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Rear axel issues.. I think?

Discussion in 'Differential & Driveline' started by Black Beauty, May 5, 2019.

  1. Black Beauty

    Black Beauty Junior Member

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    So I’ve done some reading and asked around. Been getting mixed opinions and thought I’d ask any and everything here their opinion. I have a 1985 K20 4x4 that SLAMS and I mean slams into reverse. Doesn’t shift hard in any other gear or when driving down the road. Like I’ve explained it to guys and then showed them and they were in disbelief. Explaining it doesn’t even begin to describe how hard it is(hehe)jk. I haven’t delved into the truck too much yet and I’m not a mechanic by any means but I know enough to get myself into and out of trouble. I don’t want to say what other guys have said just so I get can your guys honest opinion without guiding anyone. Fluid level is good btw

    I’m pretty sure the guy before me or before him etc etc.. regeard the axles. Only thing that makes me think that is because the dif covers aren’t the same as the axle. Could be wrong but idk. Any ideas/advice you guys got is much appreciated. I’m gunna pull the cover off probably next weekend and take a peek but if I can avoid doing that that’d be great lol. [​IMG][​IMG][​IMG][​IMG]


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  2. Black Beauty

    Black Beauty Junior Member

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    I should also tell you guys that the U joints are good too. No play in those at all.


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  3. 1987 GMC Jimmy

    1987 GMC Jimmy Automobile Hoarder

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    Does it clunk real loud when you shift into reverse? I’d check the pinion bearing for play at the differential input, pull the cover, and see if there’s an adverse wear pattern on the ring and pinion. The pinion backlash may have been set up wrong, you have a torquey motor with brakes that are not holding the truck steady, and it’s lurching you until things mesh in there. Also, I’ve only able to diagnose u-joints visually by pulling the caps and looking at the needle bearings chewing into the trunnions. That slip test didn’t do anything for me. Maybe it just has to get real bad but just thought I’d share that piece of info. Hopefully more rear end savvy people will chime in.
     
  4. Black Beauty

    Black Beauty Junior Member

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    Oh yeah she pops real hard. I’ll post a video on YouTube tomorrow and post the link here. My fear is that it wasn’t shimmed correctly. Like I said I can figure shit out but if I don’t have to rebuild it that would be awesome. Just a hunting truck for my grandfather and I. Want to make sure we can get out of wherever we are and not be stranded lol


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  5. PrairieDrifter

    PrairieDrifter Full Access Member

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    What’s your idle set at? Just curious.
     
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  6. Black Beauty

    Black Beauty Junior Member

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    Couldn’t tell you, don’t have a tach. But one of the guys I was talking to said that could be it. I’ll prob mess with that first before I crack the diff


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  7. 1987 GMC Jimmy

    1987 GMC Jimmy Automobile Hoarder

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    Didn’t think of that. If it’s excessively high, like 1000+ RPMs, that’s not helping you.
     
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  8. HotRodPC

    HotRodPC Administrator Staff Member

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    Dammit, I forget what causes this now. But you have a Turbo 400 and it's the transmission. For now your U joints and gears are fine. But that reverse slamming will eventually take it's toll. I know exactly what you're talking about. I had a Turbo 400 do it and I mean SLAAM!!! It's brutal. I forget now what I did to fix it. I'm wanting to say I had to pull the valve body out and work on a valve that was hung up. I won't swear to it though. Let me get out one of my books if I can find it and see if I can find the problem.
     
    Last edited: May 6, 2019
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  9. HotRodPC

    HotRodPC Administrator Staff Member

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    I haven't found my books, but it seems it was a sticky valve in the valve body. Try some Sea Foam Trans Tune in the transmission. That stuff does a good job of cleaning varnish out of the valve bores and such. See if will clean up a sticky valve. It won't happen instanly, but it's worth getting in there and giving it a try. The problem is the line pressure is to high and that's what causing the problem. I just forget what causes the line pressure to be to high like that.

    Are you shift points good when in D? Even the vac modulator valve can affect line pressure. In fact, and how true it is, I won't swear to it, but I've been told before on a Th400 if your vac modulator valve isn't functioning properly, your line pressure goes full all the way up and in short isn't regulated. It won't take that long to pull the vac modulator off, get some needle nose pliers and pull the valve out and clean it off. Make sure it's moving in it's bore smoothly. Make sure you have good vacuum going to the modulator and no holes or split in the vac line. If you happen to have a different vacuum modulator, you might even try swapping it out to see if it makes a difference.
     
  10. Black Beauty

    Black Beauty Junior Member

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    Thanks for the advice guys. I’ll start with the idle and work my way back from there. Seems logical. Be the shits I tear apart the diff and that not be it lol. Couple of dudes said that the idle could be it but with no tach it’s hard to tell exactly. Prob won’t get to it before next weekend. I’ll see if I can take that video tonight and post the link here so you can see exactly what I’m talking about


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  11. shiftpro

    shiftpro Full Access Member

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    Man it slams so hard it bent the fenders out...!
    It's the transmission... fortunately the T400 is cheap and easy to rebuild.
     
  12. Black Beauty

    Black Beauty Junior Member

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    Here’s a video of what I’m talking about


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  13. Wumbo

    Wumbo Full Access Member

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    That doesn't look good.
     
  14. Black Beauty

    Black Beauty Junior Member

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    It does not...


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  15. Wumbo

    Wumbo Full Access Member

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    Ever rebuild a rear end?
     

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