part of the circuit between the front and rear propshaft speed sensors (mounted on the transfer case) and the transfer case control module.

Chevy technical service bulletin 05-04-21-003C, dated April 15, 2008, discusses a water intrusion condition affecting the harness and gives part numbers for modified harness assemblies and connectors (part numbers vary by vehicle option codes). This problem has been setting DTC C0374 and causing erratic 4x4 system operation on certain Silverado, and additional GM built full-size and mid-size trucks and SUVs ranging from 2003 to 2007. If you post this to the forum I can get you the service bulletin and other information you request.
15 Feb 2015, 09:21
My apologies, I meant the ATC fuse in my last post.
15 Feb 2015, 09:18
The "service 4wd" light came on my 2000 Silverado and I can no longer engage in 4wd. I tried replacing the selector switch and still nothing. The ACM fuse is burnt and every time I plug a new fuse in, it instantly burns out. I tried replacing the acuator and the fuse still burns out every time. I believe the harness needs to be replaced because of the possibility of water hetting into it. What do you think? Where can I see how to replace the harness?
23 Nov 2014, 10:22
RE: Tyson,The first procedure you should do is to have a scan tool put on your truck to see if there are any trouble codes stored in the modules memory. Repair any trouble codes that may come up.

Chevy technical service bulletin 05-04-21-003C, dated April 15, 2008, discusses a water intrusion condition affecting the harness and gives part numbers for modified harness assemblies and connectors (part numbers vary by vehicle option codes). This problem has been setting DTC C0374 and causing erratic 4x4 system operation on certain Silverado, and additional GM built full-size and mid-size trucks and SUVs ranging from 2003 to 2007.
22 Nov 2014, 13:38
having problems with 05 silverado, service 4 wheel drive comes on in the cluster and stays on, no lights on the switch. truck blows the fuse in the fuse box. Thinking its the motor by the transfer case that switches the truck from 4wd to 2wd. can anyone let me know what they think thanks.
15 Oct 2014, 20:10
RE: Todd McMullan,

To properly check the system the truck would have to be scanned for any trouble codes related to the transfer case system. There a few related parts that can be checked like the propeller shaft sensors, control module issues which include wire harness modification and water getting into the module. That's just the tip of the problems. That is why scanning the system is so important before repairing or replacing any parts.
Todd McMullan
18 Aug 2014, 00:36
05 Silverado has transfer case issues, it will slip into neutral while going any speed unless the truck is in auto 4wd. that has been going on for months, lately it will go into 4wd for no reason, usually on a slow tight turn. Any advice?
14 Mar 2014, 15:33
RE: macie
10 Mar 2014, 13:13

The 4WD control unit DTCs are often manufacturer-specific, and cannot be read with a typical diy scanner.

Each DTC has different values as to when the warning light goes out. One may go off with three cycles of the ignition, while another must be cleared with the appropriate scan tool.

14 Mar 2014, 14:28
RE: Larry
10 Mar 2014, 17:21

If the technician at the dealership believes the problem is in the harness, then that's most likely where it is.

I've seen many times where a recall or TSB does not "technically" apply to a specific vehicle, but does to other very similar models -- and it's still the correct repair for both.
10 Mar 2014, 17:21
I have a similar problem to Alex... I have a 2005 1500HD Chevrolet Silverado and my SERVICE 4WD light comes on. When the light is on, the truck won't shift into or out of 4-wheel drive.

I took it to the dealer ship and they say that it had the error code C0374 and suggested that I replace the wire harness.

Is it only me that thinks that wires should not ware out? This is about a $600 repair for parts and labor, mainly labor.

Has anyone had this done and if so did if fix the problem?

I have read the bulletin #05-04-21-003C, and called Chevy Customer assistance, but they tell me that based on my VIN number, my vehicle is not one effect by this issue. So I am confused. Help...
10 Mar 2014, 13:13

As others, service 4wd light comes "on"
turn ignition off/ on seems to reset being able to engage from 2wd into 4 hi/lo..
yada yada yada...

1) Is the dtc read from obd2 connector or somewhere else?

2) will the dtc code stay present (set) to be read if the key is cycled,
( allowing the 4wd to engage again) or will the dtc be cleared because of turning the ignition off-- (As if disconnecting batteries on OBD 1 cars)?

Thanks, M
Fred Portrum
31 Jan 2014, 22:01
I have '05 1500 hd silverado with 6.0 having same issues with service 4wd light. Sometimes it works fine sometimes not at all. Tranny feels like its slipping sometimes with no pulling power at all. Gages not working correctly. Have had hot air on one side & cold air on other side of dual temp controls blowing at same time. Amazingly just fixed itself. I think I have a serious wiring issue. The truck is nice, but I really miss my trouble free Toyota......
04 Sep 2013, 14:33
Thanks for the update, Tom. Glad you got it fixed.
Tom Watson
14 Aug 2013, 18:52
Service-4WD came in again. Turned out to be a 4WD Transfer Case Motor Encoder Ring/Sensor 88962315
Tom Watson
13 Aug 2013, 11:29
2005 Silverado Crew Cab Z71 (37K-miles). Trip odometer display replaced by Service 4wd. Toggle through displays & it was gone. 30-miles later drove through a Driving-Rain storm. After that another 50 miles stopped for Coffee. Back in truck & headed down road. Red-Neutral light on, but transmission acting normal. Cycled to 4-HI & red light out, then to 2-Hi. Drove 700+miles and never acted up again. Dealership cleared Codes, will see if it happens again. Opinions???
01 Apr 2013, 18:39
Hey Justin,

Sorry about your bad experience at the dealership. Like you said, it's the people. There are the good, bad, and ugly.

Transmission DTCs and data can't be viewed on your typical low-dollar OBD scanner. You need the manufacture's hard/software, or a very expensive scanner package from folks like SnapOn tools.

If you're only interested in clearing the codes to see if the DTCs erase and lights go out -- do a hard reboot.

That means disconnect both battery cables (+ and -), and jump them together with a jumper wire. Turn the ignition to On, and let is sit for about 30 minutes.

Then, turn off the ignition, hook up the battery, and fire the engine. If the 4WD and/or warning lights are still not working correctly -- you have another problem which requires further diagnosis.
Justin (a different one)
26 Mar 2013, 16:27

I'd agree with your comments if dealers were honest. After spending 1.5k at Ken Garff in La Quinta, mine is unchanged. Had an independent mechanic look at it, he showed me they never bothered to open the transfer case, showing me the cause of the obvious and horrid grinding noise I had developed. I had two bearings left and the rest were all shattered and messed up. Parts cost alone was horrendous. The independent guy doesn't have a full shop and can't run codes however, so even though the problems are actually fixed now, I can't use the 4x4 and having the same 4low issues as others. Chevy is probably milking wealthier appearing customers by trashing the transfer cases on the freeway. Actually went back to KG, it is $200 for diag not $100.

Chevrolet is not amusing me. Fiesta Ford also has a really bad reputation with my father, however they were friendly enough to me and helpful versus deceptive, but I really think it's down to the people.

Anyone remotely near SoCal have this issue and have a remotely acceptable experience at the dealer? Please reply if so. Admin, please go into detail on scanning equip/ procedure needed if you can otherwise, I don't care how advanced. My guy has rebuilt a Freightliner from the ground up, so I am certain of his competence. I built my computer myself, so I am sure I can handle the equip.

Of course, I'd prefer honest dealer listings. But I am hardly willing to continue thus.
david Lamer
27 Feb 2013, 05:53
Ive had same issue with my truck as well.
It first appeared just before transmission went out.
Has occured ever sence. Have no controle in changing from 2 to 4 wd
However after being stuck and needing it it has "appeared " and allowed me to shift.
Probably just lucky.
Drives me nuts!
Also prevents me from seeing miles driven until i restart.
25 Dec 2012, 13:50

Your problem most likely has nothing to do with the 4WD system.

It could be a breakdown of a module inside the instrument cluster, a poor ground, or a whole bunch of other possibilities.

Also, instrument panels can do some crazy things when the alternator isn't charging properly.

Something you can try yourself is disconnect the battery, jump the two battery terminals together, and let it sit with the ignition on for about 30 minutes. This does a hard reset on all the electronic modules, and once in a while corrects problems.
16 Dec 2012, 03:21
service 4wdlight comes on while driving, scares me door locks go up and down, all needles on gages go back and forth, what could it be i hope i can afford the repair.
15 Nov 2012, 08:54
Too many possibilities to guess on that one. As with most, gotta start with retrieving the DTCs, check for any service bulletins, then follow manufacturer troubleshooting.
brad yurek
14 Nov 2012, 07:10
4 wd drive drops into 4wlow trucks lugs down then stsrts to pick again. Lights on dash switch go from neutral to 2w high randomly?
12 Nov 2012, 18:55

That's true. A malfunction light may not always produce a diagnostic trouble code, and the manufacturers have diagnostics which apply to that also.

It's usually the control module, or a poor connection in the power or ground circuits to the module.
12 Nov 2012, 18:49
Best bet is getting it diagnosed at a Chevy dealership. You must find the 4WD trouble code before going any further.
01 Nov 2012, 12:46
i have an 05 chev silverado nd the service 4 wheel drive was flashing on the dash but the 4 wheel drive still worked. sometimes to use the 4 wheel drive you would have to turn on and off the truck. now it doesnt work. how to i fix this.
29 Oct 2012, 22:49
And when it was all said and done, they had to replace my front right hub while I left it when I was out of town. My last mechanic told me he thought they nicked my CV boot while installing the hub as it was throwing grease everywhere. Needless to say this was the icing on the cake for me. I ended up talking to the service manager who looked at it and agreed to replace it on them so I have to make one more trip there. But hopefully, this will be the last of it.
29 Oct 2012, 22:42
I am just doing research now that I "believe" my problem has actually been fixed and I came across this thread. You might be interested my experience. I am not a mechanic and I didn't sleep at a holiday inn express last night so I will describe it to the best of my ability.

I, too, have a 03 GMC Sierra Z71. For the past few months, every time I turned on my truck, it would pick a light on my push button 4WD indicator and tell me I was in 4H, 4L, 2H, or N. I was always actually in 2H but the light would display wherever it wanted. I would also get the service 4WD code on my dash and I could not shift out of 2H.

I have a friend who owns a Chevy truck he works on and owns an auto repair shop. He replaced the encoder motor. It worked for about 5 minutes. Then back to the same routine. Through the course of 3 weeks, we replaced my 4WD push button switch assembly on the dash, did a complete computer reprogram on the truck (interestingly my dealer who serviced it regularly had never done a single update on a 9 year old truck), and he replaced a sensor on the front driveshaft related to the 4WD.

The last time I was in, he replaced the automatic transfer case module part #12580061. This immediately put my truck in 4WD and it would not shift out of 4WD at all. He put my old part back in and it shifted back into 2WD and did all the same stuff from before. I decided to take it to the dealer and dropped it off when I was out of town. When I returned, the dealer said the previous mechanic (my friend), had not cleared the codes out of the computer and that was the problem.

I drove the truck home 10 miles and again, it showed my transfer case was in neutral when it wasn't. Interestingly though, my service 4WD message never came up on the dash this time. Only red glowing N showed me their was an issue. To say I was beyond frustrated is an understatement. I had a very long talk with my service advisor who brought out one of their most thorough mechanics and I had a long conversation with him about everything I had done to fix the problem thus far.

He proceeded to replace the encoder motor again and had to drive it a fair distance before it yet again, started randomly choosing lights. After several days diagnostics, he said my automatic transfer case module in my truck would not hold a code in its memory. Therefore, there was nothing for him to read to tell him what was going on. The encoder motor my friend used was a remanufactured one. Apparently there is a bulletin out about faulty sensors in the early encoder motors. The problem is, they will test fine as they are being rebuilt. The problem doesn't appear till later on when installed on the truck. He replaced it with a new encoder motor which apparently comes with a new sensor and this combination of new sensor/encoder motor and module solved my problem (so far).

All this to say, you say to check for codes and thats all good and may work 99% of the time, but maybe someone else may have the issue like mine where the module isn't storing a code at all and their mechanics are stumped. I hope this helps someone in diagnosing their issue. I know how frustrating the last month has been for me.
15 Oct 2012, 10:15
You need a scan tool to check the 4x4 diagnostic trouble codes to begin diagnosis. Otherwise, you're paddling upstream.
steve krichten
23 Sep 2012, 11:46
My 2005 silverado is stuck in 4wd low the service 4wd light was on but not now. Is there any way I can get it back to 2wd my self?
17 Sep 2012, 21:20
2005 chev silverado 4x4 with a 5.3 v8. my problem is that i cant get my truck to go into 4 wheel drive. when i turn on the ignition all the lights on the indication buttons light up for just a few seconds and then go off. when i try to put it in to 4 wheel drive nothing happens. it just stays in 2 wheel. the service 4x4 comes on. what could it be? need help. on tight budget and cant waste my funds, i am disabled.
18 Jul 2012, 14:33
Yeah I have the same prob. 01 silverado
27 May 2012, 15:01
Hey Eric,

Gotta get it to a technician with the proper scan tool (Chevy Service).

The 4WD light coming on means there's a DTC (Diagnostic Trouble Code) stored in the module's memory.

Gotta pull the DTC before going any further. Otherwise you're paddling upstream.
25 May 2012, 21:07
I have a 07 Silverado and the 4X4 Service light came on today. When I went to start it it got stuck in 4L and had a hard time going. The motor was fine just seemed like it jerked a little bit and then accelerated just fine. The 4X4 lights sometimes flashes. I got home and turned the truck and restarted it. It stayed in 2H. Thoughts?
25 Mar 2012, 14:09
I can't see worn tires causing the problem.

It's a tricky transfer case control system to diagnose. You have to start by checking for trouble codes.
21 Mar 2012, 07:42
I have worn Mud tires on my 05 4X4 could this cuse a shifting probelm?..
My indacator Lights will go from 2-4 and sometimes get stuck in 4HI...I have already re-placed the Encoder Moter..(Not a good idea But if you get stuck in 4Lo or 4Hi With the Key in the on postion But not with the Engine running a QUICK Tap across the + to - on the Battery will clear it...Just to get you out of a JAM!let me know what you think please
27 Feb 2012, 15:40
Thanks I will bring it to the dealer
27 Feb 2012, 11:21
Gotta scan, get the diagnostic trouble code, and diagnose the problem from there.

The transmission and 4x4 codes usually require an advanced scan tool -- as opposed to OBD II codes.

So you really need a qualified technician to do it right.
27 Feb 2012, 07:37
I used 4whl auto 2 days ago and now the sevice light is on I put it back to 2 whl but the auto light stays on. I have 44k miles on the truck. Is this a problem?
21 Jan 2012, 11:45
Hi Chris,

The scanner they use at Autozone is a cheap model and may not pick up all DTCs.

Foe example, there's a technical service bulletin involving your model truck where the the 4WD light is on and DTC B2725 is set. The "B" stands for a body code, and Autozone may only being picking up "P" (engine codes).

The fix is replacement of the transfer case shift control switch.

But your best bet is getting it to a Chevy service department, and paying about a $100 diagnostic fee.

That will confirm the failure, as opposed to buying parts on a hunch.
16 Jan 2012, 16:44
I have a 2000 Silverado 1500 4x4 with push button 4wd. 246 transfer case. The service 4wd light is on and never goes out. However all 4wd drive functions seem to be working. I had Autozone check for codes an it has none. Do I need to check with a different scanner? Thiers was OBDII. Just today it slipped out of gear but said it was still in 2-HI. I pushed 4-HI and then back to 2-HI and it was fine again. Is the actuator motor going bad or not holding like it should? Please help....

Ron white
26 Dec 2011, 16:25
11 Dec 2011, 11:48
The "SERVICE VEHICLE SOON" or "SERVICE ENGINE SOON" indicator tells you there is a problem in the powertrain control system. There's a diagnostic trouble code stored in the powertrain control module's memory which will lead a technician to the cause of the failure. If the engine still runs OK it's not an emergency but should be taken care of as soon as you can.

The "CHANGE ENGINE OIL SOON" message simply means it's time for service. Bring your vehicle in for an oil change and any other service required when you get a chance.

If the "Service 4 Wheel Drive" message is displayed, again there is a trouble code stored in memory to help diagnose a problem in the 4x4 control system. No emergency, especially if 4-wheel still works.
sharon johnson
10 Dec 2011, 18:00
I have a 2008 silverado 4x4 and light come on for service.. is this ASAP repair or what. service dept told me to replace the control on the dash area and service tran when I can. The lights are not always on just now and then. I don't even use the 4 wheel in the city. thank you.
29 Nov 2011, 19:35
This advice applies to both Ray and Aaron.
29 Nov 2011, 19:33
Bring it to a qualified repair shop and pay the fee for an accurate diagnosis of the problem.

They will start by pulling up the transmission control system's diagnostic trouble code to begin diagnosing the problem.

This is more complex than a simple engine control problem.
29 Nov 2011, 11:27
my service 4wd light comes up on my dash regularly and goes out as well. and i rarely use my 4wd. but now its actually feeling like it is in 4wd sometimes. and some times not. any suggestions??
I dont want to cause damage , but dont want to pay for unnecessary service either. just want it corrected as winter is just ahead.
28 Nov 2011, 18:43
my 2005 chevy silvarado has the service 4wd display on the dash. the other night it said that it was in 4wd low but when i made a tight turn in a parking lot it was not skipping like they will usually do when the four wheel drive is engaged. and now none of the lights will light up on the pannel. is this most likley due to a bad connection?
28 Nov 2011, 11:31
My 05 Silverado also has the light come on each time I start it. At times it goes into 4 L and won't come out until the truck is shut off and started again later. You can feel the 4 L stopping and going. The indicator light doesn't light up at all now.
14 May 2011, 13:33
You really need a scan tool to pull up the transmission/transfer case DTCs (Diagnostic Trouble Code). The DTC may very well lead you to the suspect circuit.

There is a Technical Service Bulletin -- #05-04-21-003C -- it deals with DTC C0374. This DTC is caused by an insulation breakdown in the wiring to the speed sensor (s) (1 to 3 sensors depending on what transfer case you have). Not the sensors themselves. This insulation breakdown can also cause a short circuit (blown fuse).

If you would like the complete Service Bulletin, you can get that and all the service information for your truck at ALLDATAdiy.com
10 May 2011, 17:57
Hey thanks for the reply. I haven't replaced the actual encoder motor yet. I only replaced the encoder motor SENSOR. The TREC fuse shorted twice. The first going into 4 wheel drive, and then the 2nd going back to 2wd. I replaced the fuse and hasn't done it since, and the light has stayed steady in 2wd since. Now, today I tried going into 4wd because I still get the "Service 4wd" message on the dash to see if it worked. When I push 4HI, the light flashes for about 7 seconds and then nothing. The 2wd light stays on the whole time, and it IS in 2wd. No definite noise from actuator either (although I have really bad hearing). This was the original problem I was having a few weeks ago that started this whole mess. To fix it short term, I guarantee I could go underneath, and jiggle the harness going into the encoder motor, while a friend pushes the 4HI buttons, and it would work. That is how we got it to work a few weeks ago with a coworker helping me on it. Also, it is worth noting that it DID rain today very hard on a long drive from OK to Dallas, so I don't know if that holds any merit. So, should I check out that harness as the 1st step, or is it looking like I need to replace the whole encoder motor? Because I wouldn't think the sensor would be bad yet again. Thanks in advance for the for sure solution...hahaha. No, really, thanks for the help, and let me know what you think.
10 May 2011, 06:18
Hey Justin,

If you've already replaced the encoder motor, then chances are that's not the source of the short.

You really need to take a close look at the schematic, and try to duplicate the short (blown fuse). Did it blow going in 2 or 4-wheel? Then follow the circuit (s) which receives voltage at that time.

It's most likely a wire shorted to ground, somewhere.
03 May 2011, 15:13
I have a problem, where the 4wd lights sometimes illuminate, and sometimes don't. I have replaced the encoder motor sensor 5 months ago, changed the selector switch near the instrument panel 2 wees ago. After both things, it would be fine for a short time. Now, the lights didn't illuminate, so I checked the TREC fuse, and it was blown. Replaced it with a 25A fuse, because it was the only one I had at the time, and put the correct 30A fuse in yesterday, and all was fine, until today, when I shifted back and forth from 2 to 4, and the lights went out. I haven't checked to see if that fuse blew again, but if so, could it be the wiring harness, or would it be a short in the Encoder Motor itself. Thanks a bunch for the reply.
02 May 2011, 06:13
Not something you want to attempt to diagnose without the right tools and experience.

The procedure would begin by viewing the switch inputs and sending command outputs from the shift control module to verify operation (with the right scan tool), before investigating mechanical failures.
29 Apr 2011, 18:29
My 4 wd lights don't come on at all. It shifted into 4 lo from 2 hi. Went into 4 hi, now it won't come out and the lights don't blink or anything. Replaced the transfer case (np246) due to the leak from the output shaft pin. Now I have no control over this transfer case.
17 Apr 2011, 18:04
I presume you're talking about changing the transfer case fluid, and no, that's not going to fix the problem.

When the 4WD light came on there was a diagnostic trouble code stored in memory. To start, you need the correct scan tool to read the code and begin diagnostics.

If you're not familiar with the procedure, your best bet is getting it to a qualified technician to check out.
louis berryhill.
17 Apr 2011, 14:55
I to have an '05 Silverado and my service 4wd light comes on, but I can still use my 4wd. What do you recommend I do? I'm thinking that changing the oil might fix it. Please let me know what to do.


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