When the ignition is switched ON the Antilock Braking System (ABS) control unit performs a self test. The ABS and if equipped, the Automatic Stability Control with Traction Control System (ASC+T) indicator lights will illuminate for about two 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 pump motor of the hydraulic unit as a dynamic self test.
If a fault is detected, the ABS and if equipped, the ASC+T are switched off, and the ABS and if equipped, the ASC+T indicator lights are illuminated.
Unlike the E30 models, the E36 models do have a fault memory and the ability to store fault codes. If the ABS light is illuminated on an E36 model, the fault(s) will be stored and can be retrieved with the proper diagnostic equipment. The E36 models have a diagnostic link which can be accessed with diagnostic equipment such as the BMW Service Tester or MoDiC. The fault memory cannot be interrogated with the 16-pin OBD II Diagnostic Link Connector (DLC) using a Data Scan Tool (DST).
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, they 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 and if equipped, the ASC+T indicator lights 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.
- Raise and safely support the vehicle.
- 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.
- 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 BMW dealer or a certified BMW ABS brake specialist.