Speed Signal

Engine Speed Signal

The engine control module sends out an engine speed signal from pin 47.

The signal is a pulse train which varies between 0 and B+. Its frequency is about 30 Hz at idling speed and about 85 Hz at 2500 rpm. This corresponds to 3 pulses per crankshaft revolution.

The engine speed signal is used by the main instrument display panel and the transmission control module.





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 engaged at the time
^ 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.