Cruise Control Servo: Description and Operation

Fig. 1 Servo Unit:

The servo unit, Fig. 1, operates the throttle in response to signals from the electronic controller.
During a steady speed cruise condition, both vacuum and vent valves are closed or sealed. The servo holds a constant vacuum on the diaphragm and places no flow requirements on the vacuum source.
During vehicle deceleration, the vacuum solenoid is energized by the controller to open vacuum valve to vacuum source. Throttle angle is increased by increased vacuum level in the servo, and the vent remains closed.
During vehicle acceleration, the vent solenoid is de-energized by the controller to open the vent valve to atmosphere. This reduces vacuum in the servo and allows throttle return spring to decrease throttle angle while the vacuum valve remains closed.