Speed Signal

The speedometer sends a speed signal to pin 39 of the Trionic control module. This signal is a square wave which varies between 0 V and battery positive voltage (B+). At 19 mph (30 km/h) the frequency is about 20 Hz.

This speed signal is used to:
^ Ascertain which gear is engaged at the time
^ Activate the SHIFT UP lamp
^ Limit boost pressure in 1st gear (all turbo versions except B234E)
^ Activate speed limitation by limiting boost pressure when maximum speed is attained (all turbo versions except B234E)
^ Prevent adaptation of idle speed control when the car is in motion.
If there is no speed signal the above functions are dropped and boost pressure is limited to the lowest gear-dependent pressure.