How much to adjust vacuum modulator?

658Chevy · Oct 2, 2006

I did a search and found that to make my TH400 shift later I need to tighten the flathead screw in the vacuum modulator clockwise. Is this a micro-adustment or does it take full turns to get a significant difference? For example, the secondary valve spring in a Quardajet is a micro-adjustment -- every 1/8th to 1/16th of a turn makes a huge difference. Is the vacuum modulator that sensitive, or does it take 1/4, 1/2, full turns before a change is noticed?

I know trial-and-error would give me the answer, but since I don't have a space to work on it (street parking and scissor jack) I'd like to have a baseline to save time before I jack the car up-and-down a dozen times.

Thanks.

72chevy · Oct 3, 2006

From the Summit Adjustable Vacuum Modulator instructions....

"To adjust the shift points of the transmission, disconnect the vacuum hose. With a flat blade screw driver turn the screw on the inside of the modulator end clockwise to raise the shift points and firm up the shifts. To lower the shift points turn the screw counter clockwise, this will also soften the shifts. Only turn the screw two full turns at a time, reconnect the vacuum line to the modulator, and then test drive the vehicle. Re-adjust if necessary."

658Chevy · Oct 3, 2006

Exactly what I needed. Thanks.

Oldani Motorsports · Oct 3, 2006

Modulator adjustments will not have a huge impact on shift points. Mainly they work to control part throttle shifting qualities. Major moves in shift points are most easily controlled with governor weight/spring modifications.

How much to adjust vacuum modulator?

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