TH350 vacuum line

Discussion in 'The "Juice Box"' started by -JJH-, Apr 13, 2019.

  1. -JJH-

    -JJH- "Works like a Buick" - a Finnish proverb

    Hi all,
    Yesterday I picked up my '70 GS from winter storage, did the normal drill of visually checking the undersides while lowering the car from jack stands, checked fluid levels, charged up the battery etc. etc.

    Started beautifully, filled fresh 98 (RON) octane fuel and drove 80 miles back home, without problems. Back home, I noticed a small rubber line touching ground, on passenger side.

    Looks like a vacuum line would have been torn from engine end, the other end is still going into tranny 'hump'. Cannot see at the moment where it goes exactly and doing a very quick check under the hood, but couldn't tell where I am missing a vacuum line.

    Now, the odd thing is that the car runs perfectly normal, except the kick down does seem a bit hesitant;

    - I understood that there should be just one vacuum line running from intake manifold to a transmission modulator. If this vacuum signal is missing, the modulator should assume WOT situation ie. max. line pressure, hard shifts and shifting closer to red line, am I correct? My tranny works still like normal...

    - The latter, kick down issue, I think, is controlled by kick-down cable and requires just re-adjusting at the carb end? Not related to any vacuum line in TH350?

    Will jack up the car in near future, but any idea what this line might actually be?


    PS. Would someone happen to have a vacuum line diagram for 1970 Skylark/GS?
    Last edited: Apr 13, 2019
  2. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD"

    If the vacuum modulator line was disconnected, not only would you have a vacuum leak causing the engine to idle rough and hesitate, but the transmission would not shift except at WOT. The kick down cable is adjusted at the gas pedal end. There is a sliding clip that adjusts the cable. You slide the clip to the minimum length adjustment and then floor the gas pedal to self adjust the clip.

    The vacuum line consists of a metal line with vacuum hose on each end. It runs from the back of the intake manifold to the modulator. The modulator is at the back of the THM350.

    Vacuum diagrams along with a lot more information is in the Chassis Manual. It's a must have IMO. This one is a steal on Ebay,
    Last edited: Apr 14, 2019
  3. 1973gs

    1973gs Well-Known Member

    I can't see it being the hose to the modulator if the transmission still shifts normally. I have seen vacuum hose on the vent tube, but mostly on newer vehicles. I believe that they were added most of the time so that when the trans was hot and probably over filled, the fluid would drip on the ground and not run down the trans case.
  4. -JJH-

    -JJH- "Works like a Buick" - a Finnish proverb

    My thoughts exactly - it cannot be the modulator hose, since tranny works beautifully (except the kick-down, which is a separate issue). At the same time, I seem to have no issues with vacuum leak either ie. no rough running (see uploaded video w/ sound @idle) and no hesitation either, regardless am I driving 20 mph or 50 mph.

    Will need to lift that car up and see where that hose goes on the tranny side. Or it's just a practical joke by previous owner, leaving an unattached hose there to throw me off :D


  5. gstewart

    gstewart Well-Known Member

    if the vacuum line is on the passenger side of the tranny, then the vacuum facilitates the up-shift under normal acceleration. it then attached to a hardline that attached to a port on the passenger side of the intake at the rear.

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