12.5 Cruise Control

The components of the cruise control system are shown in Fig. 12-4. These are an electronic control unit, a speed sensor at the speedometer, a vacuum servo at the throttle valve, a vacuum pump, and vacuum vent valves at the brake and clutch pedals. The cruise control operates at speeds above 22 mph (35 km/h).

The control unit, using the signal from the speedometer speed sensor, compares the car's true speed to the cruise control speed selected by the driver. The control unit then either turns on the vacuum pump which causes the vacuum servo to open the throttle, or opens a vent at the pump which bleeds air into the vacuum lines to release the throttle and slow the car. If the driver steps on either the brake pedal or clutch pedal, a combination vacuum vent valve and electric switch vents the vacuum system and switches off the cruise control.

Table k lists for troubleshooting the electrical functions of the cruise control system at the control unit harness connector. On cars with CIS-E fuel injection, the control unit is located behind the center console in the passenger compartment. On all other cars, the control unit is located on the passenger side under the instrument panel. For more information on fuel systems, see FUEL SYSTEM-GASOLINE.

Other system faults may be caused by leaks in the system's vacuum lines or other components. Check vacuum lines and connections carefully. To leak-test the vacuum servo, remove the vacuum line, squeeze the servo until it is fully depressed, and then plug the vacuum line connection. If the servo does not hold its depressed position, it is faulty and should be replaced.

Fig. 12-4.

Components of cruise control system.

Performing the series of tests in Table k in the order listed will give a complete check of system operation. If there are no vacuum leaks, the results of these tests are as specified, and the system still does not function properly, then the control unit is faulty and should be replaced.

Table k. Cruise Control Troubleshooting

- All test at control unit harness connector unless otherwise specified