Front Dif Toast

Discussion in 'GM Powertrain' started by BobbyB26, Jan 7, 2011.

  1. BobbyB26

    BobbyB26 New Member


    Sorta a newby here, so please bear with me as I haven't posted allot on forums before.

    I've got a 2002 GMC 2500HD, 4WD, Crew Cab, 8.1L, Alison Tranny. About a month ago I had to use the 4x off and on and since then the truck has been feeling sluggish. Last week I took it into the dealer and said it felt like the 4x was partially was a guess, but that's what it felt like. Turns out the front dif wasn't fully disengaging the 4x system.

    So I felt pretty good that my instincts were on about the prob, but they let me feel the dif oil and was like someone had dumped sand in the oil...gritttttty. Limped it home and now am looking at the repair.

    Since I know next to nothing about a dif, can you guys give me some advice on what to do? I know I'll tear it out myself, but should I have someone else rebuild it? Is it even worth rebuilding? One guy told be to just pick up a use front axle assembly from a salvage yard and put it in? Just no experience here to base a decision on????

    Also, does anyone out there have a pdf of a service manual for this repair?

    Thanks for the help.
  2. 2COR517

    2COR517 Epic Member 5+ Years 1000 Posts

    My assumption is that "partially disengaging" means the CAD (Central Axle Disconnect) did not disconnect. Even if that's the case, that isn't what caused the diff failure. Could have been low fluid, or something simply let go.

    I would either rebuild the one you have, or replace with a used one. If you go with a used one, open it up so it can be inspected before installing. Fresh seals and Mobil 1 75W-90 and you should be good to go.
  3. 2COR517

    2COR517 Epic Member 5+ Years 1000 Posts

    Great idea to flush the diff. ATF is the stuff to use. You will be surprised how well it works, and it's relatively inexpensive.

    Drain the lube. Fill with ATF. Drive it for a couple days. Drain and refill. Repeat till it comes out as clean and pink as it goes in. Should be just two or three times.

    It would be good to get the R&P moving during this. If the CAD is stuck, it will take care of it for you.

    With the case in 2hi, can you turn the front driveshaft by hand? Do you have the pushbutton case? With Auto4wd setting?

    Once the diff is clean, refill with some cheap 80W-90 gear oil. Drive for a couple days, drain and refill with the M1
  4. BobbyB26

    BobbyB26 New Member

    Please excuse my ignorance but can you clarify for a dif dummy what a CAD is? Is it the thing that engages and disengages the 4x in the dif???

    I'll try to give the front driveline a turn in the am. It has been in 2x for awhile, so shouldn't be locked up.

    I have the push button 4x to the left side of the dash, but am not familiar with what you mean by "With Auto4wd setting?"???


    ---------- Post added at 07:05 PM ---------- Previous post was at 06:56 PM ----------

    No roaring noise at highway speeds. There was a grind and clunk noise that seemed to come from the front end.......seemed to be about 1 hr after I had "push buttoned" the 4x off. Had been on the freeway about 1 hr after being in 4x and then after exiting for fuel it did the grind and clunk routine....loud enough so that my buddies looked at me and asked me if it always did that?? Nope.

    I can understand the recommendation to flush the dif out, but that metal grit I felt was pretty substancial. Didn't smell burnt, but pretty chunky. Still worth the flush procedure?
  5. stephan

    stephan Rockstar 4 Years 5000 Posts

    Hey Bobby, when you saw this junk that came out of your diff, could you see any shiny metal particles, or was it just colorless grit?
    To answer one of your first questions, rebuilding a diff is not really a first time job. You would need a book (a really good book) & a dial indicator, a 0 to 1" micrometer, & bearing pullers to get the bearings off the ring gear carrier, & the 2nd bearing on the pinion shaft. They are a press/interference fit.
  6. 2COR517

    2COR517 Epic Member 5+ Years 1000 Posts

    The CAD is the central axle disconnect. It basically connects the right CV shaft with the differential. The left CV shaft is always connected to the diff side gear. When the CAD is disconnected, it allows the side gears and spider gears to rotate in the carrier, but the carrier, Ring & Pinion, and front driveshaft stay stationary. This saves fuel and wear and tear on these components. When you put the case in 4wd, the CAD is activated, and both side CV shafts are connected to the diff. Now when torque is applied to the front driveshaft, it is divided between both sides so both tires can be driven under power

    No electronic hubs on GM trucks.....
    Last edited: Jan 8, 2011

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