400 Turbo Kickdown Woes

[skokie]

Had the 400 turbo tranny rebuilt 2 months ago but didnt have the kickdown setup on it. Well I got one on there now and it kicks down to 2nd but wont go back to 3rd after I left off of the gas. To get it back to 3rd I have to take down to 2nd then back to D. Tranny shop said that it could be a bac kickdown switch in the tranny so I take it back down there. They said that they swapped it out but guess what same problem. I dropped it off tonight so they can look at it tomorrow. The tranny shifts just fine with no problems until you punch it and it kicks down. Any ideas on what could cause this?

Thanks

[COBALT]

The kickdown (or detent) for the TH400 is electrical. There's a solenoid under the pan that activates the valve body change. If they're telling you the "switch" is bad in the tranny they're not telling you the complete story. All you need is a simple electrical switch OUTSIDE the transmission to provide the 12v to the terminal on the driver's side of the case above the pan. That provides the electrical power to the solenoid so it can do its job.

My guess is your kickdown switch (which provides the 12v to the terminal) isn't adjusted right, or something's wrong with the rebuild and the solenoid isn't allowing the transmission to upshift. The kickdown should be adjusted so it closes right before WOT.

Is your transmission moving through all the gears correctly under normal operation?

[skokie]

Kickdown switch is adjusted to right before WOT. Tranny is shifting fine under normal operation. My luck its something with the rebuild. Me and trannys dont get along sometimes.

[COBALT]

Try this:

Get a simple 12v probe tester at your parts store (screw-driver looking thing with an ice pick end, and a wire with an aligator clip hanging out the back, and a bulb in the handle). Disconnect the wire that goes to the transmission, and connect it to the aligator clip on the tester. Put the tester to ground on the frame (shove it in a frame hole or something), and manually actuate the throttle with the key on and the truck off. Does the light come on? Drop your throttle back down. Does the light go off?

I installed an aftermarket lokar switch. You can literally hear it click from above standing over the engine.

If the light is coming on and off with the throttle like it's supposed to (and you've got the vacuum modulator set up with full manifold vacuum) then I'd have to say somebody goofed with the rebuild. It's entirely possible for the kickdown switch to be initially set up right, but get loose and start to make the detent act funny.

Mine slipped after I installed it, and since the switch has a spring in it it closes automatically. Imagine my surprise when the transmission kicked down while I was strolling down the street going 25mph at low cruise. That woke me up. Turns out the set screw on the cable was loose.

[skokie]

Thanks for the info Cobalt but I have already dropped it off at the tranny shop. I am going to call him tomorrow and see what he has found. I will post back what I find out. Hope its something minor like a vac modulator.

[skokie]

Just talked to the tranny guy. He took it for a test drive and its not intermittent. Hes going to start with the gov and vac modulator. Should hear back from him in a couple hours.

[skokie]

Talked to the tranny guy. He said that the kickdown switch in the tranny is bad. Mentioned something about the spring being stuck. Not sure what he is talking about but they replaced it with another switch and are going on a roadtest. We will see what happens.

[skokie]

Well the drama continues. They swapped the kickdown switch and it behaved for about 5 mins then started acting up again. They are looking at the valve body now.

[COBALT]

That's kinda what I thought it would be. If the detent solenoid under the pan is doing it's job then the valve body is probably having a problem, and THAT my friend is part of the rebuild.

FYI the governor helps determine shift points based on output shaft rotation. It works with the modulator (which is a diaphram that changes shape due to intake vacuum) to do this: high rotation + low vacuum = acceleration; high rotation + high vacuum = deceleration...etc.

It's an icecream cone shaped thing with a gear on one end, and a set of counter weights on the other. The farther the counter weights spread apart (the faster it spins)....well you can figure out the rest. It helps with downshift and upshift while going from idle to cruise. Detent (kickdown) is for cruise to WOT.

Here's a link to an excellent book on TH400 transmissions. I'd give it a read, and keep it on the shelf for later:

How to Work With and Modify the Turbo Hydra-Matic 400 Transmission, By Ron Sessions

Good luck.

[skokie]

Thanks Cobalt. I will pick that up and put it right next to my "how to rebuild a small block chevy book"

[skokie]

Got the final word. Bad valve body in the tranny. Will go test drive it tonight. No charge to me.

[skokie]

Well the kickdown is still acting up after the shop gave me another tranny that was fully rebuilt. I tested the kickdown as cobalt said and its working. The light lights up when the kickdown activates and turns off when I let off of it. The tranny guy I am going to swears its not the tranny and I want to believe him but I keep hitting these issues. After thinking for a bit I realized that the entire tranny was swapped but not the vac lines. Could it also be the vac modulator or the governor causing this?

[skokie]

I tested the vacuum at the modulator and its 18-19.

[skokie]

Well got some more news here I tried this. I put the truck in 2nd and it kicks down to 1st and lets go no problem. But not 3rd to 2nd.

[skokie]

Ttt