TH 400 electric vs vacuum kick-down?

Discussion in 'Sparky's corner' started by 12lives, Oct 26, 2018.

  1. 12lives

    12lives Engage! - Jean-Luc Picard

    Seems I'm finding some mysteries on this car. This plug is behind the bell housing next to the trans dip stick tube. Saw it when I pulled the exhaust manifold off. Seems I've never had the electrical kick down hooked up. What is the difference between the switch on the throttle and the vacuum downshift? 1970 GS 455 auto. When I floor it in 3rd it will go to 2nd....

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    Last edited: Oct 26, 2018
  2. philbquick

    philbquick Founders Club Member

    I think the vacuum modulator has more to do with up-shift.
  3. Bens99gtp

    Bens99gtp Well-Known Member

    when you floor it at highway speeds does it still drop to 2nd
  4. 12lives

    12lives Engage! - Jean-Luc Picard

    I don't remember how fast I was going when I floored it, maybe 30-40. Reading up on it, it sounds like the vacuum will drop it under 35 mph and the electric is for downshifting over 35. One more thing to fix! :rolleyes:
  5. telriv

    telriv Well-Known Member

    The downshift wire is a single wire in your year car & is on the LEFT side going down to the trans. on the left side. I'll almost bet that is a sensor for the heating system that doesn't allow the blower motor to start working until the engine reaches a certain temp.
  6. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD"

    That looks like the downshift connection alright. I would hook that up, but before I did, I would make sure that the gas pedal switch is getting the 12 volts and passing it down to that connection. Should be pretty easy with a test light. Listen for the click of the downshift solenoid in the pan by applying 12 volts directly to the transmission.

    BTW, the downshift is accomplished hydraulically or electrically. The valve body has a 3-2 valve unless it is a switch pitch valve body which lacks it. The valve body uses input from the modulator (vacuum as an indicator of engine load), the governor (road speed indicator), and gas pedal kick down switch (throttle position indicator). It's like a push and pull between those 3 inputs which lets the valve body determine where the shift points should be under various conditions. When the kick down is disconnected on a THM300/400, the transmission will frequently short shift at WOT.
  7. 12lives

    12lives Engage! - Jean-Luc Picard

    Thanks Larry - interesting - it does short shift! What rpm should I be looking for, at WOT, if everything is working correctly?
  8. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD"

    Not sure, I'd say 4800-5000. I'd want it that high at least if it's an all stock engine. With a decent cam, I'd get it shift at 5500.

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