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TH350 Vacuum HELP

Discussion in 'The Garage' started by bouncytruck, Aug 20, 2003.

  1. bouncytruck

    bouncytruck 1/2 ton status

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    HELP I rebuilt my TH350 at the end of last summer and it never shifted into 3rd properly. Last night was the first time I've had a chance to really test it out (we had a baby in January). It appears to be shifting into third if I'm not giving it any gas. When I step on the gas it shifts down to 2nd and won't shift up again until I start coasting again. I think it might be a vacuum line issue. right now, the vacuum line to the tranny is going to a port in the Edelbrock Performer intake manifold - the same one as the power brake vacuum. I switched it there because someone told me that is where it should be hooked up. Before the rebuild, it was attached to the carb (quadrajet). Where should the vacuum lines all be hooked up? I don't have a vacuum gauge. I really want to drive my truck again before the weather turns to rain
     
  2. Larry_in_Tx

    Larry_in_Tx 1/2 ton status

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    On mine the power brake comes off the carb and the tranny comes off the tree on the manifold.
    I could guess why it might act up if the where both coming out of the intake but I can’t say for sure...I’m half monkeyfied when I work on my truck and I just put the lines back where they where on the 350 when I did the engine swap.

    I’m sure someone can help out more.
     
  3. bouncytruck

    bouncytruck 1/2 ton status

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    which postion on the carb is it hooked into? My vacuum advance for the disty & PCV on the are hooked into the carb. I capped off one of the ports.
     
  4. dyeager535

    dyeager535 1 ton status Premium Member

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    It needs full manifold vacuum, which is what you have it hooked up to.

    Could be a vacuum modulator issue I guess, have you checked that its working correctly?
     
  5. bouncytruck

    bouncytruck 1/2 ton status

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    The vacuum modulator is brand new (came with the rebuild kit). It appears to be working because last night I adjusted the shift point earlier. Is it a problem having a "T" in the vacuum line and having it go to both the power breaks and the transmission vacuum modulator?
     
  6. diesel4me

    diesel4me 1 ton status Premium Member

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    I dont think it matters too much where you get the vacuum from,as long as its manifold vacuum and not "ported"vacuum(above the throttle plates in the carb)you can check that with the engine running,ported vacuum has no suction at idle,only when accelerating and throttle is open--if you reverse the hoses it may act like you described--but it sounds to me that tour detent downshift cable is adjusted too tight and is pulling the tranny back into second gear(or passing gear)too soon--make sure its adjusted properly. /forums/images/graemlins/grin.gifAlso-its not a good idea to hook up your pcv and power brakes together unless you have the little charcoal canister thing (looks like inline gas filter)hooked up inbetween the 2,because the blowby vapors can ruin the diagphram in the brake booster after awhile!~ /forums/images/graemlins/frown.gif
     
  7. bouncytruck

    bouncytruck 1/2 ton status

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    Thanks, I was think the same issue with the detent cable so I disconnected it. Still the same issue. The PCV is hooked into one of the ports in the carb. The brakes and the transmission modulator are hooked up to the intake manifold with a plastic "T" from NAPA.
     
  8. dyeager535

    dyeager535 1 ton status Premium Member

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    Shouldn't really be a problem, don't usually hit the brakes while you are trying to upshift. /forums/images/graemlins/smile.gif

    If it's new, you can't always be sure it's working correctly, but if it wasn't full of fluid when you pulled the line off, it's probably good.
     
  9. Calclips

    Calclips 1/2 ton status

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    Yeah, dorian is correct. If you pull the vacuum line off the modulator and tranny fluid comes out, its bad... if not... its good. At least thats what I was told.
     
  10. bouncytruck

    bouncytruck 1/2 ton status

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    No fluid came out when I pulled the vacuum line off the modulator.
     
  11. Beast388

    Beast388 1/2 ton status

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    FWIW, my modulator and power brakes are T'd together and hooked to manifold vacuum as well and it shifts just fine. I rebuilt my TH350 myself too. /forums/images/graemlins/thumb.gif

    If you are having a lack of vacuum for the modulator problem, it would act the same for the 1-2 and 2-3 shifts, not just 2-3.
     
  12. mudhog

    mudhog THEGAME Staff Member Super Moderator

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    it could be a stuck valve in the valve body or you could have pinched or broken a seal internally when putting it together /forums/images/graemlins/eek.gif
     

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