Anti-Lock Brake System: Description: Notes

The Anti-Lock Brake System (ABS) consists of an ABS control unit, hydraulic modulator, system relays, speed sensors and necessary wiring. See Fig 1.

When brake pedal is applied, the ECM monitors input signals from each wheel Vehicle Speed Sensor (VSS). If ECM senses wheel lock-up, it will output a signal to appropriate solenoid valve. Each solenoid valve allows hydraulic pressure to increase or decrease to the appropriate brake caliper.

NOTE: For more information on conventional brake system, see BRAKE SYSTEM article in BRAKES.

If a system failure occurs, the ANTILOCK warning light, located on instrument panel, will illuminate. System will be deactivated, but conventional brake system will still operate.

Fig 1: Locating ABS Components

Courtesy of AUDI OF AMERICA, INC.

CAUTION: See ANTI-LOCK BRAKE SAFETY PRECAUTIONS article in GENERAL INFORMATION.

CAUTION: The following is an additional list of manufacturer's recommended ABS cautions.

Turn ignition off before connecting or disconnecting ABS control unit connector.
DO NOT drive vehicle with ABS tester connected.
DO NOT repair hydraulic modulator except for replacing relays. Replace hydraulic modulator if it is defective.
DO NOT fabricate brakelines. Use only genuine replacement parts when making repairs.