The only Ford transmission that uses a vacuum modulator is the A4LD. It is used in the 1985 through 1991 F-150. The transmission has three speeds and is considered light duty. The vacuum modulator takes the place of the TV cable. It controls up or down shift when power is being applied. The TV stands for throttle valve. The modulator does the job of the TV cable by increasing the pressure on the second and third upshift spring, preventing bypass until a higher pressure is obtained allowing for a higher shift point.
Raise the passenger side of the truck by placing the floor jack behind the tire and on the frame. Place jack stands under the truck, one just to the rear of the floor jack and the other just forward of the leaf spring. Remove the vacuum hose that is plugged into the back of the vacuum modulator, which is located on the passenger side of the transmission, next to the tailshaft.
Remove the hold-down clamp securing the modulator to the transmission by removing the bolt in the bracket using a wrench. Pry the vacuum modulator out of the transmission housing. Make sure that the rod comes out with the modulator or pull the rod out if it remains. It is long enough to get a hold on it.
Inspect the new vacuum modulator and make sure it has the same color circle around its outside circumference. The color denotes the vacuum pressure necessary to actuate the module. Look at the two O-rings on the shaft that get inserted into the transmission and make sure they are not out of place or torn. Install the rod end into the vacuum modulator and insert the modulator into the transmission. Push it hard enough for the O-rings to enter the transmission for a good seal.
Install the hold down bracket and bolt and tighten with a wrench. Install the vacuum hose on the modulator. Lower the truck.
