Theory Of Operation

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The vehicle speed sensor rationality is a continuous test that monitors the vehicle speed sensor for lack of activity. The Powertrain Control Module (PCM) receives this information from the Anti-Lock Brake System over the CAN C bus. The CAN C bus is a high speed communication bus that allows real time control capability between various controllers. The rationality test will not run if a limp-in exists for MAP, Throttle Position, Crankshaft Sensor, Camshaft Sensor, and Engine Coolant Temperature, if the brake is applied or any Brake Switch codes are set. If vehicle speed signal is below a minimum threshold for a calibrated period of time after the vehicle has been operating within the modeled parameters that would indicate it is being driven, a failure will be indicated.