My th400 used to work quite well. Lately, it has developed a tendency to not kickdown when i stomp it. Sometimes, but rarely. I've had it on jackstands and had my son stomp it with key on engine off. I do get 12 volts at the terminal outside the transmission, and I hear the solenoid click. So I pulled the pan and the solenoid. Checked it all out. Nothing clogged or sticky or anything. While I had it apart, I checked on the governor, looks okay. Got it all back together. Took it for ride and no change. Still doesn't kick down. Any ideas?
It's been almost 15 years since I had a th400 in my trans am, so memory is questionable. But back then I was drag racing it and had the "kickdown" switch on a toggle, it would kickdown any time I had that switch on. If the switch is clicking you know you have good voltage and ground circuitry. I'd guess your solenoid is mechanically bad, or the valve is stuck.