Traction control
If for example the front wheels have a tendency to spin when the car is driven off the following occurs:
- The solenoid valves (2) close and block the connection between the pump pressure side and the master cylinder
- The inlet valve (C1) for the left front wheel closes so that wheel cannot brake.
- The pump starts and then runs constantly during operation.
- The pump takes brake fluid from the master cylinder via the hydraulic valve (1) and pumps the brake fluid to the right front wheel.
- The wheel is braked to the same speed as the left front wheel and drive is split between the wheels.
- The control module then controls the wheel spin (braking power) by closing/opening the inlet valves (C4) and the outlet valve (D4).
When the control is stopped the pump is switched off, the solenoid valves (2) opens and the in- and outlet valves resume the position for normal braking.
Control is stopped if:
- Wheelspin stops, for example because of increased friction between the road surface and the tires.
- There is a risk of the brake discs overheating.
- Braking takes place (the brake pedal is depressed, signal from the stop (brake) lamp switch).
In addition the pressure from the master cylinder hydraulic valve (1) is shut off and the pump can no longer receive brake fluid from the master cylinder.
The pump motor is controlled with a signal which has a fixed frequency but a variable pulse width. This means that the engine speed (RPM) and sound vary during traction control.
Because the pump supplies more brake fluid than is normally required for control there are two by-pass valves (3) which open at a certain pressure. When the valves open the brake fluid is released back to the brake fluid reservoir via the master cylinder.