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  • Rear axle locked (Gov-lock locked up)

    '03 2500HD w/ 10.5" AAM rear axle with G80. Turning into work this morning I noticed the binding/hopping/scrubbing feel. Went into the lot and made some circles and seems like the rear end is locked up, as in the Gov-lock is locked up. I've felt this before briefly after a low traction situation and then immediately turning on pavement, but the rear axle unlocked quickly and went back to normal. Made several circles both directions and backed up but it wouldn't unlock.........hmmmm.

    Guess I get to pull the rear cover and see what is going on. Truck has 108k on it and never any major issues until the oil pump went out last month. Really hope I'm not getting stuck in this trend of major repairs.

  • #2
    Re: Rear axle locked (Gov-lock locked up)

    If the differential has been serviced and someone put in a limited slip / posi additive, that's a big no-no on the Eaton Gov-Loc unit which is what RPO code G80 is on a truck. The additive is considered a contaminant and can cause the G80 to work improperly. There's a TSB out from GM and this has been debated to death (regardless of official word from GM) but fact is, it's a problem. If you think this has happened, you could try servicing the diff again and be sure the correct fluid is used - I think in the GMT800 trucks like yours, they specify a synthetic gear oil? Check your manual; I don't have late model info.

    Richard

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    • #3
      Re: Rear axle locked (Gov-lock locked up)

      Originally posted by someotherguy View Post
      If the differential has been serviced and someone put in a limited slip / posi additive, that's a big no-no on the Eaton Gov-Loc unit which is what RPO code G80 is on a truck. The additive is considered a contaminant and can cause the G80 to work improperly. There's a TSB out from GM and this has been debated to death (regardless of official word from GM) but fact is, it's a problem. If you think this has happened, you could try servicing the diff again and be sure the correct fluid is used - I think in the GMT800 trucks like yours, they specify a synthetic gear oil? Check your manual; I don't have late model info.

      Richard
      TSB quotes that use of a ltd slip additive (friction modifier) will actually cause the G80 to chatter and lock/un-lock.

      Spec for rear diff fluid is 75w90 synthetic.

      I would pull the cover and inspect for broken parts and filings. Then refill with correct fluid.
      Last edited by Quyonmob; 12-08-2010, 05:12 PM.

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      • #4
        Re: Rear axle locked (Gov-lock locked up)

        It has 75W-90 Synthetic......I'm the only one who has ever worked on the rear axle and I bought the truck new. I don't even believe they call for changing the fluid in the service manual but I still do about every 50k (most synthetics in trucks are considered "fill for life"). It's only been in the shop twice and it was GM dealerships both times and absolutely nothing to do with the rear axle.

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        • #5
          Re: Rear axle locked (Gov-lock locked up)

          Originally posted by grayblazer View Post
          It has 75W-90 Synthetic......I'm the only one who has ever worked on the rear axle and I bought the truck new. I don't even believe they call for changing the fluid in the service manual but I still do about every 50k (most synthetics in trucks are considered "fill for life"). It's only been in the shop twice and it was GM dealerships both times and absolutely nothing to do with the rear axle.
          If that is the instance just pull the cover off the diff and look for broken pieces. Something is obviously keeping the locking plate engaged which isn't a good sign. I have personally seen a 14 bolt ( GM 10.5" Corp.) do this before as well. Those owners replaced the carrier as theirs locked solid like a spool and there wasn't much left to do with it. I would prepare to have to replace it.
          JUST FOR CLARIFICATION

          I hate GMT900 trucks period.

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