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Is my TH400 quirky, on the way out, or is that just the way it works?????

Discussion in '1982-Present GM Diesel' started by Quack_Addict, Sep 20, 2002.

  1. Quack_Addict

    Quack_Addict 1/2 ton status

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    Hi guys,

    You'll have to forgive me... I have owned and put many miles onto SM465's, TH350's, and 700R4's. This is my first driver TH400, and it's mated to a Diesel.

    I just got my `87 Suburban on the road earlier tonight after doing a "not in too much of a hurry" engine swap and a bunch of other work. Here's what the truck is... 1987 3/4 ton Suburban, newer 6.5L NA engine (~100k), TH400, NP208, 3.73 gears, 31" tires.

    Here's what the truck is doing... For being a Suburban with a Diesel, this thing's got one helluva holeshot. In first gear with mild to medium pedal, the engine winds up and holds the high R's for literally a couple seconds before it shifts into second. When it hits second gear, it tosses you back in the seat a bit then winds up, but not nearly as much, and shifts decent into third gear. It seems to do this without being too agressive on the skinny pedal.

    This is the first time I have ever driven a truck with a TH400 and I don't know if this is the way they are, whether it needs adjustment, or if this tranny is getting ready to split.

    Any insight is greatly appreciated.
     
  2. doctor4x4

    doctor4x4 1/2 ton status

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    Re: Is my TH400 quirky, on the way out, or is that just the way it works???

    sounds like a vacum problem 2 me are u getting enough vacum to the tranny to make it shift ??? check ur specs
    hope it helps
    R
     
  3. AzzKicker

    AzzKicker 1/2 ton status

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  4. Quack_Addict

    Quack_Addict 1/2 ton status

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    Thanks kicker, it sounds like you and I both have the same exact issue with our trannys. The only difference is that my TH400/NP208 is supposedly pretty new (looks fresh, too)... at least that's what the guy said that gave me the 'Burb. The guy worked at the GM proving grounds and had the TH400/NP208 replaced with a *new* setup... right before his kid chunked the engine. The way I figure, why would he lie if he's giving me the truck? I think I will probably replace the modulator when I get back home.
     
  5. k20

    k20 3/4 ton status

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    ok, way out there, but maybe you got a shift kit in it?
     
  6. Txstruck

    Txstruck Registered Member

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    My 400 does the exact same thing. Been like that since I had it going on a year now and havent had a single problem with it yet. 6.2L/TH400+NP208/4.56 gears
     
  7. azblazor

    azblazor 1/2 ton status Premium Member

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    My 86K30 with 6.2 NA TH400 and 4.56 gears does the same thing.
     
  8. desertrat67

    desertrat67 Hawk Driver

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    Do the CHevy truck diesel/th400 combos have a vacuum modulator like gasser 400's? I know the HMMWV's use a manually actuated modulator that hooks up to the throttle lever on the injector pump.
     
  9. BARRAZA

    BARRAZA 1/2 ton status

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    Diesel turbo 400s use a vacuum regulator valve on the injector pump to mimic engine vacuum because turbo 400's sense load by vacuum. Low vacuum - high load - harder and later shift; high vacuum - low load - softer, earlier shift. If it is shifting hard at part throttle, it may be that your vacuum pump or regulator valve is not providing the proper vacuum signal(too low).
     
  10. doctor4x4

    doctor4x4 1/2 ton status

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    Re: Is my TH400 quirky, on the way out, or is that just the way it works???

    Thank You Barazza:
    Mabe they will listen 2 u as u see they
    sure as hell didnt listen to me :-)
     
  11. K10ANDYKHAMNIC

    K10ANDYKHAMNIC 1/2 ton status

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    i am told that th400s have a wide range between first and second so maybe thats it .
     
  12. Quack_Addict

    Quack_Addict 1/2 ton status

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    I towed my 18' boat up north with the truck this past weekend (200 miles and back, the last 40-50 miles through hill country) and didn't have any problems with the transmission. It kept doing the same thing, shifting hard into second at about 25-30 MPH and shifting into third at exactly 45 MPH every time. I am going to replace the vacuum can on the transmission anyway - I noticed a drop of tranny fluid on the nipple when I was doing the engine swap a while back.

    I don't know if the transmission has a shift kit or not. I guess it might have one. I know the previous owner had transmission and transfer case problems with the last unit that was in there.

    I'll post again after I get the new modulator in there.
     

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