Front-wheel sensors: Notes
The front left wheel sensor sends a wheel-speed signal to pin 7 on the control module. The sensor is grounded via pin 6.
The front right wheel sensor sends a wheel-speed signal to pin 5 on the control module. The sensor is grounded via pin 3.
A toothed wheel with 29 teeth is mounted on the left-hand and right-hand outboard drive shaft CV joints. A sensor of inductive type is mounted on each steering swivel member. The distance between sensor and toothed wheel should be 0.3-1,3 mm and cannot be adjusted.
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 respective wheel.
A wheel speed of 20 km/h gives about 2 VAC, 70 Hz.
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.