Theory Of Operation

The purpose of the diagnostic is to detect rationality failures of the wheel speed signal via the Controller Area Network (CAN). This diagnostic checks if the wheel speed signal is within the calculated tolerance bands.

The upper threshold of the first tolerance band is the calculated wheel speed plus a calibrated offset. The calculated wheel speed is based upon the Input Speed Sensor signal multiplied by the gear ratio factor. This factor corresponds to either the target gear or current gear depending on which gear provides the greater ratio.

The lower threshold of the first tolerance band is the calculated wheel speed, minus a calibrated offset. The calculated wheel speed is based upon the Input Speed Sensor signal multiplied by the gear ratio factor. This factor corresponds to either the target gear or current gear depending on which gear provides the lesser ratio.

The thresholds of the second tolerance bands are calculated using the same method described above. In place of the Input Speed Sensor signal, the Output Speed Sensor signal is used for the calculation.