Brakes (ETC operation): Description and operation

ETC operation

ETC uses brake intervention to prevent wheel spin. The ETC function operates on all four wheels and is automatically enabled at speeds up to 31.3. mph (50 kph) provided the brakes are off.

While the ETC function is enabled, if the ABS ECU detects a wheel accelerating faster than the average, indicating loss of traction, it operates the ABS modulator in the active braking mode for the affected wheel. The ABS ECU also illuminates the ETC warning lamp, for a minimum of 2 seconds or for the duration that ETC is active. If the brake pedal is pressed while ETC is active, the ABS ECU reverts to ABS mode.

HDC operation

HDC uses brake intervention to provide a controlled descent ability in off road conditions when engine braking is insufficient to maintain a comfortable speed. The HDC function operates with the vehicle travelling forwards or backwards and is selected on/off by a switch on the gear lever. When selected, HDC is enabled at speeds of 3.6 mph (5.7 kph) and above provided the vehicle is in first or reverse gear with the clutch engaged. In off road conditions this allows the driver to leave HDC selected and to control the vehicle’s descent speed, down to the system’s minimum target speed, using only the accelerator pedal.

When HDC is selected, the HDC information warning lamp illuminates permanently if HDC is enabled or flashes if HDC is not enabled (i.e. the vehicle is not in first or reverse gear, or the clutch is disengaged). When HDC is enabled, the ABS ECU calculates a target speed from the throttle position element of the engine ECU input, and compares this with actual speed. If the actual speed is higher than the target speed, the ABS ECU operates the ABS modulator in the active braking mode to achieve and then maintain the target speed. While the braking force is being applied, the ABS ECU also energizes the ABS brake lamp relay to put the brake lamps on. Active braking is discontinued while vehicle speed is below the target speed or if the foot brakes are applied. Applying the foot brakes during active braking may result in a pulse through the brake pedal, which is normal.

Minimum target speeds with the throttle closed are 6 mph (9.6 kph) in first gear and 4 mph (6.5 kph) in reverse gear. The first gear target speed is decreased to 4.4 mph (7 kph) if rough terrain or sharp bends (detected from ABS sensor inputs) are encountered while already travelling at the minimum target speed. Minimum target speeds are increased at cold idle to prevent conflict between the brakes and the engine caused by HDC trying to impose a lower vehicle speed than is normal for the increased engine speeds at cold idle. Minimum target speeds at cold idle are 7.5 mph (12 kph) in first gear and 4.4 mph (7 kph) in reverse gear.

During active braking, the brakes are operated in axle pairs on one or both axles. The braking effort is distributed between the front and rear axles as necessary to maintain vehicle stability. Distribution of the braking effort is dependant on direction of travel and braking effort being applied. To prevent wheel lock, anti-lock braking is also enabled during active braking.

The ABS ECU incorporates a fade out strategy that, if a fault occurs or HDC is deselected during active braking, provides a safe transition from active braking to brakes off. The fade out strategy increases the target speed at a low constant acceleration rate, independent of actual throttle position. If active braking is in operation, this causes the braking effort to be gradually reduced and then discontinued. The HDC information warning lamp flashes while fade out is in progress.