Cruise Control: Description and Operation

Vehicle Cruise Control





The ECM operates the vehicle cruise control system, in all forward gears, in response to driver inputs through the master, SET+, SET-, CANCEL and RES switches.

When the master switch is pressed, two 12 V ignition supplies are connected to the ECM, one direct and one via the brake switch. The LED in the switch comes on to indicate the system is armed.





With the system armed, a momentary press of either the SET+ or the SET- switch engages cruise control, provided the vehicle speed is 28 km/h (17.5 mph) or above. The ECM memorizes the current road speed as the set speed and signals the vacuum system to operate the mechanical guard. The inputs from the position sensors in the electronic throttle allow the ECM to monitor and adjust the mechanical guard and the throttle valve to maintain the set speed.

The vacuum components are installed in a line connected between the induction elbow and the vacuum actuator on the electronic throttle. In the vacuum system:
- the check valve isolates the system from the induction elbow at wide open throttle when there is little or no vacuum available
- the reservoirs extend the time (to approximately 20 minutes) that, at wide open throttle, sufficient vacuum can be maintained in the system to operate the vacuum actuator
- VSV 1 regulates the vacuum applied to the vacuum actuator to control the position of the mechanical guard
- VSV 2 connects the vacuum actuator to atmosphere to release the mechanical guard when cruise control is disengaged
- VSV 3 is a back-up for VSV 2 (because cruise control disengagement is safety critical)
- filters on the atmospheric ports of VSV 2 and VSV 3 prevent moisture ingress.

Once cruise control is engaged, a further momentary press of the SET+ or SET- switch increments or decrements the set speed by 1.6 km/h (1.0 mph). The ECM then accelerates or decelerates the vehicle to the new set speed.

The ECM stores a maximum of five incremental or decremental commands at any one time. Once the ECM has actioned the first stored command, a further command can be added. On momentary operation of the opposite sense switch, the ECM deletes the last command from memory.

Pressing and holding the SET+ or SET- switch causes the ECM to increase or decrease the set speed, and smoothly accelerate or decelerate the vehicle, until the switch is released. To protect against switch failure in the closed position, if either switch is held for longer than 10 minutes, the ECM disables cruise control until the next ignition cycle.

The throttle pedal can be used to accelerate the vehicle beyond the set speed, without disengaging cruise control (since the diaphragm actuator is holding the mechanical guard, there is a noticeable reduction in accelerator pedal load up to the point at which the input shaft in the electronic throttle begins to turn the mechanical guard). When the pedal is released the ECM returns the vehicle to the set speed in a controlled manner. Alternatively, a momentary press of the SET+ or SET- switch causes the increased vehicle speed to be adopted as the new set speed.

On receipt of an input from the CANCEL switch, the ECM disengages cruise control and clears the set speed from memory. Disengagement is in a controlled manner to provide a smooth return of throttle control to the driver.
The ECM disengages cruise control, clears the set speed from memory and immediately returns control of the throttle to the driver if any of the following occur:
- the master switch is pressed, to switch the system off
- a fault is detected in the electronic throttle system, the brake switch or the cruise control switches
- the parking brake is applied
- the engine overspeeds.

The ECM disengages cruise control, immediately returns control of the throttle to the driver and retains the set speed in memory, if any of the following occur:
- the brake pedal is pressed
- the vehicle decelerates too fast (ie. as under heavy braking, to guard against sticking contacts in the brake switch)
- the gear selector moves to neutral, park or reverse
- after resuming cruise control the vehicle accelerates to only 50% of the set speed (eg. due to a steep hill)
- the stability/traction control system operates
- vehicle speed decreases below 26 km/h (16 mph).

On receipt of an input from the RES switch, if a set speed is stored in the ECM memory, the cruise control function is re-engaged and the vehicle accelerated or decelerated to resume the set speed in a controlled manner.

The ECM continuously monitors the cruise control switches. If a switch fails closed, on the initial failure the input is treated as a driver command and the system responds accordingly. However, after subsequent disengagement then re-engagement of the system, the input from the failed switch is diagnosed as a fault. On detection of a switch fault, operation of cruise control is inhibited until the fault is corrected.

The ECM also conducts internal checking procedures to ensure that only permitted outputs are generated for given input conditions.