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My TH400 is shifting sooooo BAD.

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by AzzKicker, Sep 4, 2002.

  1. AzzKicker

    AzzKicker 1/2 ton status

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    Truck is in SIG below.

    Heres what happens. I have no problems taking off, in fact I'm impressed with the off the line this 6.2 produces.
    Anyway, what happens is I'll be taking off and engine sounds like its passing shift point but never shifts until you really go at it and then it just JERKS into 2nd. Its a pretty bad jerk too. What could cause this. I hope I dont need a new tranny. from 2nd to 3rd it'll sort of do the same thing but not as bad.
     
  2. wayne

    wayne 3/4 ton status

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    I'm no tranny expert but I would say try and new govener (sp) on it.
     
  3. 83ZZ502_Jimmy

    83ZZ502_Jimmy 1/2 ton status

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    Its most likely a bad vacuum modulator. Either the mod. is bad itself or, most likely the vacuum line came off somewhere.

    It is located on the pass. side of the tranny near the front. Should be a bell shaped object.. It removes with one bolt.

    Hope it helps,
    John
     
  4. AzzKicker

    AzzKicker 1/2 ton status

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    Thanks. I'll replace it right away.
     
  5. Calclips

    Calclips 1/2 ton status

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    Pull the vac hose off the modulator and if tranny fluid comes out, replace it because its bad. If no tranny fluid comes out it is probably good, but they're only like 10-15 bucks. fire up the motor and check for vac down at the modulator too
     
  6. AzzKicker

    AzzKicker 1/2 ton status

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    Hmmm well when i pulled off hose, no fluid came out. I'll check the vac later today.
     
  7. dyeager535

    dyeager535 1 ton status Premium Member

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    You should have vacuum at the modulator with the vehicle idling. I *believe* all TH400's used straight manifold vacuum, no "ported" setups.
     
  8. Blue85

    Blue85 Troll Premium Member

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    Yes, I would guess modulator. The tranny thinks that you have the throttle wide open so it shifts at the highest point it can with maximum pressure. Could be the hose, could be the modulator, like everyone already said.
     
  9. AzzKicker

    AzzKicker 1/2 ton status

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    Well I'm getting vaccume. So Last result is modulator. I'll go replace it and let you all know how it goes.
     
  10. AzzKicker

    AzzKicker 1/2 ton status

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    DAMN DAMN DAMN!!!

    Replaced Modulator, got VACCUME and still it shifts rough. I must admit its not as bad as before. Tranny does have almost 200k on it. Maybe a rebuild or shift improvment kit is in order.
     
  11. Blue85

    Blue85 Troll Premium Member

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    Is the modulator adjustable?

    Before a rebuild you should try cleaning the governor and the valve body. 200K is a lot of miles to wear the frictions and parts, so the odds are pretty low of having a squeaky clean valve body. Any little bit of junk can hang a valve up.

    If you can get the valve body out and back in without destroying the gaskets, it doesn't even cost you anything. You just have to be careful about check balls and torque specs. If you have all of the gears and it doesn't slip, then your frictions, oil pump and apply pistons must not be entirely shot. Everything else can be worked on with the tranny in the vehicle.

    A transmission flush should also be done, if it hasn't already.

    If the clean out fixes the problem and then it comes back fairly soon, then chances are something is already failing and spitting out junk. 200K means it should probably be rebuilt anyway, but this is a good way to stretch out the life for some more miles. A TH400 is pretty stout.
     
  12. Calclips

    Calclips 1/2 ton status

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    My Turbo 350 was acting similar to the way your 400 is acting now, I did a bunch of valve body work, flushed it out, did a filter, made a poor-man's shift kit, boosted main line pressure all in an attempt to get the dang thing to shift right. Well, long story short, it didn't work so I pulled it out of the truck and tore it down. Upon tearing it down I found a lot of worn out clutch plates, and a nearly 100% TOASTED front pump. With 200k on your 400, the valve body work will only be a temporary fix I think a rebuild is imminent. Tear the valve body down and try to get it to shift better, and save up some $$$ for a rebuild. If you pull the tranny out yourself you can get it rebuilt fairly cheap, tranny shops really start to stick it in your behind when they have to do removal and reinstallation.
     
  13. AzzKicker

    AzzKicker 1/2 ton status

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    I was gonna ask.

    Taking it out is no problem. Where would I buy a rebuild kit. My uncle is a mechanic and always did work on my 700 when i had an older truck and he did a pretty good job. So maybe he can do the rebuild for me.
     

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