Service 4wd light is on

Does yours have The Selectable Four Wheel Drive (S4WD) Front Axle or
Automatic Four Wheel Drive (A4WD) Front Axle is
The SERVICE 4WD indicator lit ? do you have any other lights lit ?
anything like this happen Transfer Case Shift Control Switch Indicator Flashes, then Returns to Previous Mode ? Your going to need to take it to a repair shop , have scan tool hooked up an check for codes in the BCM an all the modules !

Could be a bad dash switch but Envoy's and Chevy Trailblazers are notorious for bad 4wd actuators, which are located on the right side of the front differential. There is an electrical box that moves the shaft back and forth to engage the 4wd when you push the button inside. Look for a black box on the front axle with a wire going into it. Check that the plug is clean, apply a light coat of dielectric grease(available at most auto parts stores at the counter in a little packet) and plug it back in. If that doesn't work I would see a mechanic and get a professional dianoses.

I had TONS of issues with my 2001 Jimmy 4wd. A few things to check; the 4wd actuator switch, the vacuum switch, hub assembly, and the electronic 4wd dash switch.

To start with, a basic engine computer code reader will not connect to the body computer which is why they are not getting any codes.
The 4wd lights will not come on as long as the warning light is on, the system is disabled until it is fixed.
You may need someone with a professional scanner to connect to the body computer.

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

I would find an independant shop that has a Tech 2 or equal scan machine....these can communicate with your TCM (Transfercase Control Module) and narrow down what the problem is.

I just had the switch replaced on the dash and that helped for a while. Now the service 4wd light comes on but i have no problems shifting into 4wd or no problems with the switch lighting up. cost about $150 to have this done. Still not convinced the problem is solved b/c the service light is still coming on.

MIne will do that only when I'm switching my 4WD to 4low or 4hi, or back to 2hi AND the TB has good traction, ie pavement or rock OR the tires are cranked to one direction. After my wheels are straight, I just turn the engine off, and turn it back on, then the light goes off. Mine won't do that if I'm in mud, snow, or dirt where there's no traction. My guess is that it may be binding when the traction is good, therefore, I shouldn't be in 4WD. Anyway, my 2cents and I don't think you have to worry about it.

take it back, make them fix it right.

Check the actuator motor on the front axle to see that it is getting voltage,if so,the motor is bad.