P0848
DTC P0843, P0848, or P0873
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 P0843
- Transmission Pressure Switch 1 Circuit High Voltage
DTC P0848
- Transmission Pressure Switch 2 Circuit High Voltage
DTC P0873
- Transmission Pressure Switch 3 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 (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, pressure switch 3 (PS3) monitors shift valve 3 positioning, and pressure switch 4 (PS4) monitors the position of the manual selector valve. The pressure switches 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. When energized, SS1 directs control main pressure to shift valve 1, SS2 directs control main pressure to shift valve 2, and SS3 directs control main pressure to shift valve 3, which strokes the shift valves. When the valve is stroked, an exhaust passage is blocked and fluid pressure to the normally open PS1, PS2, or PS3 increases. The TCM provides a signal circuit to PS1, PS2, and PS3. 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
P0843
* Ignition voltage is between 9-18 V.
* Engine speed is greater than 200 RPM for 5 seconds.
* Shift valve 1 is commanded to ON, or stroked position.
P0848
* Ignition voltage is between 9-18 V.
* Engine speed is greater than 200 RPM for 5 seconds.
* Shift valve 2 is commanded to ON, or stroked position.
P0873
* Ignition voltage is between 9-18 V.
* Engine speed is greater than 200 RPM for 5 seconds.
* Shift valve 3 is commanded to ON, or stroked position.
Conditions for Setting the DTC
P0843
The TCM detects that PS1 is OFF or High when shift solenoid 1 (SS1) is commanded ON for 1 second.
P0848
The TCM detects that PS2 is OFF or High when shift solenoid 2 (SS2) is commanded ON for 1 second.
P0873
The TCM detects that PS3 is OFF or High when shift solenoid 3 (SS3) is commanded ON for 1 second.
Action Taken When the DTC Sets
P0843, P0848, P0873
* DTCs P0843, P0848, and P0873 are Type A DTCs.
* If DTC sets with the transmission in a forward range, the transmission will shift to another forward range. If the transmission was in reverse, the transmission will remain in reverse or shift to neutral.
* The TCM inhibits torque converter clutch (TCC) engagement.
Conditions for Clearing the DTC
DTCs P0843, P0848, and P0873 are Type A DTCs.
Diagnostic Aids
P0843
* You may have to drive the vehicle in order to experience a condition.
* This DTC may indicate that SS1 is mechanically defective.
* When DTCs P0751 and P0843 are set in combination, this may indicate an open circuit condition on the pressure switch 1 circuit or the shift valve 1 is stuck in the destroked position.
* This DTC can be caused by a loss of prime.
* This DTC can be caused by a plugged control main filter.
P0848
* You may have to drive the vehicle in order to experience a condition.
* This DTC may indicate that SS2 is mechanically defective.
* When DTCs P0756 and P0848 are set in combination, this may indicate an open circuit condition on the pressure switch 2 circuit or the shift valve 2 is stuck in the destroked position.
* This DTC can be caused by a loss of prime.
* This DTC can be caused by a plugged control main filter.
P0873
* You may have to drive the vehicle in order to experience a condition.
* This DTC may indicate that SS3 is mechanically defective.
* When DTCs P0761 and P0873 are set in combination, this may indicate an open circuit condition on the pressure switch 3 circuit or the shift valve 3 is stuck in the destroked position.
* This DTC can be caused by a loss of prime.
* This DTC can be caused by a plugged control main filter.
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. If DTC P0701 is set, diagnose first. Refer to DTC P0701 (P0701).
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.
2. Perform the Transmission Fluid Level and Condition Check (Transmission Fluid Level and Condition Check) to verify correct fluid level and condition.
3. Parking brake ON, engine running at idle, shift lever in drive, 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).
4. 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 1, TFP Switch 2, and TFP Switch 3 parameters are High.
• If not the specified value, test the affected pressure switch signal circuit for a short to ground. If circuits tests normal, replace the TCM.
3. Connect a 3A fused jumper wire to 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 test 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 when the pressure switch is pushed.
• If not the specified value, test the internal wire harness circuits for an open/high resistance. If the circuits test normal, replace the pressure switch module.
8. If all circuits test normal, remove the control valve body. Refer to Control Valve Body Replacement (Allison - Automatic Transmission).
9. Perform the 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 associated with the shift valve and pressure switch 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))
* 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.