How TRACS Works When One or Both Wheels Are Spinning

How TRACS works when one or both wheels are spinning




When the car is moving and one or both drive wheels are threatening to spin, the control module detects this by comparing the signals from the wheel sensors with an estimated reference speed. The control module then closes the TRACS inlet valve (1) and the pump cuts in.
The TRACS inlet valve then blocks the connection between the pump's output side and the master cylinder primary circuit. The inlet valve (2) to the non-spinning wheel then closes so as not to brake the wheel. The pump (3) sucks back brake fluid from the brake fluid reservoir via the TRACS valve (4) and increases the pressure until the overflow valve (5) opens. The hydraulic pressure then reaches the spinning wheel and brakes it, dividing the engine power between the drive wheels so that most engine power reaches the road via the wheel with the best grip.
As the pump supplies more flow on average than TRACS needs, the excess brake fluid is drained off by the overflow valve to the master cylinder or sucked up directly by the pump.
Under TRACS control, the pump motor is controlled by a pulsed signal. By changing the signal's pulse rate, the engine speed can be controlled. The noise from the pump may therefore vary while TRACS is in use; this is completely normal.
If TRACS is disabled manually, it can be switched back on while the front wheels are spinning. It then cuts in more gently than normal.
As soon as the spinning wheel has been braked down to a relatively normal slip, the inlet valve closes. Depending on the wheel's acceleration, either the outlet valve will open (sending brake fluid back to the inlet side of the pump) so that the pressure in the brake circuit falls, or the outlet valve will stay closed to maintain the pressure. To increase the pressure in the brake circuit again, the outlet valve closes, the inlet valve opens and the pressure in the brake circuit increases.

This keeps the wheel in a state of optimum slip until one of the following events occurs:
- The spin has nearly stopped because the friction against the road has increased.
- The control module cuts TRACS out to prevent the brakes overheating.
- Braking occurs.
- TRACS is switched off via the switch on the panel.
If the rear wheels lock under TRACS control (e.g. the parking brake is applied), the control module will disengage TRACS and switch to ABS and the TRACS warning light will come on.
The TRACS inlet valve is activated continuously while TRACS is on.
When the brakes are applied, which the control module recognizes by the signal from the brake light switch, TRACS cuts out and the TRACS inlet valve opens. The pressure in the master cylinder shuts the hydraulically-operated valve, so that the pump cannot take brake fluid from the master cylinder. The hydraulic unit now acts as a normal ABS hydraulic unit.
Depending on how fast the car is moving, there has to be a certain difference between the drive wheel speeds (overspin) before TRACS will cut in. If the car is travelling at 0 km/h, it takes approx. 18 km/h (11 mph) overspin on either wheel before TRACS cuts in. At 20 km/h (12 mph), TRACS cuts in at approx. 8 km/h (5 mph) overspin, and at 40 km/h (25 mph), it cuts in at approx. 25 km/h (15 mph) overspin. If the speed is any higher, so much overspin is needed that TRACS virtually never cuts in.