Function

Since the control modules are interconnected with a Controller Area Network bus (CAN-bus), always start troubleshooting by pressing buttons -0- and -0- to insert "Automatic Test Sequence" address word 00. This checks DTC memory for all control modules in the vehicle, including those that may influence the ABS system in case of malfunction.

On Board Diagnostic (OBD) relates to the electrical and electronic components of the ABS; in other words, only malfunctions pertaining to the electrical connections to the control module are recognized (e.g. wheel speed sensor, open circuit).

The 25-pin ABS control module -J104- and the ABS hydraulic unit together form a compact module. The control module is equipped with DTC memory, and is located on the left side of the engine compartment. The Data Link Connector (DLC) is located left of the steering column under the instrument panel. Only early production cars DLC is in the center console below the heating and air conditioning controls.

The control module recognizes malfunctions during vehicle operation and stores them as Diagnostic Trouble Codes (DTC) in permanent memory. The DTCs remain in memory if the voltage from the battery is interrupted.

Sporadic malfunctions will also be recognized and stored. If these do not occur again within the next 50 vehicle starts and driving cycles DTC memory will be erased (with the exception of "Control Module Malfunctioning").

Sporadic malfunctions are identified as such by "/SP" appearing on the VAG1551 Scan Tool (ST) display.

After switching the ignition on and/or starting the engine, ABS warning light -K47- and brake system warning lamp -K1 18- (red) lights up for approximately 2 seconds. During this period a test sequence (selftest) is run in the control module, checking the following:
^ Voltage supply (at least 10.0 volts)
^ Control module, including valve windings
^ Wheel speed sensor; electrical check fully completed at 20 km/h (approximately 13 mph)
^ Control module coding
^ If after driving off and exceeding a speed of approximately 20 km/h (approximately 13 MPH) the speed signal is not OK The ABS warning light -K47- will light up again.