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Ask Away: How Do You Hook up a Turbo 400 Kickdown? (Hint: It’s Not Like Other 3-Speeds)

I have a TH400 trans that I just bought to replace the ailing TH350 in my ’70 big-block Camaro. A friend told me that there’s a special way to set up the kickdown linkage on this transmission but I can’t find any kind of linkage. Another friend told me that it’s electrical. I really don’t know what I should do. Does it matter if I hook it up at all? Thanks

D. L.

Jeff Smith: The TH400 transmission is unlike all the other typical three-speed automatic transmissions from the late 1960s and in through the ‘70s and ‘80s. Most of the other transmissions used a mechanical linkage or cable to indicate wide-open-throttle (WOT). This WOT trigger achieves two things: it ensures the transmission line pressure is at maximum for strongest application of the clutches and then downshifts the transmission to the next lowest gear, depending upon vehicle speed. Above a certain speed, as determined by the governor, the transmission will not downshift from third to second or second to first gear. This is to protect the transmission from damage. If you want to see what this input is worth, attach a pressure gauge to the transmission (it’s an 1/8-inch pipe thread plug just above and behind the shifter shaft).

Instead of a linkage or cable, the TH400 uses an electrical switch that is most often mounted on the carburetor. The 12-volt connection is a single spade connector just above the oil pan rail on the driver’s side about halfway back–at least that’s where it is on my TH400. When the linkage hits WOT, this completes the circuit to connect the transmission to full system voltage. This electrical input powers up a solenoid in the transmission that accomplishes the same results, with higher line pressure and a signal to downshift the transmission.

Often when a TH400 is swapped into a different vehicle, this downshift electrical switch is overlooked. Besides the loss of any kind of downshift function, the real danger is that under WOT the transmission has not been commanded to increase line pressure. This causes not only a soft upshift, but also damage to the clutches because the lower line pressure increases the potential for slippage on upshifts. There are several ways to solve this problem.

The factory used a switch located on the carburetor or on the throttle linkage under the dash that at WOT supplied voltage to the transmission. You can purchase this from any of the reproduction companies like Original Parts Group, but these are somewhat expensive and clunky looking. B&M makes a TH400 Kickdown switch but it requires some fabrication work and is still a bit pricey. Edelbrock makes a nice little red aluminum bracket that will work for both 4150 Holley and Edelbrock carburetors that is a little less expensive and looks cool. This uses a small microswitch intended for nitrous operation but could also be used as a TH400 kickdown switch. If you were to use this switch for nitrous and kickdown, I would use this switch to trigger a relay that would power up both systems. This reduces the load on the switch.

Turbo 400 Kickdown

This is the NOS microswitch mounted to a home-made bracket on the passenger side of a 4150 style Holley carburetor. This will only work on a Holley where the choke mechanism has been removed.

Finally, I’ve included a photo of a similar microswitch that came off an NOS nitrous system. I’ve used it on a 4150-style Holley vacuum secondary carburetor with no choke mechanism. I simply made a small aluminum plate that mounts to the threaded choke mechanism mounting holes in the carburetor. (Be sure to seal off the small vacuum port on some Holley carburetors that’s used to pull heated air into the choke housing.) Then I mounted the microswitch in such a way that when the primary carb linkage achieves roughly 90-percent throttle opening, the linkage triggers the switch. The microswitch is a relatively common item; NOS sells it under PN 15640NOS. It took about a half-hour to make this bracket and get it aligned properly with the carb linkage. I had to bend the actuator tab a little to make it work, but it has been on my Chevelle for about a year now with no problem.

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21 Comments

  1. i have a 72 camaro with bbc 400turbo w/o the kickdown runs fine without it & its the way its staying

  2. I have a TH400 and i used a lokar kickdown cable that plugs into the tranny drivers side then runs to the fuse box for the current. then the cables connected to the drivers side of my 750 double pumper Holley carb in front of the accelerator cable with a bracket. The one you show i have never seen. It is a 1980 transmission out of a suburban. I have this in a 1969 nova with a BB

  3. I also went with the Lokar kick down switch on my JW performance 400turbo easy to hook up. I also changed my throttle pedal and linkage to Lokar , as my stock linkage was shot and the billet floor mount set up was real easy to install. It looks great with braided stainless steel cables and the billet gas and brake pedals. I bought a B&M electric kick down switch but sent it back. The B&M switch does not have any type of bracket so you must fab one for yourself if you have the skills to make it look good. I called them a suggested them to engineer one for custom applications they told me most people use stock brackets or make them for themselves. Lokar has thought their kits out and have all parts available.

  4. I have the same issue in my 85 c10, we did a 5.3 vortec swap and its fuel injected. What should I do or how can I over come this? I don’t want to hurt my trans!!!

  5. Guys, i have rebuild a complete drive train Chevy 350 motor and TH 400 transmission into a car with zero wiring and a fresh 600Holley with no switch, any DIY advice?

  6. robb hosman says:

    I have a 1972 chevy c 10 turbo 400. it has the 12 v from the throttle pedal to the transmission check and has 12 v after checking it. truck doesn’t kick down and shifts soft. any advise on how to fix the problem? Don’t know if I need to replace the shifting modulator or the 12v solenoid in the side of the trans.could sure use some advise.Thanks Robb

    • Do you have an adjustable modulator?
      Some modulators have an adjustment screw where the vacuum hose connects.
      Turn the screw in for firmer shifts and out for softer shifts.
      It’s kind of a trial and error thing so adjust it gradually.
      Hope this helps

  7. Pingback: Kickdown solenoid problem - Rolls-Royce and Bentley Forums

  8. 68 CHEVY C10 2WD with 327. I basically bought someone else Project. I knew that going into the deal. It has 4bl 600cfm 1406 Edelbrock carb and manifold with headers. After attempting to tune up the TH400 with filter, gasket it started leaking everywhere and shifting erratically and eventual failure. I had the tranny rebuilt and now I have a new vehicle. the tech said I need the KD switch but they let me take the truck home and asked me to come back with the KD switch. I am taking it back with the switch today for installation. My question is; have I damaged the tranny in the short distance between home and shop 10 miles round trip? Based on your article , this switch is a critical component and never should have been driven without it. Your thoughts.

  9. I have a 1986 Chevy truck put 1980 th400 behind a454 where does the kick down sw go . Engine is 74 year with quadra jet

  10. what do I all need all I have is the trans. with 1 spade coming out of the trans. ?

  11. Scott Latham says:

    I have a late 80’s yo early 90’s 454 engine with quadra jet and no kickdown? th400 attached. Installed in a 1997 Chevy 3500.

  12. I have read that the TH400 has a vacuum kickdown. Holley recommends that all you have to do is connect the vacuum port on the TH400 to the manifold or carburetor. There is no mention of the electronic switch. Is this accurate?

    • Jeff Smith says:

      Julian,
      The TH400 uses both. The vacuum modulator is used to eastablish the load on the engine. As vacuum drops with high load, this changes the position of the vacuummodulator valve in the transmission and increases the line pressure and also delays the shift point. The kickdown switch is a separate 12-volt connection on the driver side of the trans as described in the original questionand answer.

  13. i have a TH400/3L80 trans that i’m putting in my ’55 chevy truck. i’d like to use the petal mount KD switch i already have. i understand that the pink/black stripe wire goes to “hot in run” power and the orange wire goes to connector on trans but, what about the other terminal at trans connector plug? does it need to be connected to something certain, or will it be ok if i leave it disconnected?

  14. Travis Hansen says:

    Can I shift the tranny myself and not hook up any kick down?

  15. I have a 72 Chevelle SS, it came with a TH400 out of a truck. I get what you are saying about the switch on the side of the carb, is there anyone that sells a hookup like you are talking about? I get the switch being energized at 90% throttle but where do I send power from? Just a main hot power?

  16. MAKES A LOT OF SENCE…COUPLE OPTIONS..I WILL SAVE THIS,,THANKS LOTZ..

  17. Im having some problems with my truck and i think the kick down is to blame, when i go wot it is very latargic so i dont think its downshifting so the secondarys will not open, is that a correct assumption, i have 12v to the solenoid but i dont think it works.

  18. What about upshifting on myth400 does the kick down play any role from 1st to 2nd gear. Mine acts like its stuck cant get above 25 mph

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