Welcome to Just Answer. We are working on a fast accurate answer Based on the info you have provided. Your time and my time are both Very important. If you have replace d all of the componants in the testing the wiring harness is bad. They do get rattled around. Would you like testing for the code so you can go over it. This is usually a wiring issue code any way.

P0778

ALLISON AUTOMATIC TRANSMISSION

DTC P0778
Circuit Description for Allison Automatic Transmission
Pressure control (PC) solenoid B is used to control oncoming, off-going, and holding pressure to any one 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 1,000 Hz. Unlike the torque converter clutch pulse width modulated (TCC PWM) 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.

DTC Descriptor
This diagnostic procedure supports the following DTC:
DTC P0778 Pressure Control Solenoid B Electrical

Conditions for Running the DTC
The components are powered and ignition voltage is greater than 9.0 volts and less than 18 volts.

Conditions for Setting the DTC
DTC P0778 sets when the transmission control module (TCM) detects one of the following conditions:

A short to power
A short to ground
A high or low resistance in PC solenoid B
A high or low resistance in PC solenoid B wiring
Action Taken When the DTC Sets

The TCM illuminates the malfunction indicator lamp (MIL) .
When in a forward range, the transmission fails to 1st, 3rd, or 5th range 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, unless the transmission is compromised by a direction change, then the transmission will shift to or lock in NEUTRAL.
If the shift selector is returned to a forward range and the transmission is compromised by over speeding or direction change, the transmission will lock in NEUTRAL.
DTC P0778 will be stored in TCM history.
The TCM freezes shift adapts.
The TCM inhibits TCC engagement.
Conditions for Clearing the DTC
A scan tool can clear the code from TCM history. The TCM automatically clears the DTC from TCM history if the vehicle completes 40 warm-up cycles without failure.

Diagnostic Aids

Inspect the wiring for poor electrical connections at the TCM. Inspect 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 condition.
Solenoids are sensitive to battery voltage spikes and poor ground connections. This condition may not be indicated with your scan tool, or with a DVOM.
The installation of aftermarket devices such as remote starters may interfere with proper TCM voltage supply.
A faulty ignition switch may cause improper voltage supply to the TCM.
Power or ground corrosion may cause voltage variation.
Voltage may vary by 0.5 volts based on the state of charge in the battery.
PC solenoid B operates on 1,000 Hz and can be measured using a DMM. Refer to Measuring Frequency.
Test Description

The numbers below refer to the step numbers on the diagnostic table.

This step tests ignition voltage to the TCM.
This step tests the supply voltage to the TCM.
This step tests the resistance value of the external harness and solenoid.
This step tests the resistance value of the solenoid, and tests the internal wiring harness for opens or shorts.
This step tests the resistance value of the solenoid.

Click here for testing and here I am pointing at internal wiring harness. Bookmark my profile and I will save you money. I will help with test if needed. It best to best . Ok I see the internal wiring harness was already replaced

Thanks Daniel

It is probably a ground wire or connection diff get the volt ohm meter out and follow instructions. let me know if you need any diagrams.