P0778 code - The shift pressure solenoid is activated when the transmission control module determines that a transmission gear shift required. The shift pressure solenoid is width modulated (PWM) controlled to allow the proper amount of hydraulic pressure to the shift elements. The shift pressure solenoid is only actived during the the shift of the trasmission. When the shift pressure solenoid is actived, hydraulic pressure solenoid is actived, hydraulic pressure is applied to the proper shift elements throught one of the shift solenoids in the transmission to allow the desired shift. Once the shift is completed the solenoid is turned off. When monitored this DTC when the shift pressure solenoid is off, or active with 25-75% duty cycle, and the solenoid supply circuit is active.

Set condition this DTC when shift pressure is turned on and the TCM detects any of the following in the 2-3 solenoid control circuit open, short to ground, short to voltage, or a faulty shift pressure control driver in the TCM.

Possible causes P0778 code:

- Shift pressure control circuit open

- Shift pressure control circuit shorted to ground

- Shift pressure control circuit shorted to another circuit

- Shift pressure solenoid

- Transmission control module (TCM)

Diagnostic test P0778 code

Step 1. Check if the DTC is current

Note : When the transmission control module (TCM) detects and stores a DTC, the TCM also stores the vehicles operating condition under which the DTC originally set. This information is located in the scan tool under environmental data. Before erasing any stored DTC, record any available data to assist in duplicating the conditions in which the DTC originally set.

Turn the ignition off to the lock position. With the scan tool, erase transmission DTCs. Using the environmental data recorded earlier, try to duplicate the conditions in which the DTC originally set. It may be necessary to test drive the vehicle. With the scan tool, transmission DTCs.

Did the DTC P0778 reset ?

Yes, go to step 2.

No, go to step 6.

Step 2. Check the Shift pressure control circuit for an open

Turn the ignition off to the lock position. Disconnect the TCM C2 harness connector. Disconnect the transmission electrohydraulic control assembly harness connector. Measure the resistance of the shift pressure control circuit between the TCM C2 harness connector and the transmission control unit assembly harness connector.

Is the resistance below 5.0 Ohms?

Yes, go to step 3.

No, repair the shift pressure control circuit for an open. Perform the transmission verification test.

Step 3. Check the Shift pressure control circuit for a short to ground

Measure the resistance between ground and the shift pressure control circuit.

Is the resistance below 5.0 Ohms?

Yes, repair the shift pressure control circuit for a short to ground.

No, go to step 4.

Step 4. Check the shift pressure control circuit for short to another circuit

Meassure the resistance between the hift pressure control circuit and every other circuit in the transmission electrohidraulic control unit assembly harness connector.

Is the resistance below 5.0 Ohms between the shift pressure control circuit and any other circuit(s) in the transmission electrohydraulic control unit assembly harness connector.

Yes, repair the shift pressure control circuit for a short to another circuits.

No, go to step 5.

Step 5. Check the shift pressure solenoid

Reconnect the transmission electrohydraulic control unit assembly harness connector. Measure the resistance of the shift pressure solenoid between the T77 shift pressure control circuit and the T78 solenoid voltage supply circuit in the TCM C2 harness connector.

Is the resistance between 2.5 and 6.5 Ohm ?

Yes, using the schematic as a guide, check the transmission control module TCM terminals for corrosion, damage, or terminal push out. Pay particular attention to all power and ground circuits. If no problems are found, replace the TCM in accordance with the service information.

No, replace the shift pressure solenoid in accordance with the service information. Perform NAG1 Transmission Verification Test.

Step 6. Check the wiring and connectors

The condition necessary to set this DTC are not present at this time. Using the schematics as a guide, inspect the wiring and connectors specific to this circuit. Wiggle the wiring and connectors while checking for any possible open or shorted circuits. Perform any service information tune-ups or technical service bulletins that may apply.

Were there any problem found ?

Yes, repair as necessary transmission verification test.

No, test complete.

P0778 Shift pressure solenoid circuit