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01 Sierra 3500 stuck in third gear

Discussion in 'Truck & Equipment Repair' started by Phillor, Dec 17, 2009.

  1. Phillor

    Phillor Junior Member
    Messages: 3

    I have a 01 GMC Sierra 6.6 turbo diesel with the Allison 6 speed trans. Just to-day I was using the truck & it was working great till I stopped for fuel. When I started it up again and put it into gear the trans. made a big bang sound in both drive & rev.. I also noticed later that the gear indicator light on the dash was not functioning properly. Now as I am driving the truck seemed very sluggish on take off and seems that it is stuck in 3rd gear and won't shift at all.

    Is this a serious problem which will require a complete trans. tear down? HELP
    I just finished paying for 8 new injectors a new injector pump and 2 new batteries this month. :mad:If it is going to cost a lot then my next purchase will be a lighter:angry:
  2. mow4cash

    mow4cash Member
    Messages: 41

    Did you check the Fluid?
  3. B&B

    B&B PlowSite Fanatic
    Messages: 12,777

    Very likely needs the NSBU switch replaced. $100 and a half hour job. See here. Pics and everything.
  4. Philbilly2

    Philbilly2 PlowSite Veteran
    Messages: 3,797

    x2 - B&B nailed it. You need a NSBU.
  5. JustinD

    JustinD Senior Member
    Messages: 343

    01 has a 5 speed not a six, NSBU is likely, does it have a programmer of any sort? if so sounds like you limped the trans, it goes to third and stays there, won't even downshift when you stop. Good luck with it, I've been there, now I have about 6k in my Allison, but its good for what ever I throw at it now!!!
  6. Philbilly2

    Philbilly2 PlowSite Veteran
    Messages: 3,797

    limp will only happen when moving, and limp will throw a P0700 code along will commonly 2 to 3 other codes indidating where your problem exists. Limp can also be cleared out by shutting the truck off and restarting it and truck will drive as long as you baby it.

    NSBU just causes an instant default to 3rd gear. and not properly working PRND321 indacator.
    Last edited: Dec 18, 2009
  7. Phillor

    Phillor Junior Member
    Messages: 3

    Thanks guys... Just found somebody that had a reader and was able to pull up the codes up for me.
    2 codes came up - NSBU ($95.00) and speed sensor ($31.00).
    I will probably freeze to-day putting them on in the drive way but the local garage wants 3 hours labour to do it.
    Thanks again ... Have a Merry Christmas and a better New Year.