GM Bonneville/Eighty Eight/LeSabre 1986-1999

General Description


See Figures 1 and 2

Cruise control is a speed control system that maintains a desired vehicle speed under normal driving conditions. However, steep grades up or down may cause variations in the selected speeds. The electronic cruise control system has the capability to cruise, coast, resume speed, accelerate and "tap up" and "tap down."

To keep the vehicle under control and to prevent possible personal injury and vehicle damage, the cruise control should not be used on slippery or winding roads or in traffic of heavy or varying volume. When traveling down a steeply graded hill, the cruise control should be disengaged by depressing the brake pedal lightly. The transmission can then be shifted into a lower gear range to help control vehicle speed.

The main parts of the cruise control system are the functional control switches, cruise control module, speed sensor and brake/clutch release switches.

There are two basic systems used on the vehicles covered in this repair guide, vacuum controlled and electronically controlled. Also, some models have a cruise control module and some use the engine control computer. With the introduction of Powertrain Control Modules (PCM) which computerize the control of a number of different systems, there are several different ways of handling cruise control functions. These systems may work the same way. The only difference being if the selected cruise is maintained by vacuum or electricity or a cruise control module or the engine control module.

The main parts of the cruise control system are the mode control switches, cruise control module or PCM, depending on the vehicle, the servo, vehicle speed sensor, vacuum supply, and electrical and vacuum release switches.

The vacuum operated cruise control system uses vacuum to operate a servo. The servo has a diaphragm which is connected directly to the throttle cable. The throttle then moves with changes in vacuum. The servo maintains a desired vehicle speed by trapping vacuum in the servo at the proper servo position. The cruise control module (or PCM, depending on vehicle) monitors vehicle speed and controls the servo vacuum and vent valves to maintain desired speed. The control module/PCM contains a low speed limiter which will prevent system engagement below a minimum speed, about 25 mph (40km/h). The operation of the cruise system is controlled by switches, usually located in a multi-function turn signal lever. The system may be disengaged by turning off the switch or by pressing the brake or clutch pedal slightly. An electrical switch, mounted on the brake and clutch pedal bracket, disengages the system electrically when the brake pedal is depressed. A vacuum release valve, mounted on the brake pedal bracket, vents the trapped vacuum in the servo to the atmosphere when the brake pedal is depressed, allowing the servo to quickly return the throttle to idle position.

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Fig. Fig. 1: Cruise control system components - vacuum operated system shown, electronic stepper shares some components

The electronic stepper cruise control system uses a cruise control module to reach the desired vehicle cruise operation. There are 2 components in the module which help do this. This first is an electronic controller and the other is an electric stepper motor. The controller monitors vehicle speed and operates the electric stepper motor. The motor moves a band and throttle linkage, in response to the controller, to maintain the desired cruising speed. The cruise control module has a low speed limit which prevents system engagement at speeds below 25 mph (40km/h). Operation of the controller is controlled by functional switches located on the multi-function turn signal lever. Cruise control release switch and stop lamp switch assemblies are provided to disengage the cruise system. The switches are mounted on the brake pedal bracket. When the brake pedal is depressed, the cruise control system is electrically disengaged and the throttle returned to the idle position.

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Fig. Fig. 2: Many vehicles have the cruise control switches on the multi-function lever