Allison 1000 & 2000 Gen 4 Fault Codes: P0734 Incorrect 4th 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. Fourth range is selected and attained.
3. DTC P0716, P0717, P0721, or P0722 is not active.

CONDITIONS FOR SETTING THE DTC:
DTC P0734 sets when the calculated fourth range ratio (steady state) differs from the known fourth range ratio.

ACTION TAKEN WHEN THE DTC SETS:
When DTC P0734 is active, the following conditions will occur:
1. The transmission will fail to 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 a direction change; then the transmission will shift to N (Neutral).
4. If the shift selector is returned to a forward range and the transmission is compromised by overspeeding or a direction change, the transmission will lock in N (Neutral).

DTC P0734 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 and C2 for 4th 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.