Cruise Control: Description and Operation

This system is used on all 1984---87 models except Cadillac. 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 mph.
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 mph. The system is engaged at the desired speed by fully depressing, then releasing the set/coast button. Cruise speed can be increased from the 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 mph 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.

SYSTEM COMPONENTS
Speed Sensor
On models with conventional instrument panels, a speedometer frame mounted optic head is used to pick up light reflected by a speedometer cable mounted blade. The reflected light is produced by a light emitting diode (L.E.D.). As the speedometer cable mounted blade enters the L.E.D. light beam, the light is reflected into the optic head. From there, the light enters a photocell in the optic head and produces a low power speed signal. This signal is sent to a buffer for amplification and conditioning, then to the controller.
On models with electronic speedometers or electronic instrumentation, a transmission mounted speed sensor is used. This sensor utilizes a permanent magnet (PM) to generate vehicle speed information to the controller. Some models use a buffer amplifier to modify the sensor output.
Electronic Controller
The electronic controller is built in the instrument display of the instrument panel and contains a digital integrated circuit and supporting analog circuits. The controller receives signals from servo unit, mode control switches and speed sensor, then signals the opening or closing of the vent and vacuum solenoid vacuum valves in the servo unit in response to these inputs.
Vacuum Release Valve
The vacuum release valve provides an additional vent to atmosphere for the servo unit when the brake pedal is held in the depressed position. The venting is spring actuated and occurs within the free travel of the brake pedal arm.
Combination Vacuum Release Valve/Converter Clutch Switch
This combination valve and switch is used on vehicles equipped with a lock-up torque converter. The vacuum release valve portion operates identically to the release valve previously described. At the same time, the converter clutch switch contacts open and the locking clutch mechanism in the transmission is disengaged.
Combination Cruise/Stop Light Switch
A separately mounted vacuum release valve is used with this combination switch. When the brake pedal is depressed, the switch resets the cruise function to a non-cruise condition and illuminates the brake lights. Two sets of electrical contacts are used in the switch: one to operate the stop lights, and the other to operate the cruise release function.
Clutch Switch
The clutch switch is used on vehicles equipped with manual transmission. When the clutch pedal is depressed, the cruise function is disconnected and will remain so after the pedal is released.

Fig. 1 Vacuum accumulator & pneumatic system (if equipped):

Vacuum Accumulator (if equipped)
The vacuum accumulator, Fig. 1, is attached to the intake manifold through a check valve. The accumulator stores the high vacuum available during periods of low engine loading. The system operates on accumulator vacuum, which is normally higher than manifold vacuum. Manifold vacuum exceeds accumulator vacuum only when being charged by a high engine vacuum condition.

Fig. 2 Servo unit:

Servo Unit
The servo unit, Fig. 2, operates the throttle in response to signals from the electronic controller.
During a steady speed cruise condition, both vacuum and vent valves are closed or sealed. The servo holds a constant vacuum on the diaphragm and places no flow requirements on the vacuum source.
During vehicle deceleration, the vacuum solenoid is energized by the controller to open vacuum valve to vacuum source. Throttle angle is increased by increased vacuum level in the servo, and the vent remains closed.
During vehicle acceleration, the vent solenoid is de-energized by the controller to open the vent valve to atmosphere. This reduces vacuum in the servo and allows throttle return spring to decrease throttle angle while the vacuum valve remains closed.