Antilock Brakes / Traction Control Systems: Description and Operation

Function
ABS with On Board Diagnostic (OBD) capability is standard equipment.

OBD functions involve only the electrical and electronic components of the system. The OBD program recognizes malfunctions that affect electrical signals.

If malfunctions occur in the sensors or other components that are monitored by the system, they are stored in memory as Diagnostic Trouble Codes (DTC).

The ABS control module is equipped with DTC memory that can be checked using the VAG1551 Scan Tool (ST).

The first step in any diagnostic procedure should always be to check DTC memory (function 02). Refer to On Board Diagnostic in System Diagnosis. Testing and Inspection

Operational Notes
The system conducts a self-test whenever the ignition is switched on and the vehicle reaches a speed of at least 5-6 km/h (3-4 mph). The switching of the ABS relay and the start-up noise of the ABS hydraulic unit return pump maybe audible. Do not replace the hydraulic unit due to complaints about this noise. Another noticeable part of the self-test is a slight vibration at the brake pedal.

If a malfunction is recognized, the system switches off automatically and the ABS warning light comes on in the instrument cluster. The normal brake system remains fully operational. ABS remains switched off for the duration of the trip, except in the case of a voltage supply malfunction. (in this case, ABS switches on again as soon as the battery positive voltage has returned to a normal level).

If no malfunction is recognized, ABS warning light goes out 2 seconds after the ignition is switched on (as soon as the ABS self-test routine has been completed by the ABS control module).

If a malfunction (which may have been recognized and stored in DTC memory during the last single trip) no longer exists after the ignition has been switched off and on again, or if it cannot be detected with the vehicle stationary (certain malfunctions can only be detected when the vehicle exceeds a road speed of 12 km1h or 8 mph), the ABS warning light also goes out 2 seconds after the ignition is switched on (after the ABS self-test).

When a malfunction is no longer recognized after switching the ignition off and on, the "static" malfunction is converted to a "sporadic" malfunction.

"Sporadic" malfunctions are specifically identified as such and indicated by "/SP" on scan tool display (right side).

A "sporadic" malfunction that is not recognized again for a certain number of single trips (ignition switched on and off) it is automatically erased.

From input signals, ABS control module distinguishes between 15 different types of malfunctions (refer to the DTC table) and stores their corresponding diagnostic trouble codes until DTC memory is erased (after checking) using the VAG 1551 scan tool. Refer to On Board Diagnostic in System Diagnosis. Testing and Inspection

The OBD program is only accessible using the VAG1551 scan tool and "rapid data transfer" operating mode 1. The data transfer is only possible when the ignition is switched on or when the engine is running.

The OBD program sequence for ABS can be used not only for checking and erasing DTC memory, but also offers "Check Control Module Version" and "Read Measuring Value Block" functions. "Blink code output" operating mode 2 cannot be used ABS control module. Other possible VAG 1551 operating modes are described in the operating instructions.

The OBD program sequence can be started only when the vehicle is stationary. If starting at more than 2.5 km1h (7-5 mph) the ABS control module does not respond.

Checking and erasing DTC memory, identifying the control module version, and reading the measuring value block are only possible up to a speed of 20 km1h (12 mph) (if this speed is exceeded, the OBD program sequence will be aborted).