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.

Keywords
GM transmission tech, TH400, Turbo 400, Turbo 400 kickdown
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Author: Jeff Smith

Jeff Smith has had a passion for cars since he began working at his grandfather's gas station at the age 10. After graduating from Iowa State University with a journalism degree in 1978, he combined his two passions: cars and writing. Smith began writing for Car Craft magazine in 1979 and became editor in 1984. In 1987, he assumed the role of editor for Hot Rod magazine before returning to his first love of writing technical stories. Since 2003, Jeff has held various positions at Car Craft (including editor), has written books on small block Chevy performance, and even cultivated an impressive collection of 1965 and 1966 Chevelles. Now he serves as a regular contributor to OnAllCylinders.