Traction Control
Components
- Separation valve, normally open
- Electronic shuttle valve, normally closed
- Hydraulic pump
- Outlet valve, normally closed
- Inlet valve, normally open
- Pressure equalizer
- By-pass valve
- Non-return valve.
Traction control is integrated in the brake system.
The system is mainly designed to assist with moving off on slippery surfaces at speeds up to approximately 60 km/h. Traction control has only a very limited effect at speeds over 60 km/h.
If the brake control module (BCM) registers, via the front wheel sensors, that any of the drive wheels are spinning quicker than the others, the separation valve (1) closes. The hydraulic pump (3) is started so that brake fluid is pumped to the caliper of the slipping wheel. Pressure builds up in the brake circuit and the brakes are applied as much as required so that the drive force is distributed between the driven wheels. The wheel which has the best traction receives the most driven force against the surface. This process is independent of engine output.
On vehicles with traction control, the brake control module (BCM) continuously registers the extent of brake usage. This allows the control module to calculate the temperature of the front wheel brake discs. If traction control is engaged and the calculated temperature becomes too high, approximately 450 °C, traction control cuts out. The general warning lamp comes on and a text is displayed in the combined instrument panel. A diagnostic trouble code (DTC) is stored in the brake control module (BCM). Traction control is blocked to prevent the brakes from overheating. The light goes out once the calculated temperature falls below approximately 300 °C and traction control is engaged again. However, ABS is always available. There is information about the calculated temperature of the brake discs in the brake control module (BCM) as long as the 30-supply is connected. If the brake system has been used too hard, this may also cause high brake temperatures.
No traction control
If no drive wheel is spinning, the brake system is unaffected. This means that the master cylinder is in the standby position and the connection between the brake fluid reservoir and the rest of the hydraulic system is open. There is no pressure in the system. In the ABS hydraulic modulator valve block, the hydraulic valves are in their rest position, the inlet valves and the separation valves are open and the outlet valves and electronic shuttle valves are closed.
Traction control when one or more drive wheels are spinning
If the car is started and moves off and one or both driven wheels are threatening to spin, the brake control module (BCM) registers this by comparing the signals from the wheel sensors with a calculated reference speed. The control module closes one or both separation valves and the hydraulic pump starts. The separation valve blocks the connection between the pump pressure side and the primary circuit /secondary circuit in the master cylinder. The inlet valve for the non-spinning wheel closes so that the brake is not applied on that wheel. The hydraulic pump draws brake fluid from the brake fluid reservoir via the electronic shuttle valve and increases the pressure until the by-pass valve opens. The hydraulic pressure reaches the spinning wheel and slows down the wheel so that the driven force is distributed between the driven wheels so that the wheel with the best traction receives the most driven force.
As the pump supplies a greater flow than required by traction control, the excess brake fluid is drained off by the by-pass valve for the master cylinder or is drawn directly up by the pump.
The inlet valve closes as soon as the spinning wheel has been slowed down by the brake to a relatively normal speed. Depending on the acceleration of the wheel, the outlet valve opens (brake fluid flows back to the suction side of the pump) so that the pressure in the brake circuit drops or the outlet valve remains closed to maintain the pressure (constant pressure). The outlet valve closes to increase the pressure in the brake circuit, the inlet valve opens and the pressure rises in the brake circuit.
This control brakes the wheel in proportion to optimum slippage until one of the following occurs:
- as friction against the surface becomes greater wheel spin stops
- control is stopped by the brake control module (BCM) to prevent the brakes from overheating
- braking takes place
- the car reaches the speed where traction control ceases.
The separation valve is activated during traction control.
Activation of the brakes, which is registered by the brake control module (BCM) via the signal from the stop lamp switch, enables the traction control to be interrupted and the separation valve to open. The electronic shuttle valve closes, so that the pump is unable to draw brake fluid from the master cylinder. The hydraulic modulator now operates as a normal ABS hydraulic modulator.
A difference in speed between the driven wheels requires traction control. This difference varies with the speed of the car. When the vehicle speed is 0 km/h, a difference in speed of 8 km/h between the drive wheels is required for traction control to engage. At a vehicle speed of 20 km/h, a difference of 18 km/h, and at 40 km/h a difference of 25 km/h is required. At higher speeds the differences required between the driven wheel speeds is so great that in practice traction control does not function above 60 km/h.