I have an ’85 C-10 pickup that we’ve installed a factory fuel-injected 5.3L truck engine. We bolted a TH400 transmission to the LS engine but I now realize that I don’t have any way to trigger the kickdown linkage on the TH400 trans. How do I do this so I don’t hurt the trans?
Jeff Smith: We’re going to assume that you have a cable-actuated throttle linkage for your 5.3L LS truck engine. We’ll also assume your pickup uses a cable throttle assembly – so it should not be difficult to build a small bracket that can mount a simple microswitch that would be triggered as the pedal linkage reaches wide-open-throttle (WOT). Chevrolet used a switch mounted off the throttle pedal assembly on a 1970 Chevelle to trigger the TH400 kickdown so you might be able to look at one of these mounts to get an idea how to make one.
With regard to wiring, it’s really simple. Place one wire from a switched, 12-volt positive source on one side of the micro-switch. Run a wire from the second connection on the switch directly to the electrical connection on the transmission. When the throttle hits WOT, the switch will complete the circuit and 12-volts will be supplied to the internal solenoid in the TH400 trans which increases line pressure and changes the shift characteristics of the trans. It’s that simple!