Rear-wheel sensors: Notes
The rear left (RL) wheel sensor sends a wheel-speed signal to pin 9 on the control module. The sensor is grounded via pin 8.
The rear right (RR) wheel sensor sends a wheel-speed signal to pin 2 on the control module. The sensor is grounded via pin 1.
The wheel sensors are integrated into the wheel hubs and consist of an inductive sensor with 29 teeth.
The inductive sensor works somewhat like a miniature alternator, the sinusoidal voltage of which increases with increasing wheel speed. The sinusoidal voltage alternates between positive and negative polarity, which is achieved through the alternating teeth and gaps on the toothed wheel. By measuring the frequency, the control module can determine the speed of the wheel in question.
A wheel speed of 20 km/h gives about 0.8 VAC, 70 Hz.
The rear wheel sensor is of different design to the front wheel sensors and is not as sensitive to wheel bearing play.
The ABS control module uses wheel speed information for:
- calculating wheel acceleration, wheel retardation and the car's reference speed, which is normally measured on the fastest rear wheel;
- comparing the speeds of the rear wheels to determine whether the car is being driven straight ahead or cornering;
- calculating wheelspin on retardation and, if the limit is exceeded, activating the ABS function.