How to Change the Modulator Valve on a Ford Truckby Don Bowman
The modulator valve on a Ford Truck is used in conjunction with the governor in a transmission to control the shift points. The governor controls the pressure in the valve body of the transmission. At a certain pressure, as the vehicle is accelerated, the pressure builds to overcome spring pressure to open passages to shift gears. The modulator valve senses vacuum pressure consistent to load on the engine and applies more pressure to the springs to allow higher shift points. When replacing a modulator valve, there are two points to remember.
How to Change the Modulator Valve on a Ford Truck
Raise and support the front of the truck on jack stands. Remove the vacuum hose that runs from the intake manifold to the vacuum modulator. The vacuum modulator is on the passenger side of the truck on the rear of the transmission just forward of the tail shaft cover. Pull the hose off and lay it aside. When the hose is removed, look for transmission fluid in the hose. Shake the end of the hose and see if any transmission fluid comes out. Modulators are known to break their diaphragm and allow transmission fluid to be literally sucked out of the transmission and into the engine through its hose.
Remove the bolt from the small bracket holding the modulator into the transmission. Remove the modulator valve by twisting and pulling. When it comes out, make sure not to lose the small straight actuating rod as it may fall out.
Install the actuating rod into the orifice of the new modulator and carefully push it into the receptacle in the transmission until it snaps which indicates that the o-ring has sealed. Install the small bracket on the modulator to hold it in and insert the bolt and tighten.
- Modulators come with a color band that varies with the year of the vehicle. Make sure to get the same color modulator. Modulators can be purchased that are adjustable. There is a small screw in the vacuum end of the modulator that, with the aid of a small screwdriver, can be set to operate at different vacuum levels.