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Vehicle speed signal

Vehicle speed signal



All cars are equipped with a factory-fitted anti-lock braking system (ABS) which senses all 4 wheel speeds via separate wheel sensors.

The front wheel sensors use a toothed wheel with 29 teeth. The signal from each wheel sensor is processed by the ABS control module.

The ABS control module sends a speed signal from the RH front wheel to pin 42 of the Motronic control module. This signal is a square wave which varies between 0 V and battery positive voltage (B+) 29 times per wheel revolution. At 30 km/h the frequency is about 130 Hz.

This speed signal is used to:


• determine which gear is currently engaged

• activate the SHIFT UP lamp on cars with a manual gearbox (not in 5th gear)

• prevent adaptation of idle speed control when the car is in motion.