Cruise Control: Description and Operation

Fig. 1 Speed Control Components:

The speed control system consists of a speed transmitter, selector switch, electronic control unit, vacuum pump with valve, vacuum controller (servo) and pedal switches, Fig. 1.
The system is turned on by the selector switch. When the selector is switched to the On position power is supplied to the vacuum pump and valve. The pump is grounded across terminal 4 and the valve across terminal 7, refer to SCHEMATIC DIAGRAMS/ELECTRICAL AND ELECTRONIC DIAGRAMS in the control unit. Power is supplied from the selector to terminal 9 of the control unit.
When the Set button is depressed, power is supplied to terminal 2 of the control unit and vehicle speed is sensed by the speed transmitter. The value is supplied to the control unit and is stored in the unit's memory. Vehicle speed must be above 23 mph for system to engage.
The vacuum pump generates vacuum in the vacuum controller, corresponding to selected vehicle speed. The vacuum controller is connected by a chain to the throttle linkage. Vehicle speed is continuously sensed and compared with the stored value. If vehicle speed deviates, the vacuum pump/valve increases or decreases the vacuum in the controller and the throttle linkage is adjusted accordingly.
When terminal 4 in the control unit is grounded, the vacuum pump will run and vacuum in the controller will increase. When terminal 7 is grounded, the vacuum valve will close causing vacuum and vehicle speed to remain constant. When terminals 4 and 7 are open (not grounded) the vacuum pump is off and the valve open, causing vacuum in the controller to drop. The throttle setting is reduced.
If either the brake or clutch pedal is depressed, the ground circuit is activated by the corresponding pedal switch. At the same time, a valve in the pedal switches opens, so that vacuum is lost. The pedal switches thereby take both the electrical and vacuum systems out of operation. The memory of the control unit is not erased, so that the selected speed may be resumed when the pedal is released. This is carried out by the selector being pressed to the Resume position. Power is then supplied to terminal 6 of the control unit. The memory of the control unit will be erased if vehicle speed drops below 23 mph, if the selector is set to the Off position, or if the ignition is switched off.