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Allison WTEC II Fault Code: 51 PID: SPN: 21 FMI: Offgoing ratio test (during shift), 2 to 1

Main code 51 indicates a failed offgoing ratio test. 2?1 downshift

An offgoing ratio test occurs during a shift and uses turbine and output speed sensor readings to calculate the ratio between them. The calculated speed sensor ratio is then compared to the programmed speed sensor ratio of the commanded range. After a shift is commanded, the ECU, after a period of time, expects the old ratio to be gone. If the ratio does not change properly, the ECU assumes the offgoing clutch did not release. The shift is retried if conditions still exist to schedule the shift. If the second shift is not successfully completed, Code 51 XX is set and the ECU returns the transmission to the previous range. Additional codes could be logged for other shifts where ?X? indicates the range from and ?Y? indicates the range to.

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