Servo Unit Functions

The cruise control servo consists of a vacuum operated diaphragm, a normally closed solenoid valve to admit vacuum to the servo diaphragm chamber, a normally open solenoid valve to vent the diaphragm chamber to atmosphere, and a variable inductance position sensor that signals the position of the servo linkage - and accordingly, the throttle setting - to the electronic control module.

When the system is engaged and operating, the servo operates the throttle as follows:

Steady Cruise: Both the vacuum valve and the vent valve are closed, trapping vacuum in the diaphragm chamber to hold a steady throttle setting.

Vehicle Losing Speed: The control module opens the vacuum valve to increase the strength of the vacuum in the diaphragm chamber. The vent valve remains closed.

Vehicle Gaining Speed: The control module opens the vent valve to the atmosphere to reduce the vacuum in the diaphragm chamber so the throttle return spring can take over and reduce the throttle opening. The vacuum valve remains closed.

The control module pulses the opening of the vacuum valve or the vent valve for minor speed corrections until the vehicle reaches the set speed. Under normal road load conditions, the vacuum valve will remain completely open, instead of pulsing, when vehicle speed drops 5 mph below the set speed.. When vehicle speed exceeds 3 mph over the set speed, the vent valve will remain completely open, instead of pulsing.

The servo will go into a completely open vent valve position, returning the throttle to idle, when the brake (or clutch) pedal is depressed, when electrical voltage to the system is lost (such as when the cruise control switch or the ignition switch is turned off) or when there is an open in the variable inductance position sensor coil in the servo.