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weird 4wd issue

Discussion in 'GM Powertrain' started by MY T FAST, Feb 21, 2011.

  1. MY T FAST

    MY T FAST New Member

    Gents,
    i'm a little new to 4wd so many of the small 'ins and outs' i'm not real familar with.
    heres the scoop:
    1994 k1500 5.0 5spd. 132xxx miles

    driving just fine in 2wd no problems. got quite a bit of snow up here today, pulled her into 4H at about 20 mph, 4H activated drove home just fine. turned onto my road and at about 15mph, kicked it back into 2H and i heard a " THUMP THUMP THUMP" coming from what sounded to be right under the 4x4 shifter. the rate of thumps was speed dependent (faster revs, more frequent thumps and vice versa) scared the bejeezus out of me. so i stopped for a couple seconds. when i took off again the noise was gone, drove in 2wd just fine. so i sped back up to 20ish, pulled her into 4H, again engaged without a hitch. kicked her to 2wd and "THUMP THUMP THUMP". stopped moving for a couple secs and then continued to drive in 2wd without a thump.

    so why/what is thumping like a monkey with a hammer on my undercarriage when i disengage 4H while moving and why does it stop when i stop the truck and then disengage 4wd before driving?!?!?! i'm so confused.. thanks in advance.
    Talon
  2. drm223

    drm223 New Member

    sounds like a bad u-joint get under your truck and shake your drive shafts if it moves replace the worn u-joint. i reccomend replaing in pairs.
  3. MY T FAST

    MY T FAST New Member

    i thought about that too, and i'lll check it here soon. (i don't have a garage so i gotta wait til the snow stops) but it sounds like it is coming DIRECTLY below the 4x4 shifter and it sounds like something is whacking my floor pan. would bad front driveshaft u-joints sound like that?
    Last edited: Feb 21, 2011
  4. drm223

    drm223 New Member

    Thats how i found out that my 95 had a bad front u-joint

    sounded like a train was directly below me.

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