TH350 Kickdown Issues

Discussion in 'The "Juice Box"' started by 68Rivi_In_Cali, May 28, 2019.

  1. 68Rivi_In_Cali

    68Rivi_In_Cali Well-Known Member

    I have a 1970 Skylark that doesn't seem to kickdown. I will be at speed and an Punch it and nothing. I have adjusted the clip at the pedal (per service manual) and still does not want to kick down. The cable also returns how it should. If I manually shift it, I get a nice response. It will even chirp when it goes into second with the new cam in it.

    It use to be a 2bbl. A 4bbl was swapped in and the throttle cable and bracket seem to work on it. Could it be related to the throttle cable?

    Vac Modulator issue? It will shift fine but I don't get that nice WOT kickdown i've heard about.

    the car has 2.56 gears which I plan on replacing but I assume it should still kickdown nicely.

    This has me scratching my head :confused: anything else I should look out for?

    70 Skylark
    350/350th
    2.56 gears
    2bbl to 4bbl swap
     
  2. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD"

    The throttle bracket and possibly the cable can be different. Have you ever actually checked you are getting WOT with your foot on the floor?
     
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  3. 68Rivi_In_Cali

    68Rivi_In_Cali Well-Known Member


    Just went out to check. I'm running the Sniper EFI system and the Throttle position sensor reads 96% when my foot is to the floor.
     

    Attached Files:

  4. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD"

    The kickdown cable should reach the end of it's travel at the same time you are at WOT. If you are using the Buick cable that goes directly from the transmission to the gas pedal, and you are sure that the cable is adjusted correctly, that leaves an internal transmission problem. You'll have to start by dropping the pan and visibly inspecting the link while someone pulls on the kickdown cable or floors the gas pedal. Maybe the link is damaged or broken? You'll have to look to be sure.
     
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  5. 68Rivi_In_Cali

    68Rivi_In_Cali Well-Known Member

    Ran out again just to double check and the Kickdown cable is at the end of its travel with the pedal at WOT. there's no slack in it. I'm guessing it is internal. I was planning on having a shift kit installed, maybe the shop can also take a look at it while they are in there.

    Thanks for the insight Larry, I tried everything possible based on what I know.
     
  6. 68Rivi_In_Cali

    68Rivi_In_Cali Well-Known Member

    I am guessing this is what I would be looking at. would it still return the cable if it was damaged?

    t-350_link01.jpg
     
  7. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD"

    That doesn't look familiar to me, but it has been 20 years since I had a THM350:)
     
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  8. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD"

    Maybe this is the same thing from a different view?

    DetentTHM350KD.JPG
     
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  9. 68Rivi_In_Cali

    68Rivi_In_Cali Well-Known Member

    Looks like it! I'm digging through my files, I think I have the 71 manual but should be the same. Thanks!
     
  10. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD"

    That picture came from the 71 manual, not that it makes a difference, all the THM350's would be the same.
     
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  11. 68Rivi_In_Cali

    68Rivi_In_Cali Well-Known Member

    Just an update, tightened up the cable again and it finally works. I've never had it work, was nice to feel the automatic kickdown on the highway.

    I still believe the 2.56 open rear is holding me back. The engine wants to run with the new cam but the gears are holding it back. Does make a nice cruiser.
     
  12. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD"

    To a certain extent, yes, the gears are holding you back. Get your self a good torque converter like one of Jim Weise's 9.5" units. That will help a lot and still be tight enough for cruising.
     
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  13. 68Rivi_In_Cali

    68Rivi_In_Cali Well-Known Member

    That would be a great upgrade. Thinking of doing the torque converter along with a shift kit and lastly adding the posi/ 3.42 gears
     

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