76projectstingray

I still have been trying to get my Th 400 trans to down shift on a regular basis. In the mean time I Changed the Vacuum modulator and wanted to make the trans shift a little later. Does anyone know which way to turn the adjuster screw in the vacuum port to make the trans shift a little later? I turned it out 1 turn ( 1/2 turn at a time) ,but it didn't seem to make the trans shift any later.

I did this years ago with a TH350 and I did notice a difference.

7T1vette

That adjustment is a rather 'coarse' one. The individual downshift points are set by spring rates in the shift valve system, inside the valve body. The vacuum modulator is just meant to modify the entire shift pattern set, either 'higher' or 'lower', depending on which way the adjustment screw is turned. Turning that screw just changed the spring load on the modulator pin which helped regulate system pressure in the valve body. If you get no change at all, the vacuum level to the modulator may be too low for you to be getting any modulation at all. It was designed to see about 18" Hg vacuum at engine idle. If you are much lower than that, yours may not be working as intended.

1970-Bronze

I've been adjusting mine lately.

I believe it is clockwide to shift later.

You need to make a complete turn to even notice.


I would crank it all the way in then back off 1/2 turn and work your way back-wards. If you leave it all the way in, it will not shift the first time out. Ask me how I know.....

It will only make 3-4MPH of difference. Any more than that, the weights in the governer need to be changed.

76projectstingray

ok, if you need to turn it clockwise to shift later than I was going the wrong way. I will try it clockwise.