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Audi Cars 1999-05

Diagnosis & Testing


When the ignition is switched ON the Antilock Braking System (ABS) control unit performs a self-test. The ABS indicator lights will illuminate for about 2 seconds and then extinguish. During this time the internal electrical circuits of the ABS control unit are checked along with the external electrical circuits of the wheel sensors, if equipped, the pedal travel sensor and the voltage supply.

When the engine is started and the vehicle begins to move for the first time, the initial movement is detected by the wheel sensors, whose pulsating electrical signal is received by the control unit, that in turn briefly energizes the solenoid valve and the pump motor of the hydraulic unit as a dynamic self test.

If a fault is detected, the ABS is switched off, and the ABS indicator lights are illuminated.

Mismatched tires could activate the Antilock Braking System (ABS) indicator light because their rotational wheel speed could be dissimilar due to slight variations of tire size between different manufacturers and/or tread patterns.

The ABS control module does have a fault memory and the ability to store fault codes. If the ABS light is illuminated, the fault(s) will be stored and can be retrieved and the light reset with the proper diagnostic equipment, such as VAG 1551 or VAG 1552, or equivalent. The control module is interrogated using the diagnostic connectors in the shifter area on pre-1996 models, and via the 16-pin OBD II Diagnostic Link Connector (DLC) on 1996 and later OBD-II compatible vehicles.

Because of their location on the suspension which is constantly moving, and exposure to heat generated by the brakes the impulse (wheel sensors) can develop an open circuit in their wiring.

The impulse (wheel speed) sensors are magnetic, and can collect a build up of fine magnetic particles on the tip of the sensor and/or become covered with debris. The exciter (tone) rings can also become built up with debris as well. The ABS control unit monitors and compares the signals from the impulse (wheel speed) sensors. If their signals differ, the ABS control unit assumes that the wheels are spinning at different speeds and illuminates the ABS indicator light and turns the system(s) off.

Quick and easy checks can be made to the impulse (wheel speed) sensors without the use of expensive diagnostic equipment.

  1. Raise and safely support the vehicle.
  3. Disconnect the impulse sensor electrical connector and check the sensor for continuity or leave the sensor connected, rotate the tire quickly and use an inductive pickup connected to an oscilloscope to check for a voltage signal. If the circuit has no continuity (open) or no voltage signal is recognized, compare the readings with the remaining wheel sensors and replace as necessary.

Compressed air can be dangerous. Wear eye protection and protective clothing.

  1. Check for a build up of magnetic particles or debris between the exciter ring and the tip of the sensor. Remove the sensor, and wearing eye protection, carefully clean with compressed. Then rotate the wheel and through the sensor mounting hole, clear the exciter ring with compressed air.

If the ABS system continues to malfunction, check the wiring between all of the components. Clean the connections thoroughly and repair if necessary. If the system continues to malfunction seek the help of a dealer or a certified ABS brake specialist.

Click image to see an enlarged view

Fig. The air gap is critical. If debris gathers between the impulse sensor and the exciter ring, or the sensor and/or exciter ring are damaged, the ABS light will be illuminated