Welcome to our knowledge base of totally random stuff relating to GM 1983- forward 'square body' truck cruise control.

We've recorded these notes and observations not only for ourselves (because we forget stuff over time), but also in case anyone else can find it useful. If you find something incorrect or wish to contribute something, just let us know here.
 Cruise control Functionality
 Cruise control Component operation
 Cruise control Troubleshoooting
 Cruise control Disengagement switch



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Functionality


Note that this article is only for the 1983 forward Custom Cruise III systems on the 'square body' style trucks. 1988 and forward newer body style trucks used a different system.

The system consists of a mode control assembly, electronic controller (module), Vehicle Speed Sensor (VSS) buffer amplifier, servo unit, and release switches and valves. The servo unit maintains vehicle speed (throttle position) by trapping vacuum in its diaphragm chamber at servo positions determined by the control module. The module monitors mode control switch position, signals from the VSS buffer amplifier, servo position and release switch operation, then operates vacuum valves within the servo unit to control servo operation and vehicle speed. The module also contains a speed limiting function which prevents system operation at speeds below approximately 25 m.p.h..

The mode control assembly consists of a 3 position slide-type switch and a set/coast switch button. To operate the system, the slide switch must be in on position and vehicle speed must be above 25 m.p.h.. The system is engaged at the desired speed by fully depressing, then releasing the set/coast button. Cruise speed can be increased from set position by accelerating vehicle to desired speed, then pressing and releasing button. In order to decrease speed, the set/coast button is held in the fully depressed position (disengaging system), then released when the desired speed is reached. The system can be disengaged at any time by depressing the brake or clutch pedal, or by moving the slide switch to off position.

If the system is disengaged by depressing the brake or clutch pedal, the last set speed will be retained in the module memory until the slide switch or ignition switch is moved to off position. Momentarily moving the slide switch to the resume/accel. position will cause the vehicle to accelerate to the last set speed and maintain that speed. If the slide switch is held in the resume/accel. position, the vehicle will continue to accelerate until the switch is released. When the switch is released, the speed that the vehicle accelerated to becomes the new set speed.

The slide switch also allows a 'tap-up' function to increase cruise set speed in 1 m.p.h. increments. With the cruise control engaged and operating, 'tapping-up' is done by pressing the slide switch to the resume position, then quickly releasing it. This procedure can be repeated 10 times before the system must be reset to a new speed in the conventional manner.

GM cruise control switch

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Component operation


Servo unit operation:
The servo consists of a vacuum operated diaphragm, a normally open solenoid valve to vent the diaphragm, a normally closed solenoid valve connecting the diaphragm chamber to the vacuum source, and a variable inductance position sensor that provides the module with servo position data. The servo operates the throttle in response to control module commands as follows: when the system is engaged and operating at a steady rate of speed, both solenoid valves are closed. Vacuum is trapped in the diaphragm chamber exerting a constant force on the diaphragm and the throttle position remains fixed. When the vehicle is losing speed due to increased load, or when the operator seeks to increase speed through the control switch assembly, the module energizes the vacuum valve solenoid. This increases the force exerted on the diaphragm, which in turn increases the throttle opening. When vehicle speed increases above the pre-set cruise speed, or when the operator seeks to decrease speed through the control switch assembly, the module de-energizes the diaphragm vent valve solenoid. This opens the valve, decreases force on the diaphragm, and allows the throttle return spring to decrease the throttle opening. During normal operation, the module will pulse the operation of the vent or vacuum valves, as needed, to maintain the set cruise speed. The average duration of each pulse is 10 milliseconds. If vehicle speed drops 5 m.p.h. below set cruise speed, the module will hold the vacuum valve in the completely open (energized) position. If vehicle speed exceeds set cruise speed by 3 m.p.h. or more, then module will hold the vent valve in the open (de-energized) position. The module will hold the valves open until the vehicle speed matches the set cruise speed. The diaphragm vent valve will also remain in the fully open position if the brake or clutch pedal is depressed, the ignition is switched off, or if an open circuit exists in the system feed circuit or in the servo position sensor.
GM cruise control servo

Vacuum system operation:
The vacuum accumulator helps even out flucuations in engine vacuum that would cause the system to run erratically. The vacuum check valve makes sure that vacuum is maintained in the system and that air cannot flow backwards from the engine into the system at times when a vacuum might not be present. The brake switch vacuum release valve will open the system to release vacuum when the brake pedal is depressed, deactivating the vacuum holding the servo diaphragm. This ensures the system is not active when the brake pedal is depressed, regardless of any electrical input.
GM cruise control vacuum system

Electrical system operation:
The electrical side of the system allows the user to control inputs on the steering column stalk, as well as regulating the system based off of vehicle speed. The speed signal comes from a unit wired to the back of the speedometer. The speedometer cable drive input is converted to an electrical signal in the Vehicle Speed Sensor, or VSS. This is sent to the Electronic Controller Module, which interprets all the electrical inputs to control the servo to regulate the throttle position during cruise control function.
GM cruise control wiring diagram

Component locations:
GM cruise control part locations

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Troubleshooting


General troubleshooting:
cruise control troubleshooting chart

Operation troubleshooting:
cruise control operation troubleshooting chart

Disengagement troubleshooting:
cruise control disengagement troubleshooting chart

Electronic Control Module troubleshooting:
cruise control Electronic Control Module troubleshooting

Servo troubleshooting:
GM cruise control servo troubleshooting

Switch troubleshooting:
GM cruise control switch troubleshooting

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Disengagement switch


The brake disengagement switch is attached to the brake pedal bracket. It provides both an electrical and vacuum signal for cruise control function. The electrical contacts of the switch are closed when the brake pedal is not depressed, and the vacuum vent is closed. When the pedal is depressed, the electrical switch contacts open and the vacuum line is vented to atmosphere.

It is a very simple mechanism. The vacuum portion merely has a plunger with an o-ring seal attached, which moves into the closed position along with the plunger depression. The electrical side is attached to the same plunger and engages the continuity between contacts when the plunger is depressed. An internal spring keeps the plunger loaded against the brake pedal to follow its travel. The at-rest position of the plunger is fully depressed, which corresponds to when the brake pedal is not depressed. I.e., when the brake pedal is not pressed down, the plunger is fully depressed within the switch, the electrical contacts are engaged, and the vacuum vent is closed.

The switch is both easily checked, and sometimes easily repaired. Both functions can be easily checked with a vacuum gauge and and meter. If the vacuum side is defective and no mechanical damage is not evident, the o-ring is easily replaceable. The electrical contacts can also wear over time, which provides a loss of continuity, or a temporary loss at times which can disengage the cruise contral erroneously. This my be able to be fixed by slightly and gently bending the contacts closer to center, or widening the connection bridge on the plunger slightly to account for wear.

Cruise control brake switch disassembled

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