Cruise Control: Description and Operation

Description
Cruise control is switched on by pressing the cruise control master switch mounted on the drivers side of the fascia. The 'Set +' and 'Res' switches mounted on the steering wheel allow the driver to either set the vehicle speed as required or resume the last cruising speed set.

The cruise control system uses mechanical, electrical and vacuum operated devices to maintain vehicle speed at a setting selected by the driver. As vehicle speed changes due to changes in road gradients, so the cruise control ECU operates the cruise control pump, continually moving the cruise control actuator to maintain the set vehicle speed.

Should the driver wish to increase the speed of the vehicle, the 'Set +' switch should be pressed and held. The vehicle will accelerate until the switch is released. As the 'Set +' switch is released, the cruise control ECU will store the new cruise speed in its memory.

With cruise control switched on, it is possible to press the accelerator pedal and accelerate through the memorised set speed. When the accelerator is released, the cruise control ECU will automatically return the vehicle to its set speed.

Note: The cruise control system will only operate between speeds of 22 mph (35 km/h) and 125 mph (200 km/h).

Cruise control operation can be suspended by one of the following ways:
- By pressing the 'Res' switch.
- By pressing the brake pedal.
- The gearbox is placed in park, neutral, or reverse.
- Hill Descent Control (HDC) is active.
- Engine rev's are too high.

Under all these conditions, the cruise control set speed will be memorised. Cruise control operation can be resumed by pressing the 'Res' switch.

Cruise control operation can be cancelled in one of two ways:
- By pressing the cruise control master switch located on the drivers side of the fascia.
- By switching the ignition off.

In both these cases, the cruise control set speed will be lost.

Warning: Do not switch off the vehicle ignition unless the vehicle is stationary and the handbrake is applied.

Operation
Feed from the positive battery terminal (C0192) is supplied to the horn relay and fusible link 3 of the engine compartment fuse box (C0632) on an R wire. Fusible link 3 (C0571) is connected to the ignition switch (C0028) by an N wire. When the ignition switch is turned to the 'ignition' position, current flows across the switch (C0028) to fuse 6 of the passenger compartment fuse box (C0588) on a G wire. Fuse 6 (C0581) provides an ignition feed to the following:
- The cruise control master switch (C0749) on a W then WK wire.
- The cruise control interface unit (C0895) on a W wire (via header 292).
- The brake pedal sensor (C0652) on a W wire (via header 292).

The ignition switch (C0028) also provides a feed to fuse 2 of the passenger compartment fuse box (C0588) on an NW wire. Fuse 2 (C0581) provides an ignition feed to the brake pedal switch (C0075) on a G wire.

Cruise Control Master Switch
The cruise control master switch (C0749) is mounted in the drivers side of the fascia and is provided an ignition feed from fuse 35 of the passenger compartment fuse box (C0580) on a W then WK wire. When the switch is pressed, current flows across the switch contacts (C0749) to the cruise control interface unit (C1959) and the cruise control ECU (C0239) on WY wires.

NOTE: When the cruise control system is switched off using the master switch, any stored road speed values will be lost.

Cruise Control ECU
When cruise control is activated, the cruise control ECU (C0239) receives a feed from the cruise control master switch (C0749) on a WY wire. The ECU (C0239) is earthed on a B wire. To enable it to control the cruise control system, the ECU receives a number of inputs from various sources as follows:

ABS ECU
The ABS ECU (C0501) provides a pulsed wheel speed signal to the cruise control ECU (C0239) on a WO wire. The rate of pulses provided by the ABS ECU increases by 2.22 Hz for every mile per hour speed increase (approximately 8,000 pulses per mile travelled). This enables the cruise control ECU to calculate the road speed of the vehicle. If vehicle speed is outside the operating limits, the cruise control ECU will not activate the system.

NOTE: If the Cruise Control ECU fails to receive a pulsed wheel speed signal from the ABS modulator, cruise control will not operate.

Set + Switch
The non latching Set + switch (C1254) is provided a feed via the horn relay (C0576) on a PY wire. When the switch is pulled back against spring pressure, the switch contacts close and a feed is provided to the cruise control ECU (C0239) via the rotary coupler (C0082) on a RW wire.

If cruise control is active, and the switch is operated for the first time, the cruise control ECU will store the current vehicle speed in its memory and maintain that speed.

NOTE: The cruise control ECU will only store and maintain the vehicle speed if it is within the operating limits given in the 'Description' section above.

If the switch is tapped a second time, the ECU will increase the vehicle speed by 1 mph. If the switch is pulled back and held against spring pressure, the vehicle will accelerate until the switch is released. The final vehicle speed in both instances will be stored in the cruise control ECU memory.

Res Switch
The non latching Res switch (C1254) is provided a feed via the horn relay (C0576) on a PY wire. When the switch is pulled back against spring pressure, the switch contacts close and a feed is provided to the cruise control ECU (C0239) via the rotary coupler (C0082) on a UW wire.

If the Res switch is pulled back against spring pressure, the cruise control ECU will alter vehicle speed to reflect the vehicle speed held within its memory. If the Res switch is pulled back for a second time, cruise control operation will be suspended

NOTE: If a road speed value is not held within the cruise control ECU memory, the ECU will not respond to the Res switch input. To enter a vehicle speed into the ECU memory, refer to the 'Set + Switch' section above.

Brake Pedal Sensor
The Hall effect brake pedal sensor (C0652) is provided an ignition feed (via header 292) from fuse 6 of the passenger compartment fuse box (C0581) on a W wire. The sensor (C0652) is earthed on a B wire. When the brake pedal is in its normal position, the sensor provides the cruise control interface unit (C0895) a low voltage (between approximately 0 V and 2 V) on a PG wire. When the brake pedal is pressed, this voltage rises to between approximately 6 V and battery voltage. When the interface unit registers this rise in voltage, it suspends cruise control operation.

The brake pedal sensor (C0652) also provides a brake pedal status signal to the cruise control interface unit (C0895) on a GR wire.

Interface Unit
The interface unit (C0895) provides a feed to the cruise control ECU (C0239) on a PG wire if the following conditions exist:
- The cruise control master switch is depressed.
- The brake pedal is not depressed.
- HDC is not activated.
- The gear selector lever is not in the park, neutral, or reverse positions.
- Engine speed is below 6,496 rpm.

The cruise control ECU uses the feed provided by the interface unit to enable the cruise control pump.

Interface Unit
Fuse 6 of the passenger compartment fuse box (C0581) provides an ignition feed (via header 292) to the interface unit (C0895) on a W wire. The interface unit is earthed on a pair of B wires, and receives inputs from a number of different components as follows:

Cruise Control Master Switch
When the cruise control master switch is depressed, it provides a feed to the interface unit (C0895) on a WY wire.

Brake Pedal Sensor
When the brake pedal is in its normal position, the sensor (C0652) provides the interface unit (C0895) a low voltage (between approximately 0 V and 2 V) on GR and PG wires. When the brake pedal is pressed, this voltage rises to between approximately 6 V and battery voltage.

Engine Control Module (ECM)
The ECM (C0371) provides a feed to the interface unit (C0895) on a BU wire if the following conditions exist:
- HDC is not activated.
- The gear selector lever is not in the park, neutral, or reverse positions.
- Engine speed is below 6,496 rpm.

Cruise Control Pump
When all conditions for cruise control activation have been met, the cruise control ECU (C0239) provides a feed to the cruise control pump motor (C0228) on a WU wire. The pump motor creates a vacuum in the pump actuator.

Current flow across the pump motor is controlled by the ECU (C0239) providing a varying Pulse Width Modulated (PWM) signal to the other side of the pump motor (C0228) on a BR wire. When the correct vehicle speed has been reached, the ECU (C0239) replaces the PWM feed with a continuous feed. As the potential difference across the pump motor is now 0 V, the pump motor stops operating.

To maintain the vacuum in the actuator, both the control and dump valves are closed by the ECU. To close the valves, the ECU (C0239) provides a feed to the control and dump valves (C0228) on a WU wire. Current flows across the control valve (C0228) to the ECU (C0239) on a PY wire. Current flows across the dump valve (C0228) to the ECU (C0239) on a BR wire.

NOTE: Both the control valve and the dump valve are normally open valves.

When a decrease in speed is requested, the control valve is opened to reduce the depression within the actuator. This is done by the ECU (C0239) providing a PWM signal to the control valve (C0228) on a PY wire. When the correct speed has been achieved, the PWM signal is withdrawn.

When cruise is cancelled, the ECU (C0239) withdraws the feed to the dump valve (C0228) on a WU wire. The dump valve now opens, allowing atmospheric pressure into the actuator.

Brake Pedal Switch
As a safety feature, the dump valve is opened if the brake pedal is depressed. If cruise control is active and the brake pedal is pressed, the brake pedal switch (C0075) provides a feed to the dump valve (C0228) on a GP wire. As the potential difference across the valve is 0 V, the valve opens allowing atmospheric pressure into the actuator.

Electronic Automatic Transmission (EAT) ECU
When all conditions for cruise control activation have been met, the cruise control ECU (C0239) provides a feed to the EAT ECU (C0932) on a WU wire. When the EAT ECU receives this feed, it enters its cruise control mode.

Instrument Pack
When all conditions for cruise control activation have been met, the cruise control ECU (C0239) provides a feed to the cruise control LED mounted within the instrument pack (C0230) on a WU wire. The cruise control LED is provided an earth by the instrument pack (C0230) on a B wire.

Diagnostic Socket
The diagnostic socket (C0040) is connected to the cruise control ECU (C0239) by a K wire, enabling the ECU to be interrogated by TestBook or T4.