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TH350 and vacuum question...

Discussion in 'The Garage' started by ak bandit, Jun 22, 2006.

  1. ak bandit

    ak bandit 1/2 ton status

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    I have a Holley Truck Avenger carb on my 396. And I think the transmission vacuum is hooked up to the timing vacuum advance port on the carb and vice versa. Would this cause my newly rebuilt TH350 not to shift into 3rd gear?
     
  2. randy88k5

    randy88k5 1/2 ton status

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    Have you taken a vacuum reading there? I dont think it should be a problem if your getting full vacuum.

    EDIT: Is it hooked to a thermal vacuum switch?
     
  3. 1979jimmy350

    1979jimmy350 1/2 ton status

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    if you are able to shift in to second gear the vaccum is working, if the vaccum modulator was not hooked up you wuld never shift out of first gear
     
  4. diesel4me

    diesel4me 1 ton status Premium Member

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    in a word...MAYBE!!...

    A vacuum modulator is always hooked up to manifold vacuum...hooking it up to a ported vacuum source MIGHT cause the tranny to stay in the lower gears longer,or not upshift into third..usually if they get hooked up to a ported vacuum source ,the tranny shifts into second late, and harder than normal,then it either refuses to go into 3rd gear ,or only upshifts into third when you let off the gas,and might go right back into second again as you get back on the throttle.....or it "coasts" or "freewheels" instead of upshifting to third,and no "engine braking" is noticed...
    .....at low speeds, enough vacuum may be delivered through the ported port to give just enough vacuum to make the modulator work "normally",but as you get on the gas more,the shifting trouble becomes apparent,as the vacuum INcreases at wider throttle openings, instead of DEcreasing,like its supposed to--its backwards!!.....

    I know this from not only putting MY modulator hose on the wrong port before,:doah: --I've seen many other vehicles with the hose on a ported vacuum port--and the owner claims it "might need a tranny",and complains of shifting problems,or slippage...many times its after a carb or intake swap that hoses get hooked up incorrectly...or left off!..and the tranny starts acting up..

    If the govenor gear fails,then first and reverse are the only gears that you'll have--it wont upshift into second or third....a TH350 will shift up with a bad modulator or no vacuum to it,but it will be eratic,have late shifts,and you probably will have to "coax" it into the higher gears by manually shifting it with the shift lever,or it will tend to "hang" in first gear.....

    A maladjusted detent cable can prevent an upshift into 3rd also,by holding it in "passing gear" ,if the cable is too "tight" in its adjustment....:crazy:
     

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