Cruise Control: Description and Operation
Design and function
The system is in two main parts:
The switch is integrated with the direction indicator switch. It controls the system by sending signals to the control unit.
The electronic control unit is mounted above the pedals. It receives signals from the switch in the direction indicator stalk, from the speedometer and from the vacuum valves. It controls the vacuum pump by means of a signal proportional to the speed.
Vacuum pump, with built-in vacuum valve, mounted in the engine compartment on the bulkhead. The vacuum pump controls the vacuum servo pressure which actuates the throttle control through a system of linkages.
Vacuum valves, which are actuated by the brake and clutch pedals and contains electrical switches. When the brake or clutch is used, the vacuum valves switch off the control unit electrically and discharge the vacuum system.
Switch (integrated in direction indicator stalk)
The SET SPEED button is pressed to record and maintain current speed. The vehicle can be accelerated by holding down the SET SPEED button.When the button is released, current speed is registered and maintained.
On the side of the direction indicator stalk is a switch with three positions: OFF, ON and RESUME. In the ON position the cruise control is switched on, in the OFF position it is switched off. The RESUME position is used to return to the previous set speed, for example, after braking.
Control unit
This contains the "brain" of the system, which is a microcomputer. It registers the speed when the SET SPEED button is pressed. It responds by controlling the vacuum motor which regulates the pressure in the vacuum servo and thus the speed of the vehicle.
The set speed is stored in the memory of the control unit either until the system is switched off (by moving the switch on the direction indicator stalk to OFF or turning the ignition OFF) or until the SET SPEED button is pressed again to change the speed stored in the memory.
Normally the control unit maintains the speed within plus/minus two km/h (1.25 mph). This accuracy is achieved by actuating the vacuum pump or vacuum valve at intervals as short as 40 milliseconds.
The control unit has several safety functions to prevent malfunction in the event of a fault developing in the system.
The speed must be above 35 km/h (22 mph), the control must be set to ON and the SET SPEED button must be pressed for the control unit to work and for its relay to feed current to the vacuum pump and to connect it to ground.
The control unit then monitors current supply and controls the vacuum pump and vacuum valve. The control unit does not activate the relay in case vacuum pump and vacuum valve are short-circuited to ground.
As a further safety measure, the system is disengaged during rapid braking, even if both brake valve and brake lights are inoperative.
Vacuum pump
This is powered by an electric motor. It starts when the SET SPEED button is pressed, causing the control unit relay to cut in. This feeds current to the motor and connects it to ground. The motor drives the vacuum pump until the vacuum servo has reached sufficient vacuum to maintain the desired speed as indicated by the connection to the speedometer. The control unit then breaks the vacuum pump ground connection.
If an increased throttle setting is required on an upgrade, the vacuum pump will start working again and increase the negative pressure until the required speed is reached. On downgrades, the vacuum valve opens and admits ambient air, reducing the negative pressure in the vacuum valve and the throttle opening.
On small speed changes the vacuum pump operates only for fractions of a second.
Vacuum valves (pedal valves)
These are identical. A spring loaded piston actuates a microswitch and a vacuum valve.
In normal position, with the pedal up, the spring is compressed,the switch contacts are closed and the vacuum valve is closed. Terminal 3 of the control unit is grounded via the brake lights and the control unit controls the negative pressure in the system.
When the pedal is depressed, for braking or declutching, the spring forces the piston back. This opens the switch contacts, so that the ground circuit of the control unit terminal 3 opens and deenergises the relay. At the same time the vacuum valve opens, admitting ambient air and disengaging the cruise control.
The vacuum valves are adjustable by means of threads on the piston sleeve, so that the vacuum valve can be unscrewed or screwed in to obtain the desired disengagement position.
WARNING! This adjustment must NOT be used as a pedal stop. If the vacuum valves are adjusted incorrectly, the cruise control may not work properly because the pedals have to be pressed too far before the cruise control is disengaged.
The brake light switch acts as an extra safety feature. If the brake pedal is pressed but the vacuum valve does not open, the current to the brake lights will break the control unit ground circuit and stop the vacuum pump.
The clutch pedal vacuum valve works in the same way as that of the brake pedal, this prevents overrevving of the engine.
Special conditions
The cruise control cannot maintain the set speed underall conditions. At a given upward gradient the power of the engine is simply insufficient even though the vacuum servo has opened the throttle fully.
On a steep downgrade the set speed may be exceeded even though the throttle is closed, because engine braking alone can be insufficient.
NOTE: Since the control unit is grounded through the brake circuit, the brake light bulbs must be functional for the cruise control system to operate. If the cruise control system develops a fault, the first thing to check is the brake lights.