TH400 kickdown switch

Discussion in 'Chevy Truck Forum' started by Boudin, Feb 6, 2012.

  1. Boudin

    Boudin Forum Moderator

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    Ok....I have been trying to figure out why when you floored my truck when cruising (like passing someone or merging on interstate), it didn't seem to have very much power. At least not enough in my book. I thought it might have to do with the secondaries not opening up on my quadrajet, but they are working from what I can tell. I came to the thought that....well....you know it isn't downshifting either. I then looked at the TH400 kickdown switch located on the gas pedal. It looks like it is not even functioning. Not because it is broken....multimeter confirmed it functions just fine. The problem is that the gas pedal rod doesn't even touch the plastic tube that protrudes out from the switch. Even when at WOT, it's still not touching it. Is it possible that someone put in the wrong one? I have looked online and can't seem to find this kickdown switch for a TH400. I'm still looking though.

    I had this idea of wiring my own switch that I would engage when I was trying to pass someone or merge onto the interstate. I was thinking of using a momentary on/off switch mounted on the dash. Does anyone see a problem with this? I'm asking because it doesn't seem like these switches are easy to come by and I think it will be pretty cool to have a pushbutton "power" button that would make the tranny kickdown from 3-2 on demand.

    What do y'all think here? Why is the rod on the pedal not touching it? What about my momentary on/off switch idea?
     
  2. Case 7230

    Case 7230 Senior Member

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    Memory not so good but I think your kickdown switch was called a "throttle position switch" (not to be confused with a throttle position sensor or TPS). Your switch should be adjustable, I think, to get it to operate at WOT.
    You could certainly wire in a dash mounted switch but you would have to hold it in while passing.
     
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  3. chevychase

    chevychase Semi-Admin
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    Just drop it down a gear ..before passing.
     
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  4. Boudin

    Boudin Forum Moderator

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    Yeah....I guess I could do that. I just am a little hesitant to do that on a regular basis just because the possibility of accidentally shifting too far either way (going to 1 or even R). The shift linkage is kinda worn and I'm afraid of exactly what I just mentioned.

    Is there some piece that I am missing that will connect it to the pedal? I just don't see how that setup would ever make contact with the switch. It doesn't look adjustable at all. I tried making it as long as the switch housing would allow and it still wasn't long enough. I just don't see how my pedal would ever touch it.

    This is why I was asking about the momentary switch. If I were to install one....would it drop the tranny down a gear no matter what speed I am traveling?
     
  5. Boudin

    Boudin Forum Moderator

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    I came across this posting on another site about the TH400 and it's electric kickdown. Is there any truth to what he is saying about premature wear without the kickdown functional?

    "A TH400 actually has an electric switch for the kickdown. It is sometimes mounted on the carb linkage (in several different places) or can be mounted under the gas pedal (most common)
    The modulator controls shift points on the 400 til WOT then the detent system takes over and actuates the electric kickdown.
    The reason for this is because there isn't enough vacuum at WOT to operate the heavier 400 components.
    WOT shifts without the kickdown hooked up will cause the clutches to slip and they will wearout before you want them to.
    If I remember right there are aftermarket kits that replace the GM electric switch.
    Russ"
     
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  6. chevychase

    chevychase Semi-Admin
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    Makes some sense ..your putting pressure on trans and it is not kicking down a gear .
    I guess it would be like going up a big long incline in overdrive ..wear trans out ..think.
    Only one i,am familiar with are the ones at carb ..you can screw out to adjust.
     
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  7. Case 7230

    Case 7230 Senior Member

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    I think what the guy there is talking about is if you are at WOT and then force a downshift with the shift lever.
    It would seem like there is something missing from your switch. Maybe do a parts search for "throttle position switch" and see what turns up.
    I would think if you wire in a momentary switch you would get a downshift at any speed when it is activated.
     
  8. Boudin

    Boudin Forum Moderator

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  9. Boudin

    Boudin Forum Moderator

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  10. Boudin

    Boudin Forum Moderator

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