My 2002 GMC Envoy wont switch into 4wd. The selector switch inside the vehicle seems to be fine, the lights switch to whatever setting I set it too but it doesnt physically switch into 4wd. From other

rough guess is that there is a connection that is not properly connected

The fact that it flashes 5 times means that it is trying to tell you what is wrong, you may have a faulty selector unit

you need to replace the tccm and get it programmed.


2-04-21-006E: Inoperative 4WD/AWD Lamps, Inoperative 4WD/AWD System, DTC C0550 Set No Communication with TCCM (Reprogram Transfer Case Control Module) - (Jul 20, 2006)
Subject: Inoperative 4WD/AWD Lamps, Inoperative 4WD/AWD System, DTC C0550 Set, No Communication with TCCM (Reprogram Transfer Case Control Module) Models: 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).
ConditionSome 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.
CauseThe 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®.
Correction

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®.
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.

Important: For the 2002 model year, the TCCM must be replaced and reprogrammed.
Warranty Information

Is the Service 4WD light coming on? Do the light on the 4x4 selector switch indicate the proper selection and are on steady after a few seconds? If there is no Service 4WD light on and the selector switch light are correct, the only place for the problem to be is the Transfer Case. Has the Transfer Case fluid been changed at 50K mile intervals? This is very important.

there are bulletins on this the diff actuator could be bad or the switch on the transfer case is sticking

About 45% of all transmission problems are due to a defective battery, its components and the charging system. Make sure you have the correct battery in the vehicle and inspect all items relating to the charging system and the battery. The vehicle should charge between 13,5 to 14.5 volts. The condition of the belts is part of this inspection. If all of this is A-ok then get a scanner and pull the codes in the PCM.
Remember once light is illuminated for a item that item no longer works. Once you've gotton to this point let me know and I'll walk you threw it. Hopefully its just a weak or incorrect battery or something along those lines. Good luck and keep me posted

no, you may have a bad switch or 4wd module,i ve done afew that had bad switches, but i would check it before just replacing it,

check the transfer case selector motor, they go bad and thats what happens, no 4wd and no other lights on switch because its not doing any changing of the tranfer case, to test remove motor ground it and with connector connected, try to switch to 4wd see if motor moves, you can use a test light to make sure its getting power, hope this helps.

Try disconnecting the battery for a few minutes, that might get you out of trouble for now. There are some common problems with these 4wd systems. Most common is the mode position sensor on the encoder motor. Sometimes it can be the switch on the dash. Fact of the matter is I can't tell you what is wrong without using a scantool to retrieve the fault codes from the computer controlling the 4wd.

Clicking sound- Possible electronic fan clutch failing. This is a typical problem with the 4.2L I6. When you hear the clicking, open the hood and listen if its coming from the fan clutch on front of the engine. this could also cause abnormal engine and transmission cooling. As for the "N" light and 4wd issue, have scanned at the dealer....Hopefully this helps