Allison - Automatic Transmission
DTC P0751, P0756, or P0761
Diagnostic Instructions
* Perform the Diagnostic System Check - Vehicle (Diagnostic System Check - Vehicle) prior to using this diagnostic procedure.
* Review Strategy Based Diagnosis (Strategy Based Diagnosis) for an overview of the diagnostic approach.
* Diagnostic Procedure Instructions (Diagnostic Procedure Instructions) provides an overview of each diagnostic category.
DTC Descriptors
DTC P0751
- Shift Solenoid (SS) 1 Valve Performance - Stuck Off
DTC P0756
- Shift Solenoid (SS) 2 Valve Performance - Stuck Off
DTC P0761
- Shift Solenoid (SS) 3 Valve Performance - Stuck Off
Diagnostic Fault Information
Typical Scan Tool Data
Circuit/System Description
The pressure switch manifold (PSM) is a multiple switch assembly made up of 3 normally open (N/O) pressure switches and 1 normally closed (N/C) pressure switch. Pressure switch 1 (PS1) monitors shift valve 1 positioning, pressure switch 2 (PS2) monitors shift valve 2 positioning and pressure switch 3 (PS3) monitors shift valve 3 positioning. The normally open pressure switches, PS1, PS2, and PS3, communicate the positioning to the transmission control module (TCM). The position of the shift valves 1, 2, and 3 is controlled by 3 normally closed shift solenoids 1, 2, and 3 (SS1, SS2, and SS3). When energized, the shift solenoids direct control main pressure to shift valve 1, 2, or 3, which strokes the shift valve. When the shift valve is stroked, an exhaust passage is blocked and fluid pressure to the pressure switch increases.
Conditions for Running the DTC
P0751
* Engine speed is greater than 200 RPM.
* Shift solenoid 1 (SS1) is commanded ON.
P0756
* Engine speed is greater than 200 RPM.
* Shift solenoid 2 (SS2) is commanded ON.
P0761
* Engine speed is greater than 200 RPM.
* Shift solenoid 3 (SS3) is commanded ON.
Conditions for Setting the DTC
P0751
SS1 is commanded ON and PS1 remains OFF for 5 to 12 seconds depending on the transmission fluid temperature.
P0756
SS2 is commanded ON and PS2 remains OFF for 5 to 12 seconds depending on the transmission fluid temperature.
P0761
SS3 is commanded ON and PS3 remains OFF for 5 to 12 seconds depending on the transmission fluid temperature.
Action Taken When the DTC Sets
P0751, P0756, or P0761
* DTCs P0751, P0756, and P0761 are Type A DTCs.
* If the DTC sets with the transmission in a forward gear, the TCM will command another forward gear.
* If the DTC sets while in reverse, the transmission will remain in reverse or shift to neutral.
* The TCM freezes shift adapts.
* The TCM inhibits torque converter clutch (TCC) engagement.
Conditions for Clearing the DTC
DTCs P0751, P0756, and P0761 are Type A DTCs.
Diagnostic Aids
* If DTC P0701 is set in any combination with DTC P0751, P0756 or P0761, diagnose P0701 prior to proceeding to DTCs P0751, P0756 or P0761.
* You may have to drive the vehicle in order to experience a condition.
Reference Information
Schematic Reference
Automatic Transmission Controls Schematics (Allison - Automatic Transmission)
Connector End View Reference
* Component Connector End Views (Connector End Views By Name)
* Inline Harness Connector End Views (Connector End Views By Number)
Description and Operation
* Transmission General Description (Transmission General Description)
* Transmission Component and System Description (Transmission Component and System Description)
* Electronic Component Description (Electronic Component Description)
Electrical Information Reference
* Circuit Testing (Circuit Testing)
* Connector Repairs (Connector Repairs)
* Testing for Intermittent Conditions and Poor Connections (Testing for Intermittent Conditions and Poor Connections)
* Wiring Repairs (Wiring Repairs)
DTC Type Reference
Powertrain Diagnostic Trouble Code (DTC) Type Definitions (Powertrain Diagnostic Trouble Code (DTC) Type Definitions)
Scan Tool Reference
Control Module References (Control Module References) for scan tool information
Circuit/System Verification
Warning
Chock the wheels to prevent the vehicle from moving. Failure to chock the wheels can cause personal injury when the electrical system is repaired.
1. Perform the Transmission Fluid Level and Condition Check (Transmission Fluid Level and Condition Check) to verify correct fluid level and condition.
2. Engine running, command the transmission through the forward ranges with a scan tool. Verify the TFP Switch 1, TFP Switch 2, and TFP Switch 3 parameter states are correct for each forward range. Refer to Pressure Switch Manifold (PSM) Logic (Pressure Switch Manifold (PSM) Logic).
3. Operate the vehicle within the Conditions for Running the DTC to verify the DTC does not reset. You may also operate the vehicle within the conditions that you observed from the Freeze Frame/Failure Records data.
Circuit/System Testing
Note: You must perform the Circuit/System Verification first.
1. Ignition OFF, disconnect the X175 harness connector at the transmission.
2. Ignition ON, engine OFF, verify with a scan tool the TFP Switch 1, TFP Switch 2, and TFP Switch 3 parameters are all High.
• If not the specified value, test the affected pressure switch signal circuit for a short to ground. If circuits test normal, replace the TCM.
3. Ignition ON, engine OFF, connect a 3A fused jumper wire between the PS1 signal circuit terminal D to ground, PS2 signal circuit terminal F to ground, and PS3 signal circuit terminal E to ground. Verify with a scan tool the TFP Switch 1, TFP Switch 2, and TFP Switch 3 parameters change from High to Low when the signal circuit is grounded.
• If not the specified value, test the affected pressure switch signal circuit for an open/high resistance. If circuits tests normal, replace the TCM.
4. Ignition OFF, connect the X175 harness connector to the transmission.
5. Remove the pressure switch module from the valve body. Leave the internal wire harness connected to the pressure switch manifold. Refer to Pressure Switch Manifold Replacement (Service and Repair).
6. Connect a jumper wire to the base of the pressure switch module and ground.
7. Ignition ON, engine OFF, gently push on the PS1, PS2, and PS3 switches with the eraser end of a pencil. Verify with a scan tool that the TFP Switch 1, TFP Switch 2, and TFP Switch 3 parameters changed from High to Low.
• If not the specified value, test the internal wire harness circuits for a short to ground or an open/high resistance. If the circuits test normal, replace the pressure switch module.
8. If all circuits test normal, refer to Control Valve Body Assembly Disassemble (83. Control Valve Body Assembly Disassemble) and Control Valve Body Cleaning and Inspection (84. Control Valve Body Cleaning and Inspection).
• If valve body inspection is normal, replace the shift solenoid indicated by the DTC. Refer to Control Valve Solenoid Replacement (SS1, SS2, SS3, TCC, and MAIN MOD) (Control Valve Solenoid Replacement (SS1, SS2, SS3, TCC, and MAIN MOD))Control Valve Solenoid Replacement (PCS1 and PCS2) (Control Valve Solenoid Replacement (PCS1 and PCS2)).
Repair Instructions
Perform the Diagnostic Repair Verification (Verification Tests) after completing the diagnostic procedure.
* Pressure Switch Manifold Replacement (Service and Repair)
* Control Valve Solenoid Replacement (SS1, SS2, SS3, TCC, and MAIN MOD) (Control Valve Solenoid Replacement (SS1, SS2, SS3, TCC, and MAIN MOD))Control Valve Solenoid Replacement (PCS1 and PCS2) (Control Valve Solenoid Replacement (PCS1 and PCS2))
* Wiring Harness Replacement (Allison - Automatic Transmission)
* Control Module References (Control Module References) for TCM replacement, setup, and programming
Repair Verification
If another DTC is present, refer to Diagnostic Trouble Code (DTC) List - Vehicle (Diagnostic Trouble Code (DTC) List - Vehicle) and perform the appropriate diagnostic procedure.