Rear-wheel sensors: Notes
The rear left wheel sensor sends a wheel speed signal to pin 9 on the control module. The sensor is grounded via pin 8.
The rear right 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 like a small alternator, the sine-wave voltage of which increases with increasing wheel speed. The sine-wave voltage alternates between positive and negative polarity, which is achieved by alternate teeth and gaps on the toothed wheel. By measuring the frequency, the control module can determine the speed of each wheel.
A wheel speed of 20 km/h (12.5 mph) will give approximately 0.8 V AC 70 Hz
The design of the rear-wheel sensor is different to that of the front-wheel sensors and the sensor is less sensitive to play in the wheel bearings.
The TC/ABS control module uses this wheel-speed information for:
- calculating wheel acceleration (speed increase), wheel retardation (speed reduction) and the car's reference speed, which is normally measured on the fastest rear wheel.
- comparing the wheel speed of the rear wheels as a basis for determining whether the car is being driven straight ahead or round a bend.
- calculating wheel slip on retardation and wheel spin on acceleration. If the limit is exceeded, ABS or TCS modulation will be activated.