TH400 help

['65 chevy lover]

I have a TH 400 that I just got in my truck and, it's not shifting right. Could it have to do with this (see pic below) is this supposed to be hooked up to a vacuum line?

[B. W.]

Yes, vacuum modulator. Hook it to vacuum & see if it shifts better.

[Captainfab]

Just to clarify, it needs to be connected to manifold vacuum.

['65 chevy lover]

Originally Posted by Captainfab

Just to clarify, it needs to be connected to manifold vacuum.

Thanks, Captain, I'll check it out.

[YoungPup1977]

Just some additional tidbits.......From the pic, I believe the modulator maybe adjustable as well....so you can take a small screw driver and adjust the set screw in the modulator....turn the screw clockwise will firm up the shift (later) and counter clockwise with soften the shifts (earlier) ....

['65 chevy lover]

Originally Posted by YoungPup1977

Just some additional tidbits.......From the pic, I believe the modulator maybe adjustable as well....so you can take a small screw driver and adjust the set screw in the modulator....turn the screw clockwise will firm up the shift (later) and counter clockwise with soften the shifts (earlier) ....

Thanks, I'll check it out.

['65 chevy lover]

Originally Posted by B. W.

Yes, vacuum modulator. Hook it to vacuum & see if it shifts better.

Thanks, will do.

['65 chevy lover]

Hooking up to vacuum fixed the problem. It shifts good now. I felt and looked around on the valve body, but didn't see or feel a screw to adjust how it shifts. Thanks to all for the help!

[Captainfab]

The adjustment screw on a vacuum modulator is inside and accessed thru the port for the vacuum hose.

[Alex V.]

Originally Posted by Captainfab

The adjustment screw on a vacuum modulator is inside and accessed thru the port for the vacuum hose.

This. And on a Turbo 400 the modulator is the sole control of shift points and firmness, except for the electric switch for WOT downshifts which most, but not all have.

['65 chevy lover]

Originally Posted by Captainfab

The adjustment screw on a vacuum modulator is inside and accessed thru the port for the vacuum hose.

Ah, I'll check it out. It shifts fairly quick from 1st to 2nd, but from 2nd to 3rd takes a bit longer. I'll have to drive it more to see if I need to adjust it. Thanks for the info.

[mattfranklin]

Originally Posted by '65 chevy lover

Ah, I'll check it out. It shifts fairly quick from 1st to 2nd, but from 2nd to 3rd takes a bit longer. I'll have to drive it more to see if I need to adjust it. Thanks for the info.

Glad you got to the root of it. I'll bet it revved way up and then slammed the 1-2 shift. I had that vacuum hose fail once because the PO put the exhaust too close to it and burned the hose until it was extremely brittle.

[DieselSJ]

Originally Posted by Captainfab

The adjustment screw on a vacuum modulator is inside and accessed thru the port for the vacuum hose.

Be very careful and make changes 1/4 to 1/2 turn at a time. You can puncture the diaphragm in them if you crank it in too far. As me how I know...

[GASoline71]

Also there was a steel line that ran down there from the factory. Attached at the intake behind the carb and to the modulator with a 4" piece of rubber hose. A long rubber vacuum hose going all the way down there can weaken and colapse.

Gary

['65 chevy lover]

Thanks to everyone for the information, it is very appreciated!