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Four Wheel Drive Service light

Our 2007 Chevy Tahoe 4 X 4 recently displayed a "Service Four Wheel Drive" panel light a couple of days ago (upon ignition on a sunny, dry day). The light had been steady which, when we read the owners manual, indicates a problem but no immediate service required. One of the possible solutions was to remove and close the gas cap in case it had been incorrectly closed during the last gas-up. That did not work. Yesterday, during a snow storm, I tried using the 4WD. The dial would move to the different settings, but no 4WD activated and no light illuminated indicating which gear it was in (2WD, Auto, 4H, 4L). We checked the fuse for the 4WD but it is OK.


  • I'd Bet That You Need To " Service Four Wheel Drive " .

    " The light had been steady . . . "

    Is the light still on or did it go out ?

    There exists a GM Technical Service Bulletin, written for GM technicians, that pertains to 2007 Chevrolet Avalanche, Suburban, Tahoe, and 2007 GMC Yukon, Yukon XL models with NVG246 Transfer Case (RPO NP8).

    The bulletin discusses an intermittent " Service Four Wheel Drive " message and a DTC (Diagnostic Trouble Code) CO378 set by the TCCM (Transfer Case Control Module).

    The bulletin advises technicians to check specific terminal pins in a specific connector at the TCCM and to update the TCCM with the latest calibration.
    Also, it advises that the vehicle should normally be driven in 2WD mode and to use Auto or 4WD modes when road conditions are appropriate.

    Rocket science ? No, but it's close. If your problem is intermittent and has CO378 set then your TCCM unit may need to be reprogrammed as outlined in the bulletin. Otherwise, it will have to be properly diagnosed (Probably starting with a code scan) by a rocket scientist or mechanic who understands this stuff to find out what went kaput.

  • In the 2003 trailblazer cleaning and reseating the Electric plug on the tranny was one solution.
  • edited December 2012
    Whether or not the service light is linked to the functionality light of the 4wd is questionable. It should not be. If it is, there is an electrical problem in the system as @csa indicates. So..... When in 4 wd on a lift, you can rotate the wheels then the transfer case shaft to see what is truly engaged. If every thing works, it's not an actuator but a position switch problem. This assume oll connections have been checked.

    You never indicated that the vehicle actually failed to function as a 4wd when the lights did not go on. If it did, it again, is a position switch problem. If it did not, the actuators ( there may be two, transfer case and front differential) could be a problem. This often occurs from lack of service And/or lack of use. Get it serviced and use it frequently as the manual suggests.
    . Bottom should be using 4 wd often when you don't need it so it will function when you do.

    These switches and actuators are very suspetable to failure do to lack of use and service. The old manual ones with manual hubs just got stiffer to use. The automatic actuators just don't have enough power when you neglect them.. Sometimes, service (fluid change and use) is all that is needed. Sometimes putting in a warm garage can get it to work too...but it could be just temporary.
  • Most dealers have financing available for these kind of problems....
  • This particular transfer case has a lot of design flaws and is used in GM and Chrysler trucks. I've experienced similar problems that just magically went away. The t-case actuator, the sensor in that actuator and the switch are all common problems. The cheapest and easiest to replace is the switch. As the miles accrue, watch for oil spots colored blue from the middle of the truck. This indicates the oil pump inside the transfer case chewing its way out of the case. The pump wears the softer case and creates a leak. The prevention is to remove, disassemble and install a wear washer into the case. Once it leaks, it can be repaired with epoxy or replace the case. Either way, get prepared for a big repair bill.
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