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Front-wheel sensors

Front-wheel sensors

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.

Mounted on the right-hand and left-hand outer drive shaft universal joints is a toothed wheel with 29 teeth. A sensor of inductive type is mounted on each steering swivel member. The gap between sensor and toothed wheel should be 0.3-1.3 mm and is not adjustable.

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 gives about 2 V AC, 70 Hz

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 wheelspin on acceleration. If the limit is exceeded, ABS or TCS modulation will be activated.




Diagnostics

If there is a break or short circuit in the front left circuit, diagnostic trouble code C1372 will be set. The TCS and ABS functions will be disabled and the warning lamps for ABS, brakes and TCS OFF will light up as well as the central warning lamp.

In the event of a mechanical fault or malfunction in the front left circuit, diagnostic trouble code C1371 will be set. The TCS and ABS functions will be disabled and the warning lamps for ABS, brakes and TCS OFF will light up as well as the central warning lamp.

If there is a break or short circuit in the front right circuit, diagnostic trouble code C1377 will be set. The TCS and ABS functions will be disabled and the warning lamps for ABS, brakes and TCS OFF will light up as well as the central warning lamp.

In the event of a mechanical fault or malfunction in the front right circuit, diagnostic trouble code C1376 will be set. The TCS and ABS functions will be disabled and the warning lamps for ABS, brakes and TCS OFF will light up as well as the central warning lamp.