Traction Control

TRACS is an anti-wheelspin system integrated with the ABS system. It is mainly designed to assist with moving off on slippery surfaces at speeds up to approx. 40 km/h (25 mph). At speeds over 40 km/h (25 mph), TRACS has only a very limited effect.

If TRACS is switched on via the switch (1) on the dashboard and the control module (2) detects via the front wheel sensors (3), that one of the drive wheels is turning faster than the other, the hydraulic pump (4) cuts in. The pump pumps brake fluid to the slipping wheel's brake caliper and the TRACS valve shuts. This builds up pressure in the primary circuit and the brakes are applied just enough to ensure that both drive wheels are turning at the same speed. This process is independent of how much power the engine is putting out. The hydraulic pump runs continuously during the process.

TRACS cuts out if:

^ the wheelslip stops.
^ the driver brakes.
^ there is a risk of overheating the brakes.
^ the system is switched off via the switch on the dashboard.

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 (0 mph), 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) 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.

The control module monitors how much TRACS and ABS are in use and calculates the theoretical front brake disc temperature. If there is a risk of the brake discs overheating, TRACS cuts out, the TRACS warning indicator comes on and a DTC is posted in the control module. The control module retains data on the estimated brake disc temperature for up to 1/2 hour after the ignition is switched off.

Traction control system (TRACS) not working

If neither driving wheel is slipping, the brake system is not affected (i.e. the master cylinder is in the resting 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 at all. In the hydraulic system valve block, the hydraulic valves are in the resting position, inlet valves and the TRACS valve are open, outlet valves are closed.

How Traction control system (TRACS) works if one or both driving wheels start slipping





If the car moves off and one or both driving wheels are threatening to slip, the control module detects this by comparing the signals from the wheel sensors with a known reference speed. The control module then closes the connection between the master cylinder primary circuit and the hydraulic system pump (1) by closing the TRACS valve (2). The control module then closes the inlet valves (C1, C2) to the wheel which is not slipping, so that wheel is not braked, and switches the hydraulic pump on. The hydraulic pressure from the pump reaches the slipping wheel and brakes it until it is turning at the same speed as the other wheel. If the braking force gets so large that it slows the wheel down too much, the brake pressure is reduced by closing the inlet valve and opening the outlet valve (D1, D2). The wheel can now speed up again. If more braking power is needed again, the outlet valve shuts and the inlet valve opens so that the brake pressure increases. This inlet/outlet valve process continues until the wheelslip stops, the brakes are used or there is a risk of the brakes overheating.

Note:

As the hydraulic pump is on all the time TRACS is on, the hydraulic system will make a different sound when TRACS is on from when ABS is in use.

When the brakes are applied, the brake light switch trips first, followed by the pressure sensor (3) in the valve unit which breaks a circuit to the control module. The control module then cuts TRACS out by stopping the hydraulic pump and opening the TRACS valve. The system then stands by for ABS. This means braking always takes priority over TRACS.

As the hydraulic pump is running all the time TRACS is in use and it delivers more brake fluid than normally required, excess brake fluid is returned to the brake fluid reservoir via the overflow valve (4) in the hydraulic system valve unit.