Theory Of Operation
The purpose of the diagnostic is to detect rationality failures of the Output Speed Sensor signal. This diagnostic checks if the Output Speed Sensor signal is within the calculated tolerance bands:
- The upper threshold of the first tolerance band is the calculated output speed plus a calibrated offset. The calculated output speed is based upon the input speed signal multiplied by the gear ratio factor. This factor corresponds to either the target gear or current gear depending on which gear provides the smaller ratio.
- The lower threshold of the first tolerance band is the calculated output speed, minus a calibrated offset. The calculated output speed is based upon the input speed 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 signal, the wheel speed signal via CAN is used for the calculation.
- This diagnostic also checks if the rotating direction of the output speed sensor signal corresponds to the actual gear position
The transmission needs to be placed into Neutral to perform a 4L to 4H transfer case shift. If the operator is performing a 4L to 4H shift the potential exists for them to shift the transmission from Neutral to Drive and accelerate before the transfer case has actually shifted into 4H. The TCM deems the transfer case is in 4H since the operator did select 4H, but the transfer case had to stay in 4L because the operator shifted to Drive too early. The TCM will set P0721-00 because of the difference of the output shaft speed compared to the expected wheel speed in this scenario.
| 1 - Upper threshold of first tolerance band (Calculated output speed from input speed plus a value) | 6 - Wheel speed |
| 2 - Upper threshold of second tolerance band (Calculated output speed from wheel speed plus a value) | 7 - Not used |
| 3 - Output Speed | 8 - Lower threshold of first tolerance band (Calculated output speed from output speed plus a value) |
| 4 - Not used | Lower threshold of second tolerance band (Calculated output speed from wheel speed plus a value) |
| 5 - Input Speed | 10 - Not used |