Vehicle Speed Signals

The vehicle speed input signal is provided by either the instrument cluster or the ABS/ASC control module. The vehicle speed input is a digital square wave and the frequency is of the signal is proportional to the speed of the vehicle. The speed signal is used for idle speed regulation and for vehicle speed regulation on some models.







FROM INSTRUMENT CLUSTER "A": Prior to ODDII compliant engine control systems, all engine control systems received the speed signal directly from the instrument cluster (signal A). This signal is derived from a reed contact in the differential and processed by the instrument cluster for posting speed in the speedometer and for generating the output vehicle speed signal.







ABS/ASC ROAD SPEED SIGNAL: Engine control systems that are OBD II compliant (> 1- 96) receive the vehicle speed signal directly from the ABS/ASO control module. The ECM also monitors the signal for abrupt fluctuations in the signal frequency to "detect rough road surfaces" for the purpose of misfire detection plausibility.

E36 vehicles continue to be equipped with a reed contact in the differential to produce a dedicated speed signal for the instrument cluster.