Allison

Allison 1000 & 2000 Gen 4 Fault Codes: P0746 Solenoid A Controlled Clutch Stuck Off

CIRCUIT DESCRIPTION:
Trim solenoid A is used to control on-coming, off-going, and holding pressure in any one of five clutches. This solenoid is referred to as a Pressure Proportional to Current (PPG) 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 slipping. The clutch being controlled by the solenoid A will vary depending on the shift that is being completed.

CONDITIONS FOR RUNNING THE DTC:
1. The components are powered and ignition voltage is greater than 9V and less than 18V (12V TCM) or greater than 18V and less than 32V (24V TCM).
2. Turbine speed is greater than 60 rpm.
3. Output speed is greater than 125 rpm.
4. Transmission is at normal operating temperature.
5. DTC P0708, P0716, P0717, P0721, P0722, P0875, or P0876 is not active.

CONDITIONS FOR SETTING THE DTC:
DTC P0746 sets when the TCM detects an incorrect oncoming ratio (range-to-range) for an accumulated number of occurrences.

ACTION TAKEN WHEN THE DTC SETS:
When DTC P0746 is active, the following conditions will occur:
1. If failure occurs while in a forward range, the transmission will shift to the previous range.
2. If failure occurs while in N (Neutral) or R (Reverse), the transmission will lock in N (Neutral).
3. If the shift selector is moved to N (Neutral), the transmission will shift to N (Neutral) (some cases may lock in N (Neutral)).
4. If the shift selector is moved to R (Reverse), the transmission will shift to R (Reverse) or N (Neutral).
5. If the shift selector is returned to a forward range and the transmission is compromised by overspeeding or a direction change, the transmission will shift to N (Neutral).

DTC P0746 is stored in the TCM history.

The CHECK TRANS light illuminates.

The TCM freezes shift adapts (DNA).

The TCM inhibits TCC engagement.

CONDITIONS FOR CLEARING THE DTC/CHECK TRANS LIGHT:
A Scan Tool may be used to 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 the DTC recurring.

DIAGNOSTIC AIDS:
This DTC indicates the on-coming clutch being controlled by solenoid A is not applied or applied too slowly. This could indicate a leak or obstruction in a specific clutch apply circuit. Check the Scan Tool Failure Record data for previous or current range information when the DTC was set to determine the specific shift when the DTC was set. Refer to solenoid and clutch information to determine which clutch circuit is suspect.

If the condition is intermittent, connect the Scan Tool and select the speed sensor indicated by the code. If the signal is erratic, investigate and eliminate the following:
1. Intermittent wiring connection.
2. Excessive vibration (driveline or engine torsionals).
3. Irregular sensor gap (loose sensor, loose tone wheel, or damaged tone wheel).

Inspect and confirm that the OEM engine rating does not exceed the transmission model rating. Also inspect for the presence of an add-on engine power package or module. Whenever the engine horsepower or torque is increased over the transmission factory rating, a shift flare condition may occur leading to the diagnostic code indicated.

NOTE: Clutch failure due to an OEM engine rating exceeding the OEM transmission rating, or the installation of an engine power package or module will not be covered under the OEM transmission warranty.

Allison 1000 & 2000 Gen 4 Fault Codes: P0747 Solenoid A Controlled Clutch Stuck On

CIRCUIT DESCRIPTION:
Trim solenoid A is used to control on-coming, off-going, and holding pressure to any one of the five clutches. This solenoid is referred to as a Pressure Proportional to Current (PPG) 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 3 clutches are applied. The clutch being controlled by solenoid A will vary depending on the shift.

CONDITIONS FOR RUNNING THE DTC:
The components are powered and ignition voltage is greater than 9V and less than 18V (12V TCM) or greater than 18V and less than 32V (24V TCM).

One of the following conditions occurs:
1. Output speed is greater than 200 rpm.
2. Turbine speed is greater than 200 rpm.

DTC P0708, P0716, P0717, P0721, P0722, P0875, or P0876 is not active.

CONDITIONS FOR SETTING THE DTC:
DTC P0747 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 DTC P0747 is active, the following conditions will occur:
1. If failure occurs while in a forward range, the transmission will shift to the previous range.
2. If failure occurs while in N (Neutral) or R (Reverse), the transmission will lock in N (Neutral).
3. If the shift selector is moved to N (Neutral), the transmission will shift to N (Neutral).
4. If the shift selector is moved to R (Reverse), the transmission will shift to R (Reverse) or N (Neutral) (some cases may lock in N (Neutral)).
5. If the shift selector is returned to a forward range and the transmission is compromised by overspeeding or a direction change, the transmission will shift to N (Neutral).

DTC P0747 is stored in the TCM history.

The CHECK TRANS light illuminates.

The TCM freezes shift adapts (DNA).

The TCM inhibits TCC engagement.

CONDITIONS FOR CLEARING THE DTC/CHECK TRANS LIGHT:
A Scan Tool may be used to 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 the DTC recurring.

DIAGNOSTIC AIDS:
This DTC indicates the off-going clutch being controlled by solenoid A is not releasing or is slow to release. This could indicate a leak or obstruction in a specific clutch apply circuit. Check the Scan Tool Failure Record data for previous or current range information when the DTC was set to determine the specific shift when the DTC was set. Refer to solenoid and clutch information to determine which clutch circuit is suspect.

If the condition is intermittent, connect the Scan Tool and select the speed sensor indicated by the code. If the signal is erratic, investigate and eliminate the following:
1. Intermittent wiring connection
2. Excessive vibration (driveline or engine torsionals).
3. Irregular sensor gap (loose sensor, loose tone wheel, or damaged tone wheel).

Inspect and confirm that the OEM engine rating does not exceed the transmission model rating. Also inspect for the presence of an add-on engine power package or module. Whenever the engine horsepower or torque is increased over the transmission factory rating, a shift flare condition may occur leading to the diagnostic code indicated.

NOTE: Clutch failure due to an OEM engine rating exceeding the OEM transmission rating, or the installation of an engine power package or module will not be covered under the OEM transmission warranty.

Allison 1000 & 2000 Gen 4 Fault Codes: P0748 Pressure Control Solenoid A Electrical

CIRCUIT DESCRIPTION:
Trim solenoid A is used to control on-coming, 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 pressure output is proportional to the amount of controlled current commanded by the TCM.

The solenoid operates at a very high 1000 Hz frequency. Unlike the pulse width modulated Torque Converter Clutch (TCC) solenoid, where the ball follows the pulse width square wave, the PPC ball remains stationary due to the high frequency at which the solenoid operates. This allows the ball to move in a linear up and down motion proportional to the current commanded from the TCM. This supplies the desired signal pressure to control the trim valve.

CONDITIONS FOR RUNNING THE DTC:
1. The components are powered and ignition voltage is greater than 9V and less than 18V (12V TCM) or greater than 18V and less than 32V (24V TCM).
2. Engine crank time is under 4 seconds.

CONDITIONS FOR SETTING THE DTC:
DTC P0748 is set when one of the following conditions is detected for 125 milliseconds:
1. Open circuit – TCM commanded duty cycle between 31 percent and 87 percent with no current present at trim solenoid A.
2. Short to ground – TCM commanded duty cycle is over 87 percent with a current of less than 1.0 ampere at trim solenoid A.
3. Short to power – TCM commanded duty cycle is under 15 percent with electrical current present.
4. Whenever a P0748, P0778 combination is set in failure records, this is generally caused by having the transmission harness disconnected at the main transmission connector while the vehicle ignition is ON. Check the connection at the transmission and clear codes.

ACTION TAKEN WHEN THE DTC SETS:
When DTC P0748 is active, the following conditions will occur:
1. If failure occurs while in a forward range, the transmission shifts to 1st, 3rd, or 5th range.
2. If the shift selector is moved to N (Neutral), the transmission will shift to N (Neutral).
3. If the shift selector is moved to R (Reverse), the transmission will shift to R (Reverse) unless the transmission is compromised by a direction change; then the transmission will shift to or lock in N (Neutral).
4. If the shift selector is returned to a forward range and the transmission is compromised by overspeeding or a direction change, the transmission will lock in N (Neutral).

DTC P0748 is stored in the TCM history.

The CHECK TRANS light illuminates.

The TCM freezes shift adapts (DNA).

CONDITIONS FOR CLEARING THE DTC/CHECK TRANS LIGHT:
A Scan Tool may be used to 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 the DTC recurring.

DIAGNOSTIC AIDS:
The diagnostic test performed to detect this DTC is very sensitive. Therefore, there is a high probability that an intermittent circuit condition may be causing this DTC to set. Check for the following conditions at the OEM harness first, then at the transmission internal harness.

Inspect the wiring for poor electrical connections at the TCM connector and the transmission main connector. Look for the following conditions:
1. A bent terminal.
2. A backed-out terminal.
3. A damaged terminal.
4. Poor terminal tension.
5. A chafed wire.
6. A broken wire inside the insulation.
7. Inspect OEM wiring harness routing, looking for possible contact points where chafing could occur. Moving parts on the vehicle could be contacting the harness. Check for contact at the parking brake drum, suspension components, transmission shift linkage, etc.
8. Inspect the internal transmission wiring harness for possible contact areas where chafing may occur.

When diagnosing for an intermittent short or open, massage the wiring harness while watching the test equipment for a change. It may be necessary to check for shorting to ground at individual wires within a harness to isolate an intermittent condition. Refer to Wire Check information.

You may have to drive the vehicle in order to experience a fault. Use the data obtained from failure records to determine transmission range and/or certain vehicle operating variables such as temperature, run time etc. This data can be useful in reproducing the failure mode where the DTC was set.

If this DTC appears to be temperature-related, suspect a defective solenoid. A failing solenoid may be temperature sensitive causing resistance values to fluctuate. This may cause an intermittent DTC to be set.

Allison 1000 & 2000 Gen 4 Fault Codes: P0763 Shift Solenoid C Electrical

CIRCUIT DESCRIPTION:
Shift solenoid C is a normally closed (N/C) solenoid that provides control main pressure to stroke the C shift valve. The TCM determines the proper solenoid command logic to move the C shift valve to attain a particular range requested. A pressure switch, located at the end of the shift valve, sends shift valve position feedback to the TCM.

CONDITIONS FOR RUNNING THE DTC:
1. The components are powered and ignition voltage is greater than 9V and less than 18V (12V TCM) or greater than 18V and less than 32V (24V TCM).
2. TCM initialization is in process or engine speed is greater than 200 rpm and less than 7500 rpm for 5 seconds.

CONDITIONS FOR SETTING THE DTC:
DTC P0763 is set when the TCM detects one of the following conditions for more than 100 milliseconds:
1. Open circuit or short to ground detected when C solenoid is commanded OFF.
2. Short to power is detected when C solenoid is commanded ON.

ACTION TAKEN WHEN THE DTC SETS:
When DTC P0763 is active, the following conditions will occur:
1. If the failure occurs while in a forward range, the transmission shifts to 1st, 3rd, or 5th range.
2. If the shift selector is moved to N (Neutral), the transmission will shift to neutral.
3. If the shift selector is moved to R (Reverse), the transmission shifts to neutral or reverse.
4. If the shift selector is moved to a forward range or reverse and transmission is compromised by overspeeding or direction change, transmission shifts to neutral.
5. If this failure is present at the solenoid electrical feed wire 231, the response is hydraulic default.

DTC P0763 is stored in TCM history.

The CHECK TRANS light illuminates on first occurrence.

The TCM freezes shift adapts (DNA).

The TCM inhibits TCC engagement.

CONDITIONS FOR CLEARING THE DTC/CHECK TRANS LIGHT:
A scan tool may be used to 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 the DTC recurring.

DIAGNOSTIC AIDS:
The diagnostic test performed to detect this DTC is very sensitive. Therefore, there is a high probability that an intermittent circuit condition may be causing this DTC to set. Make sure you check for the following conditions at the OEM harness first and then at the transmission internal harness.

Inspect the wiring for poor electrical connections at the TCM connector and the transmission main connector. Look for the following conditions:
1. A bent terminal.
2. A backed-out terminal.
3. A damaged terminal.
4. Poor terminal tension.
5. A chafed wire.
6. A broken wire inside the insulation.
7. Inspect OEM wiring harness routing, looking for possible contact points where chafing could occur. Moving parts on the vehicle could be contacting the harness. Check for contact at the parking brake drum, suspension components, transmission shift linkage etc.
8. Inspect the internal transmission wiring harness for possible contact areas where chafing may occur.

When diagnosing for an intermittent short or open, massage the wiring harness while watching the test equipment for a change. It may be necessary to check for shorting to ground at individual wires within a harness to isolate an intermittent condition. Refer to Wire Check information.

You may have to drive the vehicle in order to experience a fault. Use the data obtained from failure records to determine transmission range and/or certain vehicle operating variables such as temperature, run time, etc. This data can be useful in reproducing the failure mode where DTC was set.

If the DTC appears to be temperature related, suspect a defective shift solenoid. It is possible for a shift solenoid to be temperature sensitive causing resistance values to fluctuate. This may cause an intermittent
DTC to be set.

If the circuit problem (open or short) is present at the 231 solenoid feed wire, other shift solenoid electrical DTCs may be present (C, D, E electrical codes).

Allison 1000 & 2000 Gen 4 Fault Codes: P0768 Shift Solenoid D Electrical

CIRCUIT DESCRIPTION:
Shift solenoid D is a normally closed (N/C) solenoid that provides control main pressure to stroke the D shift valve. The TCM determines the proper solenoid command logic to move the D shift valve to attain a particular range requested. A pressure switch, located at the end of the shift valve, sends shift valve position feedback to the TCM.

CONDITIONS FOR RUNNING THE DTC:
1. The components are powered and ignition voltage is greater than 9V and less than 18V (12V TCM) or greater than 18V and less than 32V (24V TCM).
2. TCM initialization is in process or engine speed is greater than 200 rpm and less than 7500 rpm for 5 seconds.

CONDITIONS FOR SETTING THE DTC:
DTC P0768 is set when the TCM detects one of the following conditions for more than 100 milliseconds:
1. Open circuit or short to ground detected when D solenoid is commanded OFF.
2. Short to power is detected when D solenoid is commanded ON.

ACTION TAKEN WHEN THE DTC SETS:
When DTC P0768 is active, the following conditions will occur:
1. If the failure occurs while in a forward range, the transmission shifts to 1st, 3rd, or 5th range.
2. If the shift selector is moved to N (Neutral), the transmission will shift to neutral.
3. If the shift selector is moved to R (Reverse), the transmission shifts to neutral or reverse.
4. If the shift selector is moved to a forward range or reverse and transmission is compromised by overspeeding or direction change, transmission shifts to neutral.
5. If this failure is present at the solenoid electrical feed wire 231, the response is hydraulic default.

DTC P0768 is stored in TCM history.

The CHECK TRANS light illuminates on first occurrence.

The TCM freezes shift adapts (DNA).

The TCM inhibits TCC engagement.

CONDITIONS FOR CLEARING THE DTC/CHECK TRANS LIGHT:
A scan tool may be used to 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 the DTC recurring.

DIAGNOSTIC AIDS:
The diagnostic test performed to detect this DTC is very sensitive. Therefore, there is a high probability that an intermittent circuit condition may be causing this DTC to set. Make sure you check for the following conditions at the OEM harness first and then at the transmission internal harness.

Inspect the wiring for poor electrical connections at the TCM connector and the transmission main connector. Look for the following conditions:
1. A bent terminal.
2. A backed-out terminal.
3. A damaged terminal.
4. Poor terminal tension.
5. A chafed wire.
6. A broken wire inside the insulation.
7. Inspect OEM wiring harness routing, looking for possible contact points where chafing could occur. Moving parts on the vehicle could be contacting the harness. Check for contact at the parking brake drum, suspension components, transmission shift linkage etc.
8. Inspect the internal transmission wiring harness for possible contact areas where chafing may occur.

When diagnosing for an intermittent short or open, massage the wiring harness while watching the test equipment for a change. It may be necessary to check for shorting to ground at individual wires within a harness to isolate an intermittent condition. Refer to Wire Check information.

You may have to drive the vehicle in order to experience a fault. Use the data obtained from failure records to determine transmission range and/or certain vehicle operating variables such as temperature, run time, etc. This data can be useful in reproducing the failure mode where DTC was set.

If the DTC appears to be temperature related, suspect a defective shift solenoid. It is possible for a shift solenoid to be temperature sensitive causing resistance values to fluctuate. This may cause an intermittent DTC to be set.

If the circuit problem (open or short) is present at the 231 solenoid feed wire, other shift solenoid electrical DTCs may be present (C, D, E electrical codes).

Allison 1000 & 2000 Gen 4 Fault Codes: P0773 Shift Solenoid E Electrical

CIRCUIT DESCRIPTION:
Shift solenoid E is a normally closed (N/C) solenoid that provides control main pressure to stroke the E shift valve. The TCM determines the proper solenoid command logic to move the E shift valve to attain a particular range requested. A pressure switch, located at the end of the shift valve, sends shift valve position feedback to the TCM.

CONDITIONS FOR RUNNING THE DTC:
1. The components are powered and ignition voltage is greater than 9V and less than 18V (12V TCM) or greater than 18V and less than 32V (24V TCM).
2. TCM initialization is in process or engine speed is greater than 200 rpm and less than 7500 rpm for 5 seconds.

CONDITIONS FOR SETTING THE DTC:
DTC P0773 is set when the TCM detects one of the following conditions for more than 100 milliseconds:
1. Open circuit or short to ground detected when E solenoid is commanded OFF.
2. Short to power is detected when E solenoid is commanded ON.

ACTION TAKEN WHEN THE DTC SETS:
When DTC P0773 is active, the following conditions will occur:
1. If the failure occurs while in a forward range, the transmission shifts to 1st, 3rd, or 5th range.
2. If the shift selector is moved to N (Neutral), the transmission will shift to neutral.
3. If the shift selector is moved to R (Reverse), the transmission shifts to neutral or reverse.
4. If the shift selector is moved to a forward range or reverse and transmission is compromised by overspeeding or direction change, transmission shifts to neutral.
5. If this failure is present at the solenoid electrical feed wire 231, the response is hydraulic default.

DTC P0773 is stored in TCM history.

The CHECK TRANS light illuminates on first occurrence.

The TCM freezes shift adapts (DNA).

The TCM inhibits TCC engagement.

CONDITIONS FOR CLEARING THE DTC/CHECK TRANS LIGHT:
A scan tool may be used to 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 the DTC recurring.

DIAGNOSTIC AIDS:
The diagnostic test performed to detect this DTC is very sensitive. Therefore, there is a high probability that an intermittent circuit condition may be causing this DTC to set. Make sure you check for the following conditions at the OEM harness first and then at the transmission internal harness.

Inspect the wiring for poor electrical connections at the TCM. Look for the following conditions:
1. A bent terminal.
2. A backed-out terminal.
3. A damaged terminal.
4. Poor terminal tension.
5. A chafed wire.
6. A broken wire inside the insulation.
7. Inspect OEM wiring harness routing, looking for possible contact points where chafing could occur. Moving parts on the vehicle could be contacting the harness. Check for contact at the parking brake drum, suspension components, transmission shift linkage etc.
8. Inspect the internal transmission wiring harness for possible contact areas where chafing may occur.

When diagnosing for an intermittent short or open, massage the wiring harness while watching the test equipment for a change. It may be necessary to check for shorting to ground at individual wires within a harness to isolate an intermittent condition. Refer to Wire Check information.

You may have to drive the vehicle in order to experience a fault. Use the data obtained from failure records to determine transmission range and/or certain vehicle operating variables such as temperature, run time, etc. This data can be useful in reproducing the failure mode where DTC was set.

If the DTC appears to be temperature related, suspect a defective shift solenoid. It is possible for a shift solenoid to be temperature sensitive causing resistance values to fluctuate. This may cause an intermittent DTC to be set.

If the circuit problem (open or short) is present at the 231 solenoid feed wire, other shift solenoid electrical DTCs may be present (C, D, E electrical codes).

Allison 1000 & 2000 Gen 4 Fault Codes: P0776 Solenoid B Controlled Clutch Stuck Off

CIRCUIT DESCRIPTION:
Trim solenoid B is used to control on-coming, 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 Transmission Control Module (TCM) uses information from the turbine and output speed sensors to detect if a clutch is slipping. The clutch being controlled by the solenoid B will vary depending on the shift that was being completed.

CONDITIONS FOR RUNNING THE DTC:
1. The components are powered and ignition voltage is greater than 9V and less than 18V (12V TCM) or greater than 18V and less than 32V (24V TCM).
2. Turbine speed is greater than 60 rpm.
3. Output speed is greater than 125 rpm.
4. Transmission is at normal operating temperature.
5. DTC P0708, P0716, P0717, P0721, P0722, P0875, or P0876 is not active.

CONDITIONS FOR SETTING THE DTC:
DTC P0776 sets when the TCM detects an incorrect oncoming ratio (range-to-range) for an accumulated number of occurrences.

ACTION TAKEN WHEN THE DTC SETS:
When DTC P0776 is active, the following conditions will occur:
1. If failure occurs while in a forward range, the transmission will shift to the previous range.
2. If failure occurs while in N (Neutral) or R (Reverse), the transmission will lock in N (Neutral).
3. If the shift selector is moved to N (Neutral), the transmission will shift to N (Neutral) (some cases may lock in N (Neutral)).
4. If the shift selector is moved to R (Reverse), the transmission will shift to R (Reverse) or N (Neutral).
5. If the shift selector is returned to a forward range and the transmission is compromised by overspeeding or a direction change, the transmission will shift to N (Neutral).

DTC P0776 is stored in the TCM history.

The CHECK TRANS light illuminates.

The TCM freezes shift adapts (DNA).

The TCM inhibits TCC engagement.

CONDITIONS FOR CLEARING THE DTC/CHECK TRANS LIGHT:
A Scan Tool may be used to 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 the DTC recurring.

DIAGNOSTIC AIDS:
This DTC indicates the on-coming clutch being controlled by solenoid B is not applied or applied too slowly. This could indicate a leak or obstruction in a specific clutch apply circuit. Check the Scan Tool Failure Record data for previous or current range information when the DTC was set to determine the specific shift when the DTC was set. Refer to the Solenoid and Clutch information to determine which clutch circuit is suspect.

If the condition is intermittent, connect the Scan Tool and select the speed sensor indicated by the code. If the signal is erratic, investigate and eliminate the following:
1. Intermittent wiring connection
2. Excessive vibration (driveline or engine torsionals)
3. Irregular sensor gap (loose sensor, loose tone wheel, or damaged tone wheel)

Inspect and confirm that the OEM engine rating does not exceed the transmission model rating. Also inspect for the presence of an add-on engine power package or module. Whenever the engine horsepower or torque is increased over the transmission factory rating, a shift flare condition may occur leading to the diagnostic code indicated.

NOTE: Clutch failure due to an OEM engine rating exceeding the OEM transmission rating, or the installation of a engine power package or module will not be covered under the OEM transmission warranty.

Allison 1000 & 2000 Gen 4 Fault Codes: P0777 Solenoid B Controlled Clutch Stuck On

CIRCUIT DESCRIPTION:
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.

CONDITIONS FOR RUNNING THE DTC:
The components are powered and ignition voltage is greater than 9V and less than 18V (12V TCM) or greater than 18V and less than 32V (24V TCM).

One of the following conditions occurs:
1. Output speed is greater than 200 rpm.
2. Turbine speed is greater than 200 rpm.

DTC P0708, P0716, P0717, P0721, P0722, P0875, or P0876 is not active.

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 DTC P0777 is active, the following conditions will occur:
1. If failure occurs while in a forward range, the transmission will shift to the previous range.
2. If failure occurs while in N (Neutral) or R (Reverse), the transmission will lock in N (Neutral).
3. If the shift selector is moved to N (Neutral), the transmission will shift to N (Neutral).
4. If the shift selector is moved to R (Reverse), the transmission will shift to R (Reverse) or N (Neutral) (some cases may lock in N (Neutral)).
5. If the shift selector is returned to a forward range and the transmission is compromised by overspeeding or a direction change, the transmission will shift to N (Neutral).

DTC P0777 is stored in the TCM history.

The CHECK TRANS light illuminates.

The TCM freezes shift adapts (DNA).

The TCM inhibits TCC engagement.

CONDITIONS FOR CLEARING THE DTC/CHECK TRANS LIGHT:
A Scan Tool may be used to 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 the DTC recurring.

DIAGNOSTIC AIDS:
This DTC indicates the off-going clutch being controlled by solenoid B is not released or is released too slowly. This could indicate a leak or obstruction in a specific clutch apply circuit. Check the Scan Tool Failure Record data for previous or current range information when the DTC was set to determine the specific shift when the DTC was set. Refer to the Solenoid and Clutch information to determine which clutch circuit is suspect.

If the condition is intermittent, connect the Scan Tool and select the speed sensor indicated by the code. If the signal is erratic, investigate and eliminate the following:
1. Intermittent wiring connection
2. Excessive vibration (driveline or engine torsionals)
3. Irregular sensor gap (loose sensor, loose tone wheel, or damaged tone wheel)

Inspect and confirm that the OEM engine rating does not exceed the transmission model rating. Also inspect for the presence of an add-on engine power package or module. Whenever the engine horsepower or torque is increased over the transmission factory rating, a shift flare condition may occur leading to the diagnostic code indicated.

NOTE: Clutch failure due to an OEM engine rating exceeding the OEM transmission rating, or the installation of a engine power package or module will not be covered under the OEM transmission warranty.

Allison 1000 & 2000 Gen 4 Fault Codes: P0778 Pressure Control Solenoid B Electrical

CIRCUIT DESCRIPTION:
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 at a very high 1000 Hz frequency. Unlike the pulse width modulated Torque Converter Clutch (TCC) solenoid, where the ball follows the pulse width square wave, the PPC ball remains stationary due to the high frequency at which the solenoid operates. This allows the ball to move in a linear up and down motion proportional to the current commanded from the TCM. This supplies the desired signal pressure to control the trim valve.

CONDITIONS FOR RUNNING THE DTC:
1. The components are powered and ignition voltage is greater than 9V and less than 18V (12V TCM) or greater than 18V and less than 32V (24V TCM).
2. Engine crank time is under 4 seconds.

CONDITIONS FOR SETTING THE DTC:
DTC P0748 is set when one of the following conditions is detected for 125 milliseconds.
1. Open circuit – TCM commanded duty cycle between 31 percent and 87 percent with no current present at trim solenoid A.
2. Short to ground – TCM commanded duty cycle is over 87 percent with a current of less than 1.0 ampere at trim solenoid A.
3. Short to power – TCM commanded duty cycle is under 15 percent with electrical current present.
4. Whenever a P0748, P0778 combination is set in failure records, this is generally caused by having the transmission harness disconnected at the main transmission connector while the vehicle ignition is ON. Check the connection at the transmission and clear codes.

ACTION TAKEN WHEN THE DTC SETS:
When DTC P0778 is active, the following conditions will occur:
1. If the failure occurs while in a forward range, the transmission will shift to tst, 3rd, or 5th range.
2. If the shift selector is moved to N (Neutral), the transmission will shift to neutral.
3. If the shift selector is moved to R (Reverse), the transmission will shift to reverse.
4. If the shift selector is moved to a forward range or reverse and the transmission is compromised by overspeeding or a direction change, the transmission shifts to neutral.

DTC P0778 is stored in the TCM history.

The CHECK TRANS light illuminates.

The TCM freezes shift adapts (DNA).

CONDITIONS FOR CLEARING THE DTC/CHECK TRANS LIGHT:
A Scan Tool may be used to 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 the DTC recurring.

DIAGNOSTIC AIDS:
The diagnostic test performed to detect this DTC is very sensitive. Therefore, there is a high probability that an intermittent circuit condition may be causing this DTC to set. Check for the following conditions at the OEM harness first, then at the transmission internal harness.

Inspect the wiring for poor electrical connections at the TCM connector and the main transmission connector. Look for the following conditions:
1. A bent terminal.
2. A backed-out terminal.
3. A damaged terminal.
4. Poor terminal tension.
5. A chafed wire.
6. A broken wire inside the insulation.
7. Inspect OEM wiring harness routing, looking for possible contact points where chafing could occur. Moving parts on the vehicle could be contacting the harness. Check for contact at the parking brake drum, suspension components, transmission shift linkage, etc.
8. Inspect the internal transmission wiring harness for possible contact areas where chafing may occur.

When diagnosing for an intermittent short or open, massage the wiring harness while watching the test equipment for a change. It may be necessary to check for shorting to ground at individual wires within a harness to isolate an intermittent condition. Refer to Wire Check information.

You may have to drive the vehicle in order to experience a fault. Use the data obtained from failure records to determine transmission range and/or certain vehicle operating variables such as temperature, run time etc. This data can be useful in reproducing the failure mode where the DTC was set.

If this DTC appears to be temperature-related, suspect a defective trim solenoid. It is possible for a trim solenoid to be temperature sensitive causing resistance values to fluctuate. This may cause an intermittent DTC to be set.