Cruise Control Servo: Description and Operation

This electro-mechanical device contains three solenoid valves that control vacuum in the diaphragm chamber, pulling the cable and thereby the throttle. The individual solenoid functions are:

Vacuum valve: When energized, this valve allows vacuum into the diaphragm chamber. When not energized, the valve is spring-loaded to prevent vacuum from entering the diaphragm chamber.

Vent valve: When energized, this valve prevents vacuum from bleeding off, thereby allowing speed control function. When not energized, the valve is spring-loaded to bleed off vacuum.

Safety valve: This valve functions like the vent valve, but its vacuum bleed is greater.

Power for all the solenoids is supplied through the ON/OFF and brake switches with one wire. Both the vacuum and vent solenoids are grounded by the engine controller to operate speed control. The safety valve is always grounded.