Speed Signal

The ABS control module sends a speed signal to pin 79 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 (not in 5th gear)
^ prevent adaptation of idle speed control when the car is in motion.
^ determine whether the car is being driven on a "rough road" if misfiring is detected.