Service 4wd light is on

Check connection at the 4X4 control module , check power and grounds . Do you know how to do basic electrical testing ? Does your truck have manual or automatic trans mission ? Which transfer case does yours have ?

The NVG 263 transfer case features a three button shift control switch located on the instrument panel. When the ignition is in the RUN position, the transfer case shift control module (TCCM) starts monitoring the transfer case shift control switch to determine if the driver desires a new mode/gear position. At a single press of the transfer case shift control switch, the lamp of the new desired position begins flashing to inform the driver that the TCCM has received the request for a new mode/gear position. The lamp continues to flash until all shifting criteria has been met and the new mode/gear position has been engaged. Once the new mode/gear position is fully active, the switch indicator for the new position remains ON constantly. In the event that the TCCM can not or will not complete the shift, the indicator returns to the previously selected position; this is a Transfer Case Shift Control Switch Indicator Flashes, then Returns to Previous Mode condition.

The NVG 263 transfer case provides the driver with 4 manual mode/gear positions:
• 2HI - 2 Wheel Drive high range
• 4HI - 4 Wheel Drive high range
• 4LO - 4 Wheel Drive low range
• Neutral
Have it check for DTC'S - diagnostic trouble codes ?

DTC B0790
DTC B2725
DTC C0306
DTC C0327
DTC C0359
DTC C0379
DTC C0550

Your best bet would be take it to a qualified repair SHOP

Search Results

GM 4WD Warning Light Issues - RepairPal

repairpal.com > Car Care Advice > References
Is your 4WD warning light on or a "Service 4WD System" message displayed? ...attempting to shift from 2WD high to 4WD low while the vehicle is moving), all the mode ... Resetting the TCCM may only be a temporary repair. ... The Bulb Check ...The mode switch on the dash will send a signal to the transfer case control ...

How To Override Stuck 4WD On A GMC Vehicle, Transfer ...

? 10:55
https://www.youtube.com/watch?v=qNQOt_JP78U
Dec 29, 2014 - Uploaded by JamieJones TheCarMan
How To Override Stuck 4WD On A GMC Vehicle, Transfer CaseControl ... My 07 GMC Yukon is stuck in 4 wheel drive low, can I do this same process on it? .... has been changed, the module was reset, and the actuator were all replaced. ... is being slughish when i start off from red light or stop sign but after ...

It means the transfer case control module has detected an error in the system...it could be the encoder motor, transfer case, dash switch assembly, axle engagement assembly or even the transfer case control module (TCCM) itself..you will have to take it in and have the the TCCM read for codes

On the transfer case does it have a floating wiper motor in it

Sounds like the encoder switch is bad ..it on the transfer case .it has a motor with wires going to it.when you press the button it sends a signal to that motor to switch into a certain gear..

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

There are two motors under the vehicle. The front axle actuator moves ONLY during 2HI->A4WD and A4WD->2HI transitions. The transfer case encoder motor makes a lower pitched noise in every mode transition: 2HI->A4WD, A4WD->4HI, and 4HI->4LO, and the three transitions upwards.

If EITHER of the motors fails to do what the TCCM commands it to do, the switch light will flash and the Service 4WD light on the dash will come on.

If you're a DIY mechanic, you might look into them further, or just take it to a mechanic who knows our systems. The average mechanic is not going to know the theory of operation of the trailblazer/envoy 4x4 system, which is pretty unique in the industry

Start with the simple and check the switch it self first. It's know for the the switch it self to go bad and will need to be replaced. The other common area to inspect is the wire harness at the TCCM, corrosion is an issue which can cause no current flow to your TCCM from the switch.

If all electrical systems check to be in 100% running order, good chance it's going to be your encoder motor (tranfer-case motor).

Thank you for using Fixya and keep us posted.

Check the fuse to your 4wd at the fuse panel at the end of the dash drivers side. If the fuse is good, put the transmission in neutral and listen for the Transfer case encoder motor to work when you push the button on the dash for 4wd hi then to 2wd. If you do not hear it working, inspect the wire harness at the Encoder motor to make sure there not damaged in anyway due to corrosion or disconnected.
Here's some info on how the 4wd system operates on newer Blazers for all those searching for the cause of the flashing dash switch light and no engagement.

4WD System Operation on 98 up (2nd Generation) Blazers/Jimmies

The front drive shaft always turns on “shift on the fly” systems so the drive train is at the same speed in the front as the rear or the whole thing would break when engaging 4Hi or Autotrac when moving. InstaTrac (NV243 case or NP1) has 3 buttons. AutoTrac (NV246 case or NP8) has 4 buttons and clutch packs in the transfer case and is the only system for on pavement driving! Prior were the NV231 (87-93) lever and NP233 (94-97) electric motor. Your VIN # is the only sure way the dealer can get correct parts/listings!
1. Select mode on dash switch
2. Signal goes to the Transfer case control module (TCCM) or computer located in the cab on the passenger side behind the kick panel.
3. If you want 4 Lo the TCCM commands the encoder motor on the transfer case to switch a lower gear in (requires putting the shifter in Neutral and be stopped or only going 2-3mph). Note that if 4 Hi selected no action done in the transfer case. The transfer case is chain coupled and has the low and high gear. You can put all systems into Neutral by pushing the 2Hi and 4Lo button at the same time when the key is in the run position (or running but not off or acc).
4. The left front (drivers side) CV axle is always engaged to the front differential, but the right side is not coupled in 2wd. There is an actuator located under the battery tray that when commanded is vacuum actuated and pulls a cable from it to the right side CV axle coupler engaging that axle. A sensor is on that axle that tells the TCCM the axle position.
5. On 99up there is an additional vacuum switch on the firewall that switches the vacuum on and signals the TCCM also.
6. The dash light on the selector switch flashes while all this is occurring and will not switch out of 2wd unless all parts report correctly – same on the switch to any mode or back to 2wd.

So, there are a lot of possible failure areas with actuators/computer/motor/sensors/vac hoses and a special scanner is required to read the codes when the Service 4WD light comes on. But, a few things can easily be done before going to the dealer.
1. Check all vacuum hoses from the intake to the drivers side reservoir can, then to the extra switch (99up) and down to the under battery actuator. Small hose is 5/32 and larger is 7/32”.
2. See if the actuator pulls the front axle cable to engage it. If not and all hoses are good the diaphragm may be leaking on the actuator. If it engages but the light is still flashing remove the indicator switch on the front axle (only electrical plug in there) and manually push it in with the connector plugged in and see if the light stops, if so replace.
3. Listen to the encoder motor clunk when 4 Lo selected. If not working look at the wiring going to it for any cuts/breaks and repair if any found. Another indicator switch on top of this case. The fluid in the AutoTrac case should be the newer light blue Autotrak –II synthetic P/N 12378508 ($7 qt) not the red Dexron III.
4. If the light keeps flashing after this then disconnect the battery for 30 minutes for a TCCM reset, just make sure you turn off your radio first.
5. Still not working – take it to the dealer and stop at the bank on the way for $$$$!
12577412 is the part number for the Transfer Case Control Module (TCCM), or you can do the Salvage Yard for $15.00 if that's the part that has gone bad.

HI this truck tccm and the switches when you have this your 4wd will not engage