P0778
ALLISON AUTOMATIC TRANSMISSIONDTC P0778
Test 1-6
Test 7-12
Circuit Description for Allison Automatic Transmission
Trim Solenoid B is used to control on-coming, off-going, and holding pressure to any of the 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 solenoid operates using a very high frequency pulse width modulation signal of 1000 Hz. Unlike the pulse width modulated Torque Converter Clutch (TCC) solenoid, where the ball follows the pulse width square wave, the PPC ball remains stationary. The ball clearance is balanced between a coil current force on one side of the ball and the clutch trim pressure force on the other side of the ball. The ball-to-seat clearance adjusts the bleed-off rate in the signal pocket. At high bleed levels, the resulting flow across the feed orifice results in a high pressure drop. This produces the desired signal pressure level used to trim the clutch.
DTC P0778 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.
Conditions for Setting the DTC
DTC P0778 sets when the TCM detects any of the following conditions:
^ A short to power
^ A short to ground
^ High or low resistance at a solenoid
^ High or low resistance in wiring
Action Taken When the DTC Sets
^ When in a forward range, the transmission will fail to 1st, 3rd, or 5th range 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.
^ If the shift selector is returned to a forward range and the transmission is compromised by overspending or direction change, then the transmission will lock in NEUTRAL.
^ DTC P0778 will be stored in the TCM history.
^ The TCM freezes shift adapts (DNA).
^ 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
^ 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.
^ Solenoids are sensitive to battery voltage spikes and poor ground connections. This condition might not be indicated with your Scan Tool, or even with a DVOM. The ideal way to test for these conditions would be to attach an oscilloscope.
^ If this DTC appears to be temperature-related, suspect a defective solenoid. A failing solenoid may be temperature sensitive causing resistance values to fluctuate.
Test Description
The numbers below refer to the step numbers on the diagnostic table.
2. This step tests ignition voltage.
3. This step tests the command signal from the TCM, and tests the wiring harness for open or shorts.
4. This step tests for resistance of harness and solenoid.
5. This step tests the command signal from the TCM.
6. This step tests the resistance value of the solenoid and tests the internal wiring harness for open or shorts.
8. This step tests the resistance value of the solenoid.