Road Speed Signal

Road Speed Signal:

All 3 series vehicles are equipped with a reed contact in the differential to produce a dedicated speed signal for the instrument cluster. The DME requires the road speed signal from the ABS control module for the purpose of misfire detection plausibility.

The wheel speed sensor input is processed by the ABS and made available to the M5.2 control module as a digital square wave signal. The frequency of the signal is proportional to the speed of the vehicle.

The vehicle speed input signal is continuously monitored as part of the OBD 11 emission requirements. The control module monitors the plausibility of the speed input against engine RPM and load.

The M5.2 control module also monitors the signal for abrupt fluctuations in the signal frequency to "detect rough road surfaces" for the purpose of misfire detection plausibility.

If a malfunction occurs, the control module limits the maximum RPM of the engine and the Check Engine Lamp will illuminate when the criteria for OBD II monitoring are satisfied.