Wheel Speed Sensor: Description and Operation

Wheel Speed Sensors: Hall effect direction sensing wheel speed sensors are used on the E65/E66. A special feature of this sensor is that forward and reverse rotation is detected. The sensor contains three Hall effect elements located next to each other in one housing. The signals of the first and third Hall element form a raw differential signal for determining the signal frequency (speed) and the air gap clearance to the impulse wheel.




Clockwise or counterclockwise rotation is detected by the phase offset of the signal from the middle (second) element as compared to the differential signal. The phase will shift (left or right) depending on the impulse wheel's approach to the Hall element from the left or the right.




The direction of rotation phase (3) will shift to the left or to the right (as shown) of the differential signal (2). These signals are processed in the sensor and are represented in the output pulse width digital signal (4) that is monitored by the DSC control module.




This sensor contains two external wires and the digital signal is transmitted over the combined ground and data line (D) to the DSC control module. The second wire is the power supply for the wheel speed sensor.

The flow of current is the influencing factor, not the voltage level. This provides a reoccurring data message that uses two different amp ratings. The 14 mA level contains the information of speed, direction of rotation and air gap. The 7 mA level is the evaluation current for the fault code memory. When the vehicle is stationary, a pulse is sent every 740 ms to check the sensor circuit integrity.