P0777

ALLISON AUTOMATIC TRANSMISSION

DTC P0777





Test 1-2





Test 3-11

Circuit Description for Allison Automatic Transmission

Important: DTC P0777 was previously P1724.

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 the Solenoid B will vary depending on the shift being made.
DTC P0777 is a type A DTC.

Conditions for Running the DTC
^ The components are powered and ignition voltage is greater than 9 V and less than 18 V.
^ One of the following conditions occur:
- Output speed is greater than 200 RPM.
- Turbine speed is greater than 200 RPM.

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 failure occurs in a forward range, the transmission will go to the previous range. If failure occurs in neutral or reverse, the transmission will lock in NEUTRAL while diagnostic response is active.
^ If the shift selector is moved to NEUTRAL, then the transmission will shift to NEUTRAL.
^ If the shift selector is moved to REVERSE, then the transmission will shift to REVERSE or NEUTRAL and in some cases may lock in NEUTRAL.
^ If the shift selector is returned to a forward range and the transmission is compromised by overspending or direction change, the transmission will shift to NEUTRAL.
^ DTC P0777 will be stored in the TCM history.
^ The TCM freezes shift adapts.
^ Illuminates the MIL when active.
^ TCM inhibits TCC engagement.

Conditions for Clearing the MIL/DTC
A scan tool can 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 failure.

Diagnostic Aids
^ This DTC indicates the on-coming clutch being controlled by the Solenoid B is not released or is released too slowly. This could indicate a leak or obstruction in a specific clutch apply circuit. Check scan tool failure record data for previous or current range information when the DTC was set to determine specific shift when the DTC was set.

Important: Clutch failure due to installation of an engine power upgrade is not covered under the manufacturers warranty.

^ Inspect for the presence of an add-on engine power package. When engine horsepower or torque is increased over factory rating, a shift flare condition may occur.

Test Description
The numbers below refer to the step numbers on the diagnostic table.
3. This step tests the ignition voltage.
4. This step tests for correct speed sensor operation.
5. This step tests for internal hydraulic leakage.
6. This stem tests for clutch capacity