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th400 won't shift - help

Discussion in 'The Garage' started by jstevenj, Jul 9, 2004.

  1. jstevenj

    jstevenj Registered Member

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    th400 won\'t shift - help

    I just put a th400 auto in my blazer and it doesn't shift out of first. It had been sitting up for a year or so before I put it in. Has new torque converter. I have the vacuum line attached also. Do I just need to run it a little while to loosen the valves ot what?

    Thanks, Steven
     
  2. 75-K5

    75-K5 3/4 ton status

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    Re: th400 won\'t shift - help

    I hate to be Captain Obvious, but have you checked the fluid level?
     
  3. dyeager535

    dyeager535 1 ton status Premium Member

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    Re: th400 won\'t shift - help

    If it won't shift, it's more than likely the governor.

    Take it out around the block, and shift through 1-3 over and over again with the shifter. That is about the only thing you can do for a stuck governor without disassembly.

    My car did that one time, just wouldn't come out of 2nd gear. Went between 2-3 over and over, finally it shifted to 3rd, and never had a problem again. I would imagine higher engine RPM's will help put more pressure against whatever is stuck.
     
  4. jstevenj

    jstevenj Registered Member

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    Re: th400 won\'t shift - help

    Yes, I put about a half gallon in the torque converter before I put it on, then poured about another gallon in before starting the engine, then cranked it and added until full. I have a deep pan with tranny cooler and radiator cooler, it held around 3.5 gallons total.
     
  5. Ryan B.

    Ryan B. 3/4 ton status GMOTM Winner

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    Re: th400 won\'t shift - help

    Vaccum modulator guys!
    Check the vacuum line and make sure it is hooked up at both ends, and make sure it is getting good vaccum out to the modulator. With this disconnected usually it won't shift unless you hold it at WOT.
    The modulator is also a $7 part so a cheap easy test, and sometimes if it goes bad your engine will be sucking tranny fluid through the broken diaphram and vaccum line.
    Once you know that is not the problem then i would suspect the governor not building pressure as Dorian mentioned. /forums/images/graemlins/peace.gif Not real sure about a th400 but there might be a governor pressure port back by the governor cover, in addition to the line pressure port so you could hook up a psi gauge and see if you are getting proper governor pressure rise as road speed increases... easily diagnosing it still in the truck.
    If that checks good then possibly stuck shift valves? You said its been sitting for a long time?
     
  6. mrk5

    mrk5 The Sticker Guy Moderator Vendor GMOTM Winner Author

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    Re: th400 won\'t shift - help

    You beat me to it. I had the exact same problem and it was because I forgot to hook up the vacuum line down at the modulator.
     
  7. dyeager535

    dyeager535 1 ton status Premium Member

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    Re: th400 won\'t shift - help

    I'm certainly no tranny expert, so don't take this the wrong way, but my impression is that the modulator controls shift firmness, and has almost nothing to do with shift timing.

    My only experience is with that same TH400 in my car, and at least two times the modulator vacuum line fell off (lol) and I didn't notice until the next time I crawled under the car.

    With no modulator, you are running at maximum pressure all the time.

    If you apply vacuum to the modulator, and it moves and holds in place, it's good.
     
  8. Ryan B.

    Ryan B. 3/4 ton status GMOTM Winner

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    Re: th400 won\'t shift - help

    Not real sure about on a th400.. but definatly on my th350 with the vaccum line disconnected, it will only shift really late and hard usually only at almost WOT. and no engine brakeing either.

    When you hooked back up your vaccum line your shift points were exactly the same? /forums/images/graemlins/dunno.gif
     
  9. supersize75k5

    supersize75k5 OrganDonorRacing.com

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    Re: th400 won\'t shift - help

    t-40o roik jus buy a new ine and the hell with it pm me for ifno on a co i tanny thanks
     
  10. ben427

    ben427 1/2 ton status

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    Re: th400 won\'t shift - help

    I'd say its the modulator is unhooked as well. With the modulator unhooked it will still shift but not until 6K rpms or so, which most people dont know so they assume the trannys friged. Hook a line up from manifold vacuum to the modulator and i bet it will work tits /forums/images/graemlins/thumb.gif
     
  11. dyeager535

    dyeager535 1 ton status Premium Member

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    Re: th400 won\'t shift - help

    [ QUOTE ]
    When you hooked back up your vaccum line your shift points were exactly the same?

    [/ QUOTE ]

    Yep. And if this proves anything (conclusions are up to you!) I had an adjustable vacuum modulator as well. Fully in or fully out, MPH at which shifts occurred (WOT every time) varied by only 5MPH. (no kickdown on the car)

    How would the modulator control shift timing? Governor is a fixed item with centrifugal weights...it's valve operates based on output shaft RPM. Unless the modulator can somehow redirect the flow of fluid to the governor, the modulator can't change the shift point. Kickdown solenoid certainly would based on throttle position, I don't see how it's possible for the modulator to do the same thing.

    Like I said, I'm no tranny expert, so if fluid flow to the governor is changed by the modulator, then it is, and the modulator will affect shift points.
     

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