Allison - Automatic Transmission
DTC P0877 or P0878
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 P0877
- Transmission Fluid Pressure (TFP) Switch 4 Circuit Low Voltage
DTC P0878
- Transmission Fluid Pressure (TFP) Switch 4 Circuit High Voltage
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 pressure switches and 1 normally closed pressure switch. Control main pressure is directed through an orifice to pressure switch 4 (PS4) and manual selector valve. When the selector lever is in park, neutral, or any forward position, the exhaust port is blocked and fluid pressure increases to the normally closed PS4. The transmission control module (TCM) monitors the PS4 to determine the position of the manual selector valve. The TCM provides a signal circuit to PS4. The low reference to the switches is provided through the metal base of the PSM which is connected to chassis ground through the transmission case.
Conditions for Running the DTC
P0877
* The engine speed is 200 RPM or greater for 5 seconds.
* The battery voltage is between 9-18 V.
* The transmission fluid temperature is 0°C (32°F) or greater.
* DTC P0708, P0877, or P0878 is not set.
P0878 Condition 1
* Engine speed is greater than 200 RPM.
* The battery voltage is between 9-18 V.
* The transmission fluid temperature is 0°C (32°F) or greater.
* DTC P0708, P0877, or P0878 is not set.
P0878 Condition 2
* Engine was cranking or running during this ignition cycle.
* The battery voltage is between 9-18 V.
* The transmission fluid temperature is 0°C (32°F) or greater.
* The engine speed, automatic transmission turbine speed, and transmission output speed are all less than 50 RPM.
Conditions for Setting the DTC
P0877
The TCM detects that the PS4 signal remains LOW for greater than 1 second when Park (P), Neutral (N), or a forward range is selected.
P0878 Condition 1
The TCM detects that the PS4 signal remains high for 1 second when reverse range is selected. The DTC sets after 40 occurrences.
P0878 Condition 2
The TCM detects that the PS4 signal remains high for 25 to 60 seconds, depending on the transmission fluid temperature, when engine shutdown is in process.
Action Taken When the DTC Sets
P0877
* DTC P0877 is a Type A DTC.
* The TCM will either lock the transmission to neutral or limit the transmission to reverse, 3rd, or 5th gear operation.
* The TCM freezes shift adapts.
* The TCM inhibits torque converter clutch (TCC) engagement.
P0878
* DTC P0878 is a Type A DTC.
* The TCM will either lock the transmission to neutral or limit the transmission to reverse, 3rd, or 5th gear operation.
* The TCM freezes shift adapts.
* The TCM inhibits TCC engagement.
Conditions for Clearing the DTC
DTCs P0877 and P0878 are Type A DTCs.
Diagnostic Aids
P0877
* If DTC P0701 is set in combination with DTC P0877, diagnose P0701 prior to proceeding to DTC P0877.
* If DTCs P0877, P0762 and P0708 are set in combination, the DTCs may indicate a problem with the internal mode switch.
* If DTC P0877 is set with DTCs P0701, P0843, P0848, and or P0873, a problem with line pressure may be indicated.
* You may have to drive the vehicle in order to experience a condition.
P0878
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
1. Perform the Transmission Fluid Level and Condition Check (Transmission Fluid Level and Condition Check) to verify correct fluid level and condition.
2. Ignition ON, engine OFF, verify with a scan tool the TFP Switch 4 parameter is Low.
3. Engine running, verify with a scan tool the TFP Switch 4 parameter is Low with selector in reverse and High with selector in all other positions.
4. Ignition ON, engine OFF, move the shift selector lever through all the ranges. With a scan tool, verify the TR Sw. A/B/C/P parameters match the IMS A, B, C, and P circuit states in the transmission IMS logic table. Refer to Transmission Internal Mode Switch Logic (Transmission Internal Mode Switch Logic).
• If not the expected results, refer to DTC P0708 (P0708).
5. 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
1. Ignition OFF, disconnect the X175 harness connector at the transmission.
2. Ignition ON, engine OFF, verify with a scan tool the TFP Switch 4 parameter is High.
• If not the specified value, test the PS4 signal circuit for a short to ground. If circuit tests normal, replace the TCM.
3. Ignition ON, connect a 3A fused jumper wire to the PS4 signal circuit terminal K to ground. Verify with a scan tool the TFP Switch 4 parameter changes from High to Low when the signal circuit is grounded.
• If not the specified value, test the PS4 signal circuit for an open/high resistance. If the circuit 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 PS4 with the eraser end of a pencil. Verify with a scan tool that the TFP Switch 4 parameter changes from Low to High when the PS4 is depressed.
• If not the specified value, test the internal wire harness PS4 signal circuit for an open/high resistance or short to ground. If the circuit tests normal, replace the pressure switch manifold.
8. Remove the control valve body. Refer to Control Valve Body Replacement (Allison - Automatic Transmission).
9. Disassemble the control valve body assembly and carefully inspect the separator plate for a blocked orifice. Repair or replace as necessary. Refer to Control Valve Body Assembly Disassemble (83. Control Valve Body Assembly Disassemble).
Repair Instructions
Perform the Diagnostic Repair Verification (Verification Tests) after completing the diagnostic procedure.
* Pressure Switch Manifold Replacement (Service and Repair)
* 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.