Brakes (Traction Control): Description and operation

Traction Control

This feature uses brake intervention to prevent wheel slippage (i.e. wheel speed faster than vehicle reference speed) during attempts to accelerate or on a slippery road surface. This is done by the ECU, which monitors the speed of each wheel. If any wheel is spinning faster than others, brake pressure is applied to that wheel to slow it down, bringing it in line with other wheel speeds, thus providing the optimum traction between the road surface and each vehicle tyre. Traction control will not function with speeds above 50 km/h.

If ETC is required and the brake pedal is not depressed, the ECU starts the re-circulation pump to draw fluid into the system from the master cylinder. Additional valves are required for the purpose of controlling the volumetric flow. The Continental TEVES system uses two additional solenoid valves in each brake circuit. As the pump starts up, the separation valve blocks the delivery line to the master cylinder and diverts the fluid flow to the pump circuit. The changeover, or electric shuttle valves, control fluid flow from the master cylinder and reservoir. Actual wheel control takes place in the same way as ABS via the control of the individual inlet and outlet valves. Excess volumetric flow of the pump is routed via the pressure relief valve, which is integrated into the separation valve on the Continental TEVES system.

The traction control warning lamp is amber in colour and is illuminated in the following circumstances/conditions: l It illuminates for a minimum of 2 seconds when TC is active or longer if TC is active for longer than 2 seconds l During the initialisation phase and a following test phase controlled by the microprocessor l In the event of TC fault condition l Fully 'on' when manual disable TC function is operated l Flashing when brakes are hot (over 350 °C) l When the controller is switched off, for as long as ignition voltage is supplied to the ABS ECU l During diagnostics

To allow the vehicle to be tested on two wheel rolling roads there is a feature which allows the ETC function to be disabled. To disable ETC, the brake pedal has to be operated 10 times within 10 seconds of turning the ignition on. When traction control is disabled, the ETC warning lamp will be illuminated in the instrument pack and no wheel braking will occur during this period. Also, the road speed signal will be an average of the two rotating wheels and no ABS sensor or accelerometer faults will be registered during this period.

The ETC function can be re-enabled at the start of a subsequent ignition cycle by again operating the brake pedal 10 times within 10 seconds of turning the ignition on. Alternatively, the ABS ECU will automatically re-enable the ETC function when it receives a 4.4 mph (7 km/h) speed signal from all four ABS sensors.

Should the ETC (or HDC) be active for long periods the temperature of the brakes may cause damage to the brake components (disks, drums, pads and shoes). To prevent this there is a safety feature that disables the ETC or HDC if it considers the system is overheating. The system functions by the ABS calculating the brake temperature. When the first temperature limit (350°C) is reached the ETC warning lamp will start to flash (if HDC is also 'ON' then the HDC fault lamp will also flash). When the second temperature limit (400°C) has been reached the warning lights will continue to flash but the ETC and HDC functionality will become inactive. Should HDC be active as the second temperature limit is reached the HDC will fade out gradually. System functionality will return when the brakes have returned to the third limit (300°C).

Hill Descent Control

This feature allows the vehicle speed to be controlled during a hill descent using the vehicle Brakes. This feature has to be selected using the Hill descent switch with the selected gear being 'first' or 'reverse' and the brakes below 350°C.

When HDC is selected by operating the latching HDC switch, the HDC information warning lamp is illuminated continuously to indicate HDC is available. If conditions are not met to enable HDC operation, after the switch is operated, the warning lamp flashes. When going downhill and HDC is selected the vehicle will maintain a target speed by applying the brakes. The target speed will be relative to the throttle pedal position. If the slope is not steep enough and the speed is less than the target speed, the vehicle will not accelerate to reach the target speed. The HDC function is brakes intervention only.

Minimum target speeds with the throttle closed are 6 mph (9.6 km/h) in first gear and 4 mph (6.5 km/h) in reverse gear. The first gear target speed is decreased to 4.4 mph (7 km/h) 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 km/h) in first gear and 4.4 mph (7 km/h) in reverse gear.