Cruise Control: Description and 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 the suction. The Powertrain Control Module (PCM) controls the vacuum motor and the throttle by pulsing these solenoid valves on and off.

One input to the PCM is the 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 PCM, the PCM generates pulses to change the vacuum and return the vehicle to the set speed. Other inputs to the module are from the Cruise Control Switches in the Multi-Function Lever. Disengage inputs to the PCM come from two switches located on the brake pedal, the VCC/ Antilock Brake Switch and the Cruise/Shift Interlock/Brake Switch. A separate vacuum Cruise Control shut-down comes from the Cruise Control Vacuum Release Valve on the brake pedal.

Two PCM outputs operate the Cruise Control Servo's vacuum valve and vent valve coils. These valves move the throttle using the vacuum motor.

The Throttle Position Sensor in the throttle body senses the position and motion of the vacuum motor. It feeds back information to the PCM to provide smooth throttle changes while the vehicle is in cruise control.

If the PCM diagnostics detect a fault with the Vehicle Speed Sensor (Code E24) the Cruise Control System is inhibited for the entire ignition cycle. The Cruise Control System is also inhibited if:

1) Cruise control is requested above 90 mph,

2) The transaxle is shifted to Park/Neutral (Code E60),

3) There is a very rapid increase in RPMs (Code E66),

4) The difference between the vehicle's speed and the set speed is too high (Code E67),

5) A cruise switch is stuck closed or a PCM signal wire is shorted to voltage when the system is enabled (Code E67),

6) The PCM detects battery voltage above 16 volts or below 10 volts with engine running above 500 RPM (Code E16),

7) The acceleration is too high (Code E64),

8) A Cruise Control Servo Position Sensor is shorted to ground (Code E65),

9) The Cruise Control is on, the brake is not depressed and the PCM output is HI when it should be LO, or vice versa (Code E62), [VENT SOLENOID]

10) A throttle angle greater than 20 degrees and the servo position sensor indicates a stroke greater than commanded (Code E68), [VACUUM SOLENOID]

11) The servo position sensor indicates that the servo is still engaged after Cruise Control has been disengaged (Code E99),

12) The vehicle speed cycles from 30 mph or greater to 0 mph with no VCC/Antilock Brake Switch input, and the PCM records 10 of these events consecutively (Code E90),

13) The Park/ Neutral Switch is closed when the vehicle is in 4th gear with vehicle speed between 23 and 35 mph for 10 seconds (Code E91).