Allison

CIRCUIT DESCRIPTION:
PCPS1 is a Pressure Proportional to Current (PPC) solenoid used to control oncoming, off-going and holding pressure to any one of five clutches.

The TCM sends control current to PCS1 from High Side Driver 1 (HSD1) via wire 111. HSD1 is continuously on unless the TCM detects a fault condition. The TCM regulates the amount of current to PCS1 by switching the low side driver on and off. Wire 155 completes the circuit between PCS1 and its low side driver. DTC P2730 indicates that the TCM processor has detected a short-to-battery condition in the low side of the electrical circuit.

ACTION TAKEN WHEN THE DTC SETS:
When DTC P2730 is active, the following conditions occur:
1. If the failure occurs while in a forward range a shift to 1st, 3rd, or 5th range is made.
2. While diagnostic response is active, if shift selector is moved to N (Neutral), transmission shifts to neutral; if the shift selector is moved to R (Reverse), transmission shifts to reverse. If the shift selector is moved to D (Drive) range or R (Reverse) and transmission is compromised by overspeeding or direction change, transmission shifts to N (Neutral).
3. The CHECK TRANS light illuminates (non-OBD II strategy).
4. The DTC is stored in the TCM history.
5. The TCM forces Variable Modulated Main off.
6. The TCM freezes shift adapts (DNA).

CONDITIONS FOR CLEARING THE DTC:
Use the diagnostic tool to clear the DTC 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 following procedures are not documented in any OEM procedures or in any OEM Troubleshooting or Service information and should not be used as a method to diagnose any transmission DTCs, function, or shift quality concern:

1. Back-probing any connections used for transmission features or functions may damage and/or unlock terminals from the back-probed connector creating permanent or intermittent shorts and/or open circuits. If possible, use the J-39700 Breakout Box, the appropriate harness adapters, and appropriate magnetic overlays to troubleshoot the vehicle.

2. Load-testing any transmission-related circuits with any other electrical devices such as vehicle lamps or relays, especially with the TCM connected to the harness. Use J-39700 Breakout Box and appropriate harnesses with the diagnostic tool to monitor the circuit performance in question unless otherwise specified in the various Troubleshooting information.

3. Piercing a wire to check for voltages, shorts-to-grounds or other wires anywhere in the circuit but especially at the TCM. This creates a leak path for moisture and damages the wire and insulation.

An intermittent circuit condition can allow this DTC to set. Make sure you 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 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.

Inspect OEM wiring harness routing. Look for possible contact points where chafing could occur leading to an open circuit condition. Moving parts on the vehicle could be contacting the harness; this includes parking brake drum, suspension components, transmission shift linkage, etc.

When diagnosing for an intermittent open condition, massage the wiring harness while watching the test equipment for a change. It may be necessary to check continuity at individual wires within a harness to isolate an intermittent circuit ground condition.

Inspect internal transmission harness wiring for possible contact areas where chafing may occur leading to a shorting to power circuit condition.

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 when the DTC was set.

CIRCUIT DESCRIPTION:
The TCC PCS is a Pressure Proportional to Current (PPC) solenoid used to control the TCC apply and release.

The TCM sends control current to solenoid TCC from High Side Driver 1 (HSD1) via wire 111. HSD1 is continuously on unless the TCM detects a fault condition. The TCM regulates the amount of current to the TCC by switching the low side driver on and off. Wire 178 completes the circuit between the TCC and its low side driver. DTC P2761 indicates that the TCM processor has detected an open condition in the electrical circuit. The open condition may exist in the high side (wire 111) or low side (wire 178).

ACTION TAKEN WHEN THE DTC SETS:
When DTC P2761 is active, the following conditions occur:
1. The CHECK TRANS light illuminates (non-OBD II strategy).
2. The DTC is stored in the TCM history.
3. TCM inhibits Torque Converter Clutch (TCC) engagement.

CONDITIONS FOR CLEARING THE DTC:
Use the diagnostic tool to clear the DTC 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 following procedures are not documented in any OEM procedures or in any OEM Troubleshooting or Service information and should not be used as a method to diagnose any transmission DTCs, function, or shift quality concern:

1. Back-probing any connections used for transmission features or functions may damage and/or unlock terminals from the back-probed connector creating permanent or intermittent shorts and/or open circuits. If possible, use the J-39700 Breakout Box, the appropriate harness adapters, and appropriate magnetic overlays to troubleshoot the vehicle.

2. Load-testing any transmission-related circuits with any other electrical devices such as vehicle lamps or relays, especially with the TCM connected to the harness. Use J-39700 Breakout Box and appropriate harnesses with the diagnostic tool to monitor the circuit performance in question unless otherwise specified in the various Troubleshooting information.

3. Piercing a wire to check for voltages, shorts-to-grounds or other wires anywhere in the circuit but especially at the TCM. This creates a leak path for moisture and damages the wire and insulation.

An intermittent circuit condition can allow this DTC to set. Make sure you check for the following conditions at the OEM harness first, then at the transmission internal harness:

DTC P2761 indicates an open in the electrical circuit for the TCC solenoid. In addition to the TCC, HSD1 also supplies power to solenoids PCS1 and Main Mod. If DTC P2761 is accompanied by DTC P0960 (Main Mod open circuit) and/or DTC P2727 (PCS1 open circuit, the open is most likely in the high side of the circuit).

When this DTC sets there can be a noticeable reduction of engine performance.

3. Inspect the wiring for poor electrical connections at the TCM 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.

Inspect OEM wiring harness routing. Look for possible contact points where chafing could occur leading to an open circuit condition. Moving parts on the vehicle could be contacting the harness; this includes parking brake drum, suspension components, transmission shift linkage, etc.

When diagnosing for an intermittent open condition, massage the wiring harness while watching the test equipment for a change. It may be necessary to check continuity at individual wires within a harness to isolate an intermittent circuit ground condition.

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 when the DTC was set.

CIRCUIT DESCRIPTION:
The TCC PCS is a Pressure Proportional to Current (PPC) solenoid used to control the TCC apply and release.

The TCM sends control current to TCC from High Side Driver 1 (HSD1) via wire 111. HSD1 is continuously on unless the TCM detects a fault condition. The TCM regulates the amount of current to the TCC by switching the low side driver on and off. Wire 178 completes the circuit between the TCC and its low side driver. DTC P2763 indicates the TCM processor has detected a short-to-battery condition in the low side of the electrical circuit.

ACTION TAKEN WHEN THE DTC SETS:
When DTC P2763 is active, the following conditions occur:
1. TCM inhibits Torque Converter Clutch (TCC) engagement.
2. The CHECK TRANS light illuminates (non-OBD II strategy).
3. The DTC is stored in the TCM history.

CONDITIONS FOR CLEARING THE DTC:
Use the diagnostic tool to clear the DTC 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 following procedures are not documented in any OEM procedures or in any OEM Troubleshooting or Service information and should not be used as a method to diagnose any transmission DTCs, function, or shift quality concern:

1. Back-probing any connections used for transmission features or functions may damage and/or unlock terminals from the back-probed connector creating permanent or intermittent shorts and/or open circuits. If possible, use the J-39700 Breakout Box, the appropriate harness adapters, and appropriate magnetic overlays to troubleshoot the vehicle.

2. Load-testing any transmission-related circuits with any other electrical devices such as vehicle lamps or relays, especially with the TCM connected to the harness. Use J-39700 Breakout Box and appropriate harnesses with the diagnostic tool to monitor the circuit performance in question unless otherwise specified in the various Troubleshooting information.

3. Piercing a wire to check for voltages, shorts-to-grounds or other wires anywhere in the circuit but especially at the TCM. This creates a leak path for moisture and damages the wire and insulation.

4. DTC P2763 indicates a short-to-battery in the electrical circuit for the TCC.

Inspect the wiring for poor electrical connections at the TCM 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.

Inspect OEM wiring harness routing. Look for possible contact points where chafing could occur leading to an open circuit condition. Moving parts on the vehicle could be contacting the harness; this includes parking brake drum, suspension components, transmission shift linkage, etc.

When diagnosing for an intermittent open condition, massage the wiring harness while watching the test equipment for a change. It may be necessary to check continuity at individual wires within a harness to isolate an intermittent circuit ground condition.

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 when the DTC was set.

CIRCUIT DESCRIPTION:
The TCC PCS is a Pressure Proportional to Current (PPC) solenoid used to control the TCC apply and release.

The TCM sends control current to TCC solenoid from High Side Driver 1 (HSD1) via wire 111. HSD1 is continuously on unless the TCM detects a fault condition. The TCM regulates the amount of current to the TCC by switching the low side driver on and off. Wire 178 completes the circuit between the TCC and its low side driver. DTC P2764 indicates that the TCM processor has detected a short-to-ground condition in the low side of the electrical circuit.

ACTION TAKEN WHEN THE DTC SETS:
When DTC P2764 is active, the following conditions occur:
1. If the failure occurs while in a forward range a shift to 1st, 3rd, or 5th range is made.
2. While diagnostic response is active, if shift selector is moved to N (Neutral), transmission shifts to neutral; if the shift selector is moved to R (Reverse), transmission shifts to reverse. If the shift selector is moved to forward range or reverse and transmission is compromised by overspeeding or direction change, transmission shifts to neutral.
3. TCM inhibits Torque Converter Clutch (TCC) engagement.
4. The CHECK TRANS light illuminates (non-OBD II strategy).
5. The DTC is stored in the TCM history.
6. The TCM forces Variable Modulated Main off.
7. The TCM freezes shift adapts (DNA).
8. Reverse operation is disabled.

CONDITIONS FOR CLEARING THE DTC:
Use the diagnostic tool to clear the DTC 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 following procedures are not documented in any OEM procedures or in any OEM Troubleshooting or Service information and should not be used as a method to diagnose any transmission DTCs, function, or shift quality concern:

1. Back-probing any connections used for transmission features or functions may damage and/or unlock terminals from the back-probed connector creating permanent or intermittent shorts and/or open circuits. If possible, use the J-39700 Breakout Box, the appropriate harness adapters, and appropriate magnetic overlays to troubleshoot the vehicle.

2. Load-testing any transmission-related circuits with any other electrical devices such as vehicle lamps or relays, especially with the TCM connected to the harness. Use J-39700 Breakout Box and appropriate harnesses with the diagnostic tool to monitor the circuit performance in question unless otherwise specified in the various Troubleshooting information.

3. Piercing a wire to check for voltages, shorts-to-grounds or other wires anywhere in the circuit but especially at the TCM. This creates a leak path for moisture and damages the wire and insulation.

4. DTC P2764 indicates a short-to-battery in the electrical circuit for the TCC solenoid.

Inspect the wiring for poor electrical connections at the TCM 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.

Inspect OEM wiring harness routing. Look for possible contact points where chafing could occur leading to an open circuit condition. Moving parts on the vehicle could be contacting the harness; this includes parking brake drum, suspension components, transmission shift linkage, etc.

When diagnosing for an intermittent open condition, massage the wiring harness while watching the test equipment for a change. It may be necessary to check continuity at individual wires within a harness to isolate an intermittent circuit ground condition.

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 when the DTC was set.

CIRCUIT DESCRIPTION:
Clutch Life Monitor is part of OEM 5th Generation Prognostics functionality. When enabled in the TCM, prognostic logic determines transmission health with clutch adaptive volumes. When the transmission is new the TCM stores the initial clutch volumes. Throughout the life of the transmission these initial volumes are compared to the current volumes and are correlated to free running clearance, allowing transmission life expectancy to be determined. If the TCM determines that clutch volumes are out of range for a specific clutch, a J1939 message is sent to a dash mounted SERVICE TRANS indicator lamp informing the driver the transmission needs inspection. If the SERVICE TRANS indicator lamp is ignored for 100 hours, a CHECK TRANS light illuminates and DTC P2789 sets.

NOTE: This feature was designed to provide advanced notification of clutch system concerns to help prevent internal component damage typically associated with clutch fatigue as well as provide the end user with an opportunity to manage downtime. This tool is not intended to replace current OEM troubleshooting tools and methods, but to act as a supplemental guide to speed the identification of specific transmission concerns.

ACTION TAKEN WHEN THE DTC SETS:
1. DTC P2789 is stored in the TCM history.
2. The CHECK TRANS light illuminates.

CONDITIONS FOR CLEARING THE DTC:
Use the diagnostic tool to clear the DTC 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:
1. Low oil level can result in unnecessary increases in clutch volumes leading to a false reading. Always verify oil level before removing the transmission.

2. It is possible for weak front pump, internal valve body, or trim solenoid leaks to falsely increase clutch volumes, triggering this code. Always inspect for secondary signs of clutch damage to confirm your diagnosis before removing the transmission for service.

3. Erratic speed traces, due to intermittent wiring concerns or defective speed sensor performance, may allow clutch volumes to increase without DTCs setting, leading to a false triggering of this DTC. If this concern is present look for all clutches to show “Not Okay”. Normally, one or two clutches would show “Not Okay” if an actual clutch system concern were indicated. Always inspect for secondary signs of clutch damage to confirm your diagnosis before removing the transmission for service.

4. This feature tests clutch volumes on C1, C2, C3, C4, and C5 only.

5. When transmission clutches are replaced, reset the TCM and clear any unadapted shifts and health prognostics.

CIRCUIT DESCRIPTION:
This DTC sets when the TCM detects no communication on the CAN1 backbone harness for a period of time.

ACTION TAKEN WHEN THE DTC SETS:
1. The CHECK TRANS light is not illuminated.
2. DTC is stored in the TCM history.
3. SEM operation is not active, if applicable.
4. Default values are substituted for missing data, e.g., engine throttle percentage, engine torque, engine coolant temperature.
5. The TCM freezes shift adapts (DNA).

CONDITIONS FOR CLEARING THE DTC:
Use the diagnostic tool to clear the DTC from the TCM history. The DTC automatically clears the DTC from the DTC history if the vehicle completes 40 warm-up cycles without the DTC recurring.

DIAGNOSTIC AIDS:
DTC U0073 indicates that a CAN1 bus hardware error has occurred. This code may be caused by:
1. Miswired or defective OEM controllers, OEM components, or OEM circuits attached to CAN 1.
2. Defective TCM.

An active DTC U0073 may prevent the diagnostic tool from communicating with the TCM. The J-47276 TCM Reflash Harness may be used to confirm that the TCM is operational. Connect the J-47276 TCM Reflash Harness to the TCM and leave the OEM harness disconnected. Provide input power from the J-42455-A Load Box.

4. CAN1 and CAN2 devices must not be interconnected because they are not compatible with each other. CAN1 operates at 250 kbps data rate and CAN2 operates at 500 kbps data rate.

5. For proper datalink communications, it is necessary to have two 120 Ω resistors installed in parallel across the CAN1 High and CAN1 Low terminals. Resistance across CAN1 High and CAN1 Low should measure 60 Ω when wired correctly.

6. The diagnostic tool Data Bus Viewer may be selected from the File Menu to monitor or log SAE J1939 datalink messages.

NOTE: The OEM transmission warranty, or the extended coverage for parts and labor, does not cover the repair or replacement of OEM or bodybuilder provided components or any external wire harnesses not provided by the OEM.

Before repairing or replacing these components or circuits that caused a transmission issue, always contact the vehicle OEM call center and/or the vehicle OEM or bodybuilder service representative to verify the warranty coverage and proper payment arrangements, arrange an alternate sublet or repair plan (if needed), and get troubleshooting assistance for the repair (such as vehicle schematics, DTC trees, parts assistance, etc.).

CIRCUIT DESCRIPTION:
This DTC sets when the TCM detects no communication on the CAN2 backbone harness for a period of time.

ACTION TAKEN WHEN THE DTC SETS:
1. The CHECK TRANS light is not illuminated.
2. DTC is stored in the TCM history.
3. SEM operation is not active, if applicable.
4. Default values are substituted for missing data, e.g., engine throttle percentage, engine torque, engine coolant temperature.
5. The TCM freezes shift adapts (DNA).

CONDITIONS FOR CLEARING THE DTC:
Use the diagnostic tool to clear the DTC from the TCM history. The DTC automatically clears the DTC from the DTC history if the vehicle completes 40 warm-up cycles without the DTC recurring.

DIAGNOSTIC AIDS:
DTC U0074 indicates that a CAN2 bus hardware error has occurred. This code may be caused by:
1. Miswired or defective OEM controllers, OEM components, or OEM circuits attached to CAN1.
2. Defective TCM.

An active DTC U0074 may prevent the diagnostic tool from communicating with the TCM. The J-47276 TCM Reflash Harness may be used to confirm that the TCM is operational. Connect the J-47276 TCM Reflash Harness to the TCM and leave the OEM harness disconnected. Provide input power from the J-42455-A Load Box.

4. CAN1 and CAN2 devices must not be interconnected because they are not compatible with each other.

5. For proper datalink communications, it is necessary to have two 120 Ω resistors installed in parallel across the CAN2 High and CAN2 Low terminals. Resistance across CAN2 High and CAN2 Low should measure 60 Ω when wired correctly.

6. The diagnostic tool Data Bus Viewer may be selected from the File Menu to monitor or log SAE J 1939 datalink messages.

The OEM transmission warranty, or the extended coverage for parts and labor, does not cover the repair or replacement of OEM or bodybuilder provided components or any external wire harnesses not provided by the OEM.

Before repairing or replacing these components or circuits that caused a transmission issue, always contact the vehicle OEM call center and/or the vehicle OEM or bodybuilder service representative to verify the warranty coverage and proper payment arrangements, arrange an alternate sublet or repair plan (if needed), and get troubleshooting assistance for the repair (such as vehicle schematics, DTC trees, parts assistance, etc.).

CIRCUIT DESCRIPTION:
This test detects the loss of certain messages from the CAN Bus.

ACTION TAKEN WHEN THE DTC SETS:
1. The CHECK TRANS light is not illuminated.
2. DTC is stored in TCM history.
3. Default values are substituted for missing data, e.g., engine throttle percentage, engine torque, engine coolant temperature.

CONDITIONS FOR CLEARING THE DTC:
Use the diagnostic tool to clear the DTC from the TCM history. The DTC automatically clears the DTC from the DTC history if the vehicle completes 40 warm-up cycles without the DTC recurring.

DIAGNOSTIC AIDS:
DTC U0100 indicates that a CAN bus hardware error has occurred. This code may be caused by:
1. Miswired or defective OEM controllers, OEM components, or OEM circuits attached to CAN.
2. Defective TCM.

The diagnostic tool Data Bus Viewer may be selected from the File Menu to monitor or log SAE J1939 datalink messages.

An active DTC U0100 may prevent the diagnostic tool from communicating with the TCM. The J-47276 TCM Reflash Harness may be used to confirm that the TCM is operational. Connect the TCM Reflash Harness to the TCM and leave the OEM harness disconnected. Provide input power for the J-42455-A Load Box.

CAN1 and CAN2 devices must not be interconnected because they are not compatible with each other.

For proper datalink communications, it is necessary to have two 120 Ω resistors installed in parallel across the CAN High and CAN Low terminals. Resistance across CAN High and CAN Low should measure 60 Ω when wired correctly.

NOTE: The OEM transmission warranty, or the extended coverage for parts and labor, does not cover the repair or replacement of OEM or bodybuilder provided components or any external wire harnesses not provided by the OEM.

Before repairing or replacing these components or circuits that caused a transmission issue, always contact the vehicle OEM call center and/or the vehicle OEM or bodybuilder service representative to verify the warranty coverage and proper payment arrangements, arrange an alternate sublet or repair plan (if needed), and get troubleshooting assistance for the repair (such as vehicle schematics, DTC trees, parts assistance, etc.).

Allison 1000 & 2000 Gen 4 Fault Codes: P1688 Unmanaged Engine Torque Delivered To TCM Signal

CIRCUIT DESCRIPTION:
The GM 8.1 liter gasoline engine is used in some medium duty applications. The 8.1 liter engine uses a 3-wire system to communicate engine torque data/requests between the powertrain control module (PCM) and the transmission control module (TCM).

Unmanaged engine torque is sent from the PCM to the TCM over wire 109 (GM wire 2467).

CONDITIONS FOR RUNNING THE DTC:
1. The components are powered and ignition voltage is greater than 9V and less than 18V (12V TCM).
2. Engine speed is greater than 200 rpm and less than 7500 rpm for 5 seconds.
3. Pulse Width Modulation (PWM) driver demand torque option is selected in transmission calibration.

CONDITIONS FOR SETTING THE DTC:
DTC P1688 sets if unmanaged (gross) engine torque signal (circuit 2467) is under 1.5 percent or over 98.5 percent for more than a 2 second period.

ACTION TAKEN WHEN THE DTC SETS:
1. DTC P1688 is stored in the TCM history.
2. The CHECK TRANS light illuminates on the second occurrence.
3. The TCM defaults to a calculated unmanaged torque (gross) input value using throttle and engine speed.

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:
An Intermittent condition may be the cause of the problem. Return the vehicle to the OEM for further troubleshooting.