P0777

ALLISON AUTOMATIC TRANSMISSION

DTC P0777

DTC P0777 Step 1 - 9:

DTC P0777 Step 10 - 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 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 9V 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
^ The TCM illuminates the malfunction indictor lamp (MIL).
^ 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, the transmission will shift to NEUTRAL.
^ If the shift selector is moved to REVERSE, 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.
^ 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
^ Inspect the wiring for poor electrical connections at the TCM. Look for the following conditions:
- A bent terminal
- A backed-out terminal
- A damaged terminal
- Poor terminal tension
- A chafed wire
- A broken wire inside the insulation.
^ When diagnosing for an intermittent short or open, massage the wiring harness while watching the test equipment for a change.
^ You may have to drive the vehicle in order to experience a fault.
^ This DTC indicates the off-going clutch being controlled by the solenoid B is not releasing or is slow to release. This could indicate a leak or obstruction in a specific clutch apply circuit. To determine the specific shift during which the DTC was set, refer to the scan tool failure record which contains previous or current range information.

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 refer to the step numbers on the diagnostic table.
3. This step tests the ignition voltage.
4. This step tests for erratic speed sensor readings or signal dropout.
5. This step tests for internal hydraulic leakage.
6. This step tests for clutch capacity.