Antilock Brakes / Traction Control Systems: Description and Operation

Circuit Description
The 3-channel ABS system for Quattro(All-wheel-drive) models consists of the following components:

- ABS electronic control unit
- Hydraulic modulator
- Four wheel-speed sensors, one at each wheel
- ABS combi-relay
- ABS on/off switch
- Acceleration switch

The ABS control unit processes the varying signals from each wheel speed sensor and operates the hydraulic modulator accordingly. If the signal from a wheel sensor (or sensors) indicates that a wheel is about to lock, the hydraulic modulator adjusts pressure to the brake caliper at that wheel.

The acceleration switch contains two mercury switches that will open during hard braking and interrupt a control unit ground circuit.

The ABS operates in conjunction with the standard brake components and is basically maintenance-free. The following precautions must be observed when working on ABS-equipped cars:


CAUTION: Disconnect the ABS control unit connector before doing any electrical welding on the car.

Remove the ABS control unit before subjecting the car to temperatures above 85°C (185°F) for more than two hours, such as in a paint-drying oven.

Disconnect and connect the ABS control unit connector only with the ignition off.

Disconnect the battery cables from the battery before charging the battery or replacing the hydraulic modulator.

Do not drive the car with the ABS tester connected.

Do not use mini-spare tires on ABS-equipped cars. Wheels and tires of the same size must be used.