Transmission kickdown question
 

1972 Grand Prix 455 TH400...Transmission kicks down correctly at speeds under 30mph (or so) but does not kick down at highway speeds. Anyone here have advice on what components should be checked for functionality or adjustment? Thanks for any input.

Check to see if you have 12 volts on the connector on the drivers side of the transmission with the key on/engine off and the gas pedal floored.

Is it kicking down into 1st gear below 30 mph?

I'll check voltage at the transmission connection. Depending on what's there it makes sense that I'll know whether to examine the accelerator kick-down switch or transmission solenoid (not sure of the terminology for those two components). Thanks for the tip.

Thanks for taking time to respond. Not sure but I'll check. Would the finger of suspicion be pointed in different directions if it kicks down to first vs. kicks down to second at the slower speeds?

If its kicking down into 1st at 28 mph or below that tells me the electrical part is working. When in 3rd at low speeds, it will go down into second on it own, but not into 1st if the switch isn't working. Another simple check is to find a quiet spot and turn the key to the run position (do not start it) and manually pull the on switch lever all the way (full travel) to activate the kickdown and listen for a "click" at the transmission. If you hear a click, it's working. Sometimes the pedal bracket gets bent over time just enough to not allow full travel of the switch and a simple bending of the bracket back towards the firewall will fix it.

Great information! Thanks!

Here's what I found...With key on, checked for voltage in wire that connects to throttle activated kick down switch and it was good. Checked switch for continuity with accelerator pedal to the floor and the switch was open. Removed switch and bench test showed that it remained open to within 5/16" of full travel. Thinking it was probably dirty, sprayed it liberally with contact cleaner and worked it vigorously but to no avail. Found a piece of round hard plastic 11/16" OD and drilled it to 3/8" ID for a press fit over the arm of the switch. Re-installed and test drive shows proper kick down function. Good for now. Thanks again for taking time to respond and sharing your knowledge!