kick down switch or vacuum leak

[Trippin3030]

howdy thanks for taking the time to read this I'm almost at my wits end with my truck if its not one thing its another and i'm learning as i go so firstly please don't judge as I realize i'm a amateur at best haha. im damn near finished a restore on a 1980 chevy K20 campers special its running a 350 bored 30 over bolted to a th400 she runs an drives but wont shift out of first until i hit 50 KMH unless i plug the trans into direct vacuum from the intake and bypass the kick down switch once i do that it shifts just fine and runs smooth now i just rebuilt the carb (Rochester QJ) and once installed i did the usual checks and the vacuum seal is fine on it. i tried adjusting and then eventually swapping out the modulator valve to a brand new one on the trans but to no avail. this truck is nickel an diming me to death so i thought i would try to exhaust some truck forum knowledge befor buying new parts i got giddy and started driving her anyways with it plugged directly into the intake but its gutless on those single lane highway passes so i would love to have my kick down operational. any help is greatly appreciated. once again thank you for your patience while reading this incredibly long winded post sorry bout that it kinda got away from me haha

 

[79K10]

what kind of valve body are you running in the th400? wouldn't be some kind manual valve would it? or somethings sticking, pull the pan maybe?

 

[chengny]

but wont shift out of first until i hit 50 KMH unless i plug the trans into direct vacuum from the intake

That is exactly where the vacuum signal for the modulator is designed to be obtained. There is generally a special fitting (3/8" NPT X 1/4" hose barb IIRC) that screws into the intake manifold right in front of the distributor. The modulator sensing tube is shaped to line right up with the fitting. A short length of rubber hose is used to connect the tube to the fitting - actually the tube can fit into the ID of the hose barb fitting:

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but its gutless on those single lane highway passes so i would love to have my kick down operational.

If you are having trouble with upshifts and the problem is resolved by disconnecting the detent solenoid (kickdown) switch, that would almost surely indicate that the switch is stuck closed.

That switch can be hard to source, but it is available. Before you buy a new one however, try to salvage the one you have.

Remove it from the bracket next to the accelerator pedal.

Pull it apart and clean it. Apply some white grease for lube.

DON'T FORGET TO PROPERLY SET THE INITIAL POSITION before whacking the gas pedal to the floor for final adjustment.

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Or, if you don't feel like getting involved with tearing it all apart, regaining normal function is often just a matter of resetting the switch point. I know this is going to sound crazy but, all you do to reset the switch is:

First, push the plastic plunger all the way back in (opposite the direction that the gas pedal arm normally moves it).You might have to push it pretty hard to get it back in:

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It will make a ratcheting sound and stay way in.

The to do the actual setting of the contact close point - just vigorously depress the gas pedal to the floor.

That's it. If it can be reset that will do it.

 

[Trippin3030]

wow chengny pictures an everything I really appreciate the help fella I just gotta work up the courage to cram myself under the dash to get it done now. thank you again for goin to all the trouble to help me out especially the pictures sorry if I set you back any

 

[HotRodPC]

Yep, vacuum is supposed to be hooked directly to the intake manifold. If you still have the problem. If the vacuum isn't it, then I'd say it is the switch Jerry pictured. If it was that way all the time, even when you didn't bypass the swtich, then I'd say the plunger on the switch inside the pan is stuck out. Since you can unhook the switch and it goes back to normal operation, then I'd say the switch under the dash is powering the switch inside the pan.