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ttiger79, Auto Mechanic
Category: Chevy
Satisfied Customers: 1257
Experience:  ASE Master Certified, Advanced Engine Performance
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I have a 2004 chevy silverado.. the service 4WD comes ...

Customer Question

I have a 2004 chevy silverado.. the service 4WD comes on in the service center on dash. None of the lights are on in the 4wd selector switch. Will not ingage 4wd either. I have taken to a mechanic and he is puzzled also. Chevy dealer found no communications with transfer case module. What do you think or know about this?? And where is the transfer case module located at on this truck?
Submitted: 9 years ago.
Category: Chevy
Expert:  ttiger79 replied 9 years ago.
Here's a technical service bulletin that should explain the problem you are having:

Bulletin No.: 02-04-21-006E

Date: July 20, 2006

Inoperative 4WD/AWD Lamps, Inoperative 4WD/AWD System, DTC C0550 Set, No Communication with TCCM (Reprogram Transfer Case Control Module)

2004 Buick Rainier
2002-2004 Chevrolet TrailBlazer, TrailBlazer EXT
2003-2004 Chevrolet Avalanche, Silverado, Suburban, Tahoe
2002-2004 GMC Envoy, Envoy XL
2003-2004 GMC Sierra, Yukon, Yukon XL
2004 GMC Envoy XUV
2002-2004 Oldsmobile Bravada

with Electronic Shift or Active Transfer Case (RPOs NP1, NP4 or NP8)


This bulletin is being revised to add Subject information and change the labor operation. This bulletin cancels and supersedes Bulletin Number 03-04-21-006. Please discard Corporate Bulletin Numbers 02-04-21-006D and 03-04-21-006 (Section 04 - Driveline/Axle).


Some customers may comment that the 4WD/AWD indicator lights on the selector switch do not work, or the 4WD/AWD system may be inoperative and/or the service 4WD light is illuminated. The condition is typically intermittent and always occurs at key-up. Upon investigation, the technician may find DTC C0550 set.


The Transfer Case Control Module (TCCM) has not received the wake-up call during key-up and has remained in the "sleep mode".

When the TCCM is in the "sleep mode", it will not communicate with the class II bus or the Tech 2(R).

1. Try to communicate with the TCCM. If no communication, go to step 2. If communication is possible, go to step 5.

2. Remove the underhood (ATCM or TREC) TCCM fuse.

3. Wait 30 seconds.

4. Reinstall the fuse. The module should wake up and communicate with the Tech 2(R).

5. Check for codes in the TCCM. If a C0550 is found current or in history with or without other codes, replace the module and go to step 6.

6. Reprogram the TCCM with the latest TIS software using normal SPS procedures.

7. Verify operation and that no codes are present.

The control module may have to be replaced. I believe it's located under the dash on your truck, but it's not something you can change yourself- it needs to be programmed before the truck can be driven. Follow the instructions in the TSB, and see if you can get the TCCM to wake up.

Let me know what you find!
ttiger79, Auto Mechanic
Category: Chevy
Satisfied Customers: 1257
Experience: ASE Master Certified, Advanced Engine Performance
ttiger79 and 6 other Chevy Specialists are ready to help you
Customer: replied 9 years ago.
Relist: I still need help.
ok, I had already purchased and had the Transfer case module installed before i got the info from you. I have reinstalled the old Transfer case module and Pulled the TREC fuse out and waited for over 30 seconds. The service 4wd indicater does not come on anymore but my 4wd switch is still dead and does not light up or operate the shifter, we installed a new switch also on the dash. Still dead.. Do I need to go ahead with the new transfercase module and have it programed?
Expert:  ttiger79 replied 9 years ago.
Yes, that's what you are going to have to do. This means that the module isn't powering up correctly and isn't communicating with the vehicle yet, and that would be the next step. I would probably return the switch, too!
Customer: replied 9 years ago.
Reply to ttiger79's Post: Is there anyother thing that could be causing this problem besides the transfer case module that you know of.
Expert:  ttiger79 replied 9 years ago.
The only other thing that can be happening is a break in the communication bus (electrical wiring itself.) You can check this first if you would like to make sure this is ok. Here's a couple diagrams that will help you. The first one shows the connections with the high speed communication bus, and the second one shows powers and grounds to the module itself:



I would suggest removing the negative battery cable before checking the wire for the high speed bus (grey wire between the PCM and the module) as this bus is easily shorted out, and that could cause you many more problems!
Customer: replied 9 years ago.
This vehicle only has 41,000 miles on it, i'll give the transfer case module a try and hope for the best. thanks for your help. I"ll let ya know how it goes.
Expert:  ttiger79 replied 9 years ago.
I would agree with you on that one! That's a lot of wiring to check!

Let me know how things turn out for you!
Customer: replied 9 years ago.
Reply to ttiger79's Post: Just dropped in to let ya know that the Transfer case module was the fix and having it programed. All is good now.. Thanks for your advice.

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