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"thunk" on take-off

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by fortcollinsram, Dec 22, 2001.

  1. fortcollinsram

    fortcollinsram 1/2 ton status

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    \"thunk\" on take-off

    Lately, I've notice that if I take off slowly, all is good...no wierd stuff going on at all...But If I start to take off slow then gas on it a bit, there is a "thunk" that seems to be coming from the rear end... What could this be gang...It seems to be fine if I get up top about 15-20 then stomp on it but if I go slow ro like 10 then hit the go-pedal..."thunk" and slight jerk follows [​IMG] thanks for your input...

    Chris

    Was that a speed bump? No, just a Rice Rocket </font color=red> <a target="_blank" href=http://coloradok5.com/gallery/fortcollinsram>My 87 Blazer</a>
     
  2. Esteban86K5

    Esteban86K5 1/2 ton status

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    Re: \"thunk\" on take-off

    what is yor set up in the rear? Do you have a Detroit back there?

    <font color=blue>It's easier to teach a child than to fix an adult.</font color=blue>

    <a target="_blank" href=http://community.webshots.com/user/geltyone>Webshots Pics.</a>
     
  3. BadDog

    BadDog SOL Staff Member Super Moderator Author

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    Re: \"thunk\" on take-off

    Sounds like a normally functioning detroit locker. Or maybe a gov-lock in it's death throws? Like he said, what is back there?

    Russ

    85 K30 CUCV, 350 TBI, TH400, 205, D60/C14, 4.56 Locked
    Some day: 4" lift, 44" tires, massive cutting, shorter wb and rear overhang.
     
  4. BurbinOR

    BurbinOR 3/4 ton status

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    Re: \"thunk\" on take-off

    Do you have lift blocks? Sounds like what I used to have - axle wrap. Horrible feel/sound but really doesn't seem to hurt anything.

    <font color=blue>'79 ONE TON TPI K5 - See it at---&gt;<a target="_blank" href=http://www.blazzinor.rockcrawler.com>www.blazzinor.rockcrawler.com</a>
     
  5. MudNurI

    MudNurI 1/2 ton status

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    Re: \"thunk\" on take-off

    Couldn't it just be a normal GM rear end clunk??? our 89 p/u and 94 burban do the same thing- after numerous trips to the dealer with the 89-- I finally got the answer from a technical service bullentin from GM- its NORMAL, its the "play" in the rear end. It does it more if you slow down, then jump the gas? is that the klunk? just my thoughts but with 2 rigs, 1 with over 300k miles on it, and no rear end trouble-I'm satisfied with GM's answer on it-

    --Brandy

    LOVING the new Duramax!!!
     
  6. fortcollinsram

    fortcollinsram 1/2 ton status

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    Re: \"thunk\" on take-off

    Yea, no Gov-Lok or Detroit....Its open back there...It hjas most likely been there but I may have just notices it...I do have lift blocks...so that is a possible cause as well...I will check it out some more tomorrow...I@ am replacing the tranny in a few weeks and will throw in new T-case and tranny mounts for good measure...

    Chris

    Was that a speed bump? No, just a Rice Rocket </font color=red> <a target="_blank" href=http://coloradok5.com/gallery/fortcollinsram>My 87 Blazer</a>
     
  7. Michael

    Michael 1/2 ton status

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    Re: \"thunk\" on take-off

    It has been my experience that it is caused by the slip joint on the rear drive shaft. Lifting usually exacerbates the problem...try a squirt or two of grease in the slip joint and see if that helps.

    Michael[​IMG]
    <a target="_blank" href=http://www.jmartin.net/parker/goose.htm>www.jmartin.net/parker/goose.htm</a>
     
  8. 90BlazerK5

    90BlazerK5 Registered Member

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    Re: \"thunk\" on take-off

    I have this too... part of it I'm thinking is in my lift blocks... getting rear springs though, so I'm not worried much about it. However, changed hte filter and fluid in my 700-R4 tonight, and that solved a LOT of the problem in mine... clogged filter and old fluid I guess.

    Where did all these damned dirty apes come from!?!
     
  9. vince13

    vince13 1/2 ton status

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    Re: \"thunk\" on take-off

    My guess would be you u joints on the driveshaft. i am having the same problem with my truck and same syptmos too. check the u joints. most likley what it is.

    73 Blazer 1Ton 6inch lift, 3 inch body, 36 inch swampers
    79 3/4 ton suburban, soon to be 6 inch lift, 3 inch body, and 44 in boggers. Oh ya
     
  10. talison79

    talison79 1/2 ton status

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    Re: \"thunk\" on take-off

    I had the same problem with mine, but it just took a couple of turns with the rachet on the tranny/ transfer mounts at the cross member, to get rid of it.

    Talison79
     
  11. BadDog

    BadDog SOL Staff Member Super Moderator Author

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    Re: \"thunk\" on take-off

    Hehe, I guess I've got lockers on the brain. [​IMG] Lots of good ideas here. Not sure about lift blocks, I've run 4" blocks a couple of times and never seen it. However, it could be compounding another problem. The drive shaft slip joint and/or u-joint sounds the most promising since you have an open diff. When you get on the gas, the pinion will rise, shortening the drive shaft by a small amount. Plus, acceleration squat shortens it even more. If the spline is sticking, or a u-joint is bad, that might do it. Blocks could compound/exaggerate that problem by making the pinion rise more due to the additional axle wrap from leverage on the block.

    Russ

    85 K30 CUCV, 350 TBI, TH400, 205, D60/C14, 4.56 Locked
    Some day: 4" lift, 44" tires, massive cutting, shorter wb and rear overhang.
     
  12. HarryH3

    HarryH3 1 ton status Author

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    Re: \"thunk\" on take-off

    Look under (from off to the side!) while someone starts to drive off. You'll most likely see the slip joint suddenly change in length, just as the thunk happens. When the slip joint gets old and worn, it tends to bind under load. But as you take off from a stop, the pinion rotates, forcing the driveshaft length to change. The binding splines release suddenly, with a bang. Greasing the splines up really well will reduce it, but eventually the solution is a new slip joint
     
  13. shawnboy

    shawnboy 1/2 ton status

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    Re: \"thunk\" on take-off

    You could also check your rear brakes. If you have a leaky wheel cylinder or axle seal the shoes could be contaminated and grabbing. They will make an audible thunk when they release.
    Shawnboy.

    <font color=red>If you are having too much fun it's probably illegal.</font color=red>
     
  14. jimmyjames

    jimmyjames Registered Member

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    Re: \"thunk\" on take-off

    That almost same thing happned to me. My spider gears went. That is worst case sonaro, but hopfully it is an easy fix.

    88 jimmy 350, 700R4, open 3.73, stock hight
     
  15. jimmyjames

    jimmyjames Registered Member

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    Re:

    That almost same thing happned to me. My spider gears went. That is worst case sonaro, but hopfully it is an easy fix.

    88 jimmy 350, 700R4, open 3.73, stock hight
     
  16. Danno

    Danno 1/2 ton status

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    Re: \"thunk\" on take-off

    You know, I have the same thing going on in my truck, and I'm pretty sure it's not the tranny mounts. The only thing I haven't looked at yet is the driveshaft, and this slip yoke problem sounds like it might be a possibility.

    How hard is this to fix? I'll have to crawl underneath the truck tomorrow and take a look.

    Thanks!

    1984 K5 Blazer 4x4 Silverado
    6.2L diesel, 700R4, 3.42 gears, 31x10.5" tires
     
  17. rocko

    rocko 1/2 ton status

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    Re: \"thunk\" on take-off

    My 89 does the thunk..
    only when i hit the gas hard.. feel's like it's coming from underneath my feet ..
    what could it be?

    I haven't failed, I've found 10,000 ways that don't work. www.rocko.rockcrawler.com
     
  18. HarryH3

    HarryH3 1 ton status Author

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    Re: \"thunk\" on take-off

    Danno, the "easy" fix is to grease the splines well. If that doesn't stop the binding, then the next step is to have a driveshaft shop replace the splined part of the driveshaft. [​IMG]

    <font color=black>HarryH3 - '75 K5</font color=black>
    <a target="_blank" href=http://www.angelfire.com/super/ThunderTruck>www.angelfire.com/super/ThunderTruck</a>
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  19. Danno

    Danno 1/2 ton status

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    Re: \"thunk\" on take-off

    Looks like my rear drive shaft has already been replaced once. ARgh.

    Thanks. I'll check it out.

    1984 K5 Blazer 4x4 Silverado
    6.2L diesel, 700R4, 3.42 gears, 31x10.5" tires
     
  20. johnnyK5

    johnnyK5 Registered Member

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    Re: \"thunk\" on take-off

    I have a thump up front too!! I was told it was my full-time front hubs going bad.?...dunno..

    "Baby it first, floor it second!" -correct.
    "Floor it first, baby it second!" -incorrect.
     

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