Allison 3000 & 4000 Gen 4 Fault Code: P077F Incorrect Reverse (R2) Ratio

DIAGNOSTIC RESPONSE:
DNS, Lock in Neutral

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

CONDITIONS FOR RUNNING THE DTC:
1. Hydraulic system is pressurized.
2. No shift in progress.
3. Hydraulic default condition not present.
4. Output speed is above 200 rpm.
5. Engine initialization or shutdown is not in process.

CONDITIONS FOR SETTING THE DTC:
DTC P077F sets during steady-state condition when the calculated reverse (R2) gear ratio differs from the known reverse (R2) gear ratio for more than 2 seconds.

ACTION TAKEN WHEN THE DTC SETS:
When DTC P077F is active, the following conditions will occur:
1. Locks in Neutral (N).
2. While diagnostic response is active, the TCM ignores shift selector inputs.
3. The CHECK TRANS light illuminates.
4. DTC is stored in the TCM history.
5. TCM freezes shift adapts (DNA).

CONDITIONS FOR CLEARING THE DTC/CHECK TRANS LIGHT:
Use the diagnostic tool to clear the DTC 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:
NOTE: A second “deep reverse” gear (R2) was released January 2010 for 4700RDS, 4700EVS, 4800EVS and 4700/4800 International transmission models. The 47000FS version was released June 2010. The R2 gear ratio is 17.12. The standard reverse (R1) gear ratio is 4.80. The 4th Generation selectors display a lowercase “r” when in R2 range and an uppercase R when in R1 range. Refer to the OEM Technician’s Library for R2 vehicle integration information.

DTC P077F may be caused by:
1. Output speed sensor signal circuit issues.
2. Turbine speed sensor signal circuit issues.
3. Low clutch pressures, main pressure, or control main pressure.
4. Control module issues like stuck/sticky valves, mechanically defective solenoids, broken springs, damaged valve bores, plugged suction filter, damaged gaskets, excessive leaks to exhaust, etc.
5. Transmission issues such as worn and/or damaged seals or seal rings, dislocated snap rings, worn clutch packs, rotating drum damage (affecting turbine speed pickup), debris, excessive hydraulic circuit leaks to exhaust or other hydraulic circuits.
6. Clutch system issues causing a clutch not to release.
7. Issue with TCM.