Abs Mark 20: 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- (w/EDL) 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 Scan Tool VAG1551 display.
After switching the ignition on and/or starting the engine, ABS Warning Lamp -K47- and Brake System Warning Lamp -K118- (red) lights up for approx. 2 seconds. During this period a test sequence (self-test) 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 (approx. 13 mph)
- Control module coding
- If after driving off and exceeding a speed of approx. 20 km/h (approx. 13 MPH) the speed signal is not OK. The ABS Warning Lamp -K47- will light up again.
Scan Tool VAG1551 or Vehicle Diagnostic Testing and Information System VAS 5051
The On Board Diagnostic (OBD) program must be initiated at the start of troubleshooting. Electrical malfunctions which influence braking characteristics will be stored, and can only be checked with the Scan Tool VAG1551 or with the Vehicle Diagnostic Testing and Information System VAS 5051.
The following procedures refer only to the Scan Tool VAG1551.
The malfunction information displayed is used in conjunction with a DTC table, which has information on the possible cause of malfunctions and specific corrective actions.