Allison 1000 & 2000 Gen 4 Fault Codes: P0731 Incorrect 1st Gear Ratio

Allison 1000 & 2000 Gen 4 Fault Codes: P0731 Incorrect 1st Gear Ratio

CIRCUIT DESCRIPTION:
The Transmission Control Module (TCM) uses input from the turbine speed sensor and output speed sensor to determine gear ratios. The TCM then compares the known gear ratio to the calculated gear ratio for the current range.

CONDITIONS FOR RUNNING THE DTC:
1. Output speed exceeds 200 rpm.
2. First range is selected and attained.
3. DTC P0716, P0717, P0721, or P0722 is not active.

CONDITIONS FOR SETTING THE DTC:
DTC P0731 sets when the calculated first range ratio (steady state) differs from the known first range ratio.

ACTION TAKEN WHEN THE DTC SETS:
When DTC P0731 is active, the following conditions will occur:
1. The transmission will fail to 2nd or 5th range.
2. If the shift selector is moved to N (Neutral), the transmission will shift to N (Neutral).
3. If the shift selector is moved to R (Reverse), the transmission will shift to R (Reverse) unless the transmission is compromised by overspeeding or a direction change; then it will lock in N (Neutral).

DTC P0731 is stored in the TCM history.

The CHECK TRANS light illuminates.

The TCM freezes shift adapts (DNA).

The TCM inhibits TCC engagement.

CONDITIONS FOR CLEARING THE DTC/CHECK TRANS LIGHT:
A Scan Tool may be used to clear the code from the TCM history. The TCM automatically clears the DTC from the TCM history if the vehicle completes 40 warm-up cycles without the DTC recurring.

DIAGNOSTIC AIDS:
1. Incorrect ratio codes typically indicate mechanical problems with specific clutches, i.e. C1 or C5 for 1st range.
2. An incorrect ratio code could indicate a hydraulically failed solenoid. Check the DTC information for the specific solenoid.
3. You may have to drive the vehicle to experience the fault. Clutch test mode can be used to check stall speed.
4. Incorrect TCM calibrations will cause this DTC to set. Verify that the proper TCM calibration is being used with the correct transmission family (there are different gear ratios for the 1000 and 2000 Product Families transmissions).