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famous GM 'service 4wd' indicator problem

Discussion in 'Chevy Truck Talk & GM News' started by JohnnyZJ, Jul 19, 2012.

  1. JohnnyZJ

    JohnnyZJ New Member

    hi!
    so i've done some reading already on this as the 'service 4wd' light came on, infact thats how i found this forum :)

    so when lights on the truck won't shift out of 2wd. turn it off and and back on, no light, it shifts fine. it seems to happen pretty randomly. i read a thread on here about replacing the sensor in the encoder motor which is likely where i will start. i'm hoping the TCCM isn't fried.

    second question; when the 'service 4wd' light is on, can i pull coded witha typical OBDII scanner?

    i'm not afraid of pulling stuff apart. i'm no stranger to fixin' vehicles. just everything i'm used to owning has a shifter........

    thanks for any thought/input. greatly appreciated!
  2. RayVoy

    RayVoy Active Member 1000 Posts

    No, the codes are not retrievable with an OBD2 reader, you need something much bigger, like a GM tech II

    It could be a number of things, including the TCCM. It might be an encoder motor, but first, I'd look for bad/dirty connections. Pull the connector at the transfer case and clean everything first.

    The 2nd thing you need to do, is change the transfer case fluid.

    Let us know what you find.
  3. JohnnyZJ

    JohnnyZJ New Member

    i was hoping the OBDII would work. oh well. i have a buddy at a shop and he's going to read the code for me, at a discount. so i'm likely going to do this.

    i should have also said, i will also be changing the tcase fluid this weekend as well. i just bought the truck a few weeks ago, and whenever i buy something used i usually do the fluids too. That said, what is the best fluid to use? (again, new to chev/GMs!)

    i will give the connections a thorough cleaning, and will pull the 4wd switch too, make sure all the contacts are good; however, i do not have a problem with the lights ever changing. so i think it the issues a little deeper.

    will definitely keep posted!
  4. JohnnyZJ

    JohnnyZJ New Member

    went to a buddies shop and paid to have the codes pulled.

    got: C327 and C379.

    C327 i do believe is the ecoder motor they said. or the sensor inside. they said to just replace the motor. fair enough i guess
    C379 they say is the little thing on the side of the front diff that apparently engages the 4wd.. stupid little thing it looks like.

    we had it up on the hoist, and no wires or anything were rubbing, it all looked good.

    so ya, fun times.
    someone tell me why GM got rid of the floor shift?
  5. RayVoy

    RayVoy Active Member 1000 Posts

    I very much doubt that both are bad.

    A quick over-view. When you select any of the 4x4 positions, the TCCM tells the front disconnect to engage, a signal is sent to the encoder motor when the disconnect has engaged. at this time, the encoder motor moves the clutches to the engage the front driveshift position.

    You have a problem in this circuity that is setting the codes.

    As I suggested in an earlier posts, check the connections.

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