Theory Of Operation

WARNING: This page is about a different car, the 2017 RAM 2500 HD. However, it is still accessible from the selected car via links, so may be relevant.

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 (ABS) module 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 the brakes are applied, a Brake Switch code is set, or a limp-in condition exists for Manifold Absolute Pressure (MAP) Sensor, Throttle Position, Crankshaft Position (CKP) Sensor, Camshaft Position (CMP) Sensor, or Engine Coolant Temperature (ECT) Sensor. 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.