How to Adjust a Shift Modulator

by Robert Bayly

The vacuum modulator on your automatic transmission monitors engine vacuum changes caused by throttle position or load on the engine. It works with the modulator valve in the transmission to control shift points. Most factory vacuum modulators are sealed and cannot be adjusted. Some early Ford and GM vacuum modulators are adjustable and adjustable modulators are available from aftermarket suppliers. An adjustable modulator will allow you to make the transmission shift a little bit earlier or later. This adjustment is used by people who want to try to optimize gas mileage or performance. All of these modulators are made in the same manner and the DIY mechanic can make the adjustment in just a few minutes.

Step 1

Park the vehicle on a level, paved surface and set the parking brake.

Step 2

Raise the front of the vehicle with a jack and support with jack stands.

Step 3

Remove the vacuum line from the end of the vacuum modulator. Use a small slotted screwdriver to turn the adjuster that is located inside of the vacuum fitting. Turn the adjuster clockwise for earlier shifts and counterclockwise for later shifts. Make small adjustments of 1/8 to 1/4 turn at a time.

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