Cruise Control: Description and Operation

Cruise Control System Schematic:

CIRCUIT OPERATION

The cruise control system operates a mechanical linkage to the throttle by means of a vacuum motor. This is a diaphragm moved by a vacuum applied to one side. A solenoid operated valve connects the vacuum motor to a vacuum tank. Another solenoid valve vents the vacuum to reduce suction. The Electronic Control Module controls the vacuum motor and the throttle by pulsing these solenoid valves on and off.

One input to the module is vehicle speed. This input comes from the vehicle speed sensor. If the actual speed signal is different from the speed that was set into and remembered by the module, the module generates pulses to change the vacuum and return the vehicle to the set speed. The vehicle speed sensor is mounted on the transaxle. Other inputs to the module are from the cruise on/off switch, the set/coast switch, the resume/accelerate switch and the transaxle position switch. A separate vacuum shut down of the cruise control comes from the vacuum release valve on the brake pedal.

The two outputs of the ECM operate the coils of the vacuum valve and the vent valve. Both valves are located in the cruise control servo. These valves move the throttle by means of the vacuum motor. The throttle position sensor in the throttle body senses the position and motion of the vacuum motor. It feeds this information back to the cruise control module, to provide smooth acceleration while the vehicle is in cruise control.

A number of additional features are included in the system. The ECM may detect a fault with the throttle position sensor or the vehicle speed sensor and set an ECM trouble code. In these cases the ECM may inhibit operation of the cruise control.