B&Mmakes a TH400 Kickdown switchbut it requires some fabrication work and is still a bit pricey. You can purchase this from any of the reproduction companies like Original Parts Group, but these are somewhat expensive and clunky looking. 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. 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. 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. Often when a TH400 is swapped into a different vehicle, this downshift electrical switch is overlooked. 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. When the linkage hits WOT, this completes the circuit to connect the transmission to full system voltage. 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. 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). This is to protect the transmission from damage. Above a certain speed, as determined by the governor, the transmission will not downshift from third to second or second to first gear. ![]() 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. ![]() Most of the other transmissions used a mechanical linkage or cable to indicate wide-open-throttle (WOT). Jeff Smith: The TH400 transmissionis unlike all the other typical three-speed automatic transmissionsfrom the late 1960s and in through the ‘70s and ‘80s. Does it matter if I hook it up at all? Thanks Another friend told me that it’s electrical. 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. ![]() I have a TH400 trans that I just bought to replace the ailing TH350 in my ’70 big-block Camaro.
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