The C4 automatic transmission was introduced by Ford in 1964. It is equipped with a vacuum modulator that senses change in the engine's vacuum due to throttle position and load changes. As these changes occur, the modulator adjusts the shift timing of the transmission according to the demand of the engine. You can adjust the modulator with the proper tools, equipment and basic mechanical knowledge.
Park the vehicle on a firm, flat surface. Set the shifter in the "Park" position and remove the ignition keys. Place wheel chocks under the rear tires.
Raise the vehicle on the passenger side using a jack. Position a jack stand under a solid section of the frame. Lower the car until its full weight rests on the stand.
Locate the C4 transmission modulator at the rear of the transmission. It can be identified by a small rubber vacuum hose that connects to it. The other end of the hose is attached to a steel tube that rises up to the engine.
Remove the rubber hose from the modulator nipple by pulling it straight back by hand.
Insert a narrow shank flat head screwdriver into the nipple until it seats into the slot of the adjustment screw.
Turn the modulator adjustment screw in a clockwise direction with the screwdriver to firm up and delay the upshift of the transmission. Do not adjust the screw more than four turns in either direction.
Turn the adjustment screw in a counterclockwise direction to speed up and soften the upshift.
Adjust the modulator screw one turn at a time. Replace the vacuum hose onto the modulator nipple until it is fully seated.
Raise the vehicle and remove the jack stand. Lower the vehicle to the ground and remove the jack and the wheel chocks.
Test drive the vehicle. Make any further adjustments by repeating Step 4 through 8.
Ford C4 transmissions had two modulators over the years. Early models were a screw-in type and later models were a push-in style that were secured with a bracket. Both are adjusted in the same manner. Wear safety glasses.