Allison WTEC II Fault Code: 56 PID: SPN: 66 FMI: Range verification ratio test, 6th

Main code 56 indicates a failed range verification speed sensor ratio test. Range verification ratio test (between shifts) 6

The ratio test occurs after a shift and determines if a clutch has lost torque carrying capability. If output speed is above programmed output speed for a range but the correct speed sensor ratio is not present, the DO NOT SHIFT response is commanded and a range which can carry the torque without damage is commanded or attempted. Turbine and output speed sensor readings are used to calculate the actual ratio that is compared to the commanded ratio.

Main code 56 can also be caused by the EEPROM being calibrated for a close ratio transmission and installed in a wide ratio transmission, or vice versa.