Zr1bandit

I have a 400 turbo auto that was recently rebuilt and seems to be working fine with the exception that under full throttle it will not shift from 1st to second without letting off the gas. Tranny shop claims this is how it should operate. I don't remember ever having to let off throttle to get a auto to shift gears when left in drive. Does anyone have any thoughts on this----thanks---Greg

Fun71

That is not how it should operate. Sounds just like what happens when the vacuum modulator is not connected. With no vacuum to the modulator, the trans will downshift under anything more than light throttle and not upshift until very high RPM.

svnt442

Double check the vacuum connections at both ends for the modulator.
And is this only under full throttle or all the time? I had an issue where the connector was broken on my 70 and the TCS wire was connected to the kickdown terminal on the trans and it wouldn't upshift at all. I had the trans rebuilt because I thought there was an issue with it and it did the exact same thing after I picked the car up from the trans shop. He swore that he took it for a ride and it was shifting just fine, but when I drove it it was revving to 5500 at any throttle input before it would shift to second. I took it back a second time and he still said that nothing was wrong and that it couldn't be the kickdown.
I picked the car up for the third time and it wouldn't shift. I got under the car....IN THE PARKING LOT....and unplugged the wire, and whadda know....it shifted.
I took it back to him to show him the issue and he was a bit baffled. That gave me no confidence in the guy.
The trans held up, the the cheap torque converter died on me and he did nothing for me. Such is life.

oldcutlass

If it shifts fine under normal driving, it is not the modulator.

Zr1bandit

Only does not up shift under full throttle---otherwise it's ok which connector are you referring too ??? Thanks. Greg

Zr1bandit

What then ??? Governor ??

oldcutlass

Either the governor or something wrong with the shift kit install.

Zr1bandit

wonder how to check for bad shift kit install ??

droldsmorland

First verify correct fluid level. Next verify no vac leaks in the modulator line. Look at the steel line (if still equipped) to verify its not partially plugged off. Ive seen the steel line get plugged up. It should be plumbed into a direct intake manifold vac source not on the carb. Verify the modulator is in good working service. Pull it see if it holds a vacuum and verify the vacuum side does not have trans fluid in it. That indicates a blown diaphragm.
If that all checks pull out the governor and have a look at the springs and both the drive and driven gear. If all that passes check the line pressures in PRD123 see how they compare to spec.
Then dig deeper according to what the pressures say.

droldsmorland

Also verify the Down Shift switch is working properly. Disconnect the wire at the switch road test and see if the problem goes away.

Koda

If it shifts under moderate throttle fine, it's not the modulator. A disconnected modulator will not upshift at all (maybe hellacious RPM, I don't know). I had a hose pop off mine once after some work and I took it home in first gear until I could get to it.

coldwar

Before you tear in to trans for any reason replace the modulator with another as a test, and verify vacuum connections and reliable vacuum supply. Late late upshift troubles with the 400 are very often traced to the modulator or something related to it. I like the older GM aneroid bellow type, a larger can style case if you have the room for it. Some of the aftermarket types that look like a top hat have a adjustment screw seen down in the vacuum port, allen head. I have seen those jammed all the way down right out of the box so check that out too. You can rule out the electric kick down by simply disconnecting the connector from the side of the case for testing. Good Luck

Zr1bandit

thanks guys for all the help-----gonna do some testing this weekend

Fun71

This sparked a memory with me. Back in the 80s I put a shift kit in my TH350 and inadvertently installed the spring on the wrong end of the vacuum modulator valve (the valve that goes into the trans case behind the vacuum modulator). This caused the WOT shifts to be really, really high. Right now I can't remember which end the spring was on, but putting it on the other end of the valve cured the problem.

Hopefully someone can say which end the spring should be on and you can check that on your car.

svnt442

The connector is on the left side of the case just above the pan rail to the rear of the shifter arm. It will have either 1 or 2 prongs on it depending on if the trans has either a switch pitch (I doubt you have that) or was set up with TCS. All of the factory TH400's for at least 70-72 (I think...help here guys) should have that. IF your trans is not factory and from a later year then it may only have the one connector for the kickdown. (this was my problem)
Your harness has a two wire plug on it. One has constant 12v to the TCS and the other is switched from the switch on the gas pedal for the kickdown. My "harness" was a mess and the connector was split so my trans guy plugged the TCS wire onto the single terminal on the trans and thus causing my trans to always be in kickdown mode.
Once I figured that out it was simple to correct the problem. I also ended up replacing both under hood harnesses which totally cured my harness issues.

Zr1bandit

I believe the tranny is a 1969 400. Will check this

svnt442

Zr1bandit

I have the single pole connector.