Anti-Lock Brake System: Description
The Porsche Anti-Lock Brake System (ABS) consists of a hydraulic unit with 4 solenoid switching valves, 4 speed sensors and an electronic control unit. The system contains a network of wire harnesses, relays and an overvoltage protection unit. This system functions as a supplement to conventional braking. The ABS equipped vehicles require different disc rotors and master cylinders than standard brakes.
An indicator light ("ANTILOCK" on 928S; "ABS" on 944 series) is located on the instrument panel and illuminates when the ignition is turned on. This indicator light normally goes out after the engine is started, to indicate the ABS system is ready for operation.
The ABS system is activated above 8 MPH. When vehicle speed is above 3 MPH, built-in test equipment will start checking the ABS. If a fault is found, the warning light will again illuminate. When the "ANTILOCK" light is on, ABS is deactivated and the vehicle will convert to conventional braking.
If battery voltage drops below 10.5 volts when the ignition is on and test speed is exceeded, the ABS will remain off and warning light will illuminate. When alternator voltage increases above 10.5 volts, warning light will go out.