Allison 1000 & 2000 Gen 4 Fault Codes: P0777 Solenoid B Controlled Clutch Stuck On

CIRCUIT DESCRIPTION:
Trim solenoid B is used to control on-coming, off-going, and holding pressure to any one of five clutches. This solenoid is referred to as a Pressure Proportional to Current (PPC) solenoid since the output hydraulic pressure supplied by this solenoid is proportional to the controlled current command.

The Transmission Control Module (TCM) uses information from the turbine and output speed sensors to detect if a clutch is in a tie-up condition or if three clutches are applied. The clutch being controlled by solenoid B will vary depending on the shift.

CONDITIONS FOR RUNNING THE DTC:
The components are powered and ignition voltage is greater than 9V and less than 18V (12V TCM) or greater than 18V and less than 32V (24V TCM).

One of the following conditions occurs:
1. Output speed is greater than 200 rpm.
2. Turbine speed is greater than 200 rpm.

DTC P0708, P0716, P0717, P0721, P0722, P0875, or P0876 is not active.

CONDITIONS FOR SETTING THE DTC:
DTC P0777 sets when the TCM detects an incorrect off-going ratio, range-to-range, for an accumulated number of occurrences.

ACTION TAKEN WHEN THE DTC SETS:
When DTC P0777 is active, the following conditions will occur:
1. If failure occurs while in a forward range, the transmission will shift to the previous range.
2. If failure occurs while in N (Neutral) or R (Reverse), the transmission will lock in N (Neutral).
3. If the shift selector is moved to N (Neutral), the transmission will shift to N (Neutral).
4. If the shift selector is moved to R (Reverse), the transmission will shift to R (Reverse) or N (Neutral) (some cases may lock in N (Neutral)).
5. If the shift selector is returned to a forward range and the transmission is compromised by overspeeding or a direction change, the transmission will shift to N (Neutral).

DTC P0777 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:
This DTC indicates the off-going clutch being controlled by solenoid B is not released or is released too slowly. This could indicate a leak or obstruction in a specific clutch apply circuit. Check the Scan Tool Failure Record data for previous or current range information when the DTC was set to determine the specific shift when the DTC was set. Refer to the Solenoid and Clutch information to determine which clutch circuit is suspect.

If the condition is intermittent, connect the Scan Tool and select the speed sensor indicated by the code. If the signal is erratic, investigate and eliminate the following:
1. Intermittent wiring connection
2. Excessive vibration (driveline or engine torsionals)
3. Irregular sensor gap (loose sensor, loose tone wheel, or damaged tone wheel)

Inspect and confirm that the OEM engine rating does not exceed the transmission model rating. Also inspect for the presence of an add-on engine power package or module. Whenever the engine horsepower or torque is increased over the transmission factory rating, a shift flare condition may occur leading to the diagnostic code indicated.

NOTE: Clutch failure due to an OEM engine rating exceeding the OEM transmission rating, or the installation of a engine power package or module will not be covered under the OEM transmission warranty.