P0787
DTC P0787 or P0788
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 P0787
- Downshift Timing Solenoid Control Circuit Low Voltage
DTC P0788
- Downshift Timing Solenoid Control Circuit High Voltage
Diagnostic Fault Information
Typical Scan Tool Data
Circuit/System Description
The 4-3 downshift solenoid valve controls transmission fluid pressure acting on the shift timing valve to position the valves for the appropriate gear. The 4-3 downshift solenoid valve is a normally closed exhaust valve located on the channel plate. Ignition voltage is supplied to the solenoid. The transmission control module (TCM) commands the solenoid ON or OFF by providing a ground path. When the 4-3 downshift solenoid valve is commanded ON, the TCM senses low voltage. When the 4-3 downshift solenoid valve is commanded OFF, the TCM senses high voltage.
Conditions for Running the DTC
P0787
The engine is running for 5 seconds.
P0788
The engine is running for 2 seconds.
Conditions for Setting the DTC
P0787
The 4-3 downshift solenoid valve feedback circuit indicates a continuous open or short to ground for 4.4 seconds.
P0788
The 4-3 downshift solenoid valve feedback circuit indicates a continuous short to voltage for 150 milliseconds.
Action Taken When the DTC Sets
* DTC P0787 and P0788 are Type C DTCs.
* The TCM freezes transmission adaptive functions.
* The TCM commands maximum line pressure.
Conditions for Clearing the DTC
DTC P0787 and P0788 are Type C DTCs.
Reference Information
Schematic Reference
Automatic Transmission Controls Schematics (4T45-E - Automatic Transmission)
Connector End View Reference
Component Connector End Views (Connector End Views By Name)
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 (Programming and Relearning) for scan tool information
Special Tools
J 44152 Jumper Harness (20 pins)
Circuit/System Verification
1. Verify that DTC P0961, P0973, P0974, P0976, or P2764 are not set.
• If any of the DTCs are set, refer to Diagnostic Trouble Code (DTC) List - Vehicle (Diagnostic Trouble Code (DTC) List - Vehicle).
2. Ignition ON, engine OFF, observe the 4-3 Sol. CKT Status parameter while commanding the 4-3 downshift solenoid ON and OFF with a scan tool. The reading should display OK or Indeterminate.
3. Operate the vehicle within the Conditions for Running the DTC. You may also operate the vehicle within the conditions that you observed from the Freeze Frame/Failure Records data.
Circuit/System Testing
Important: Additional DTCs may set when disconnecting the automatic transmission inline 20-way connector.
1. Ignition OFF, disconnect the automatic transmission (AT) inline 20-way connector at the transmission.
2. Install J 44152 on the engine side of the AT inline 20-way connector.
3. Ignition ON, verify that a test lamp illuminates between the ignition circuit terminal E and ground.
• If the test lamp does not illuminate, test the ignition circuit for a short to ground or an open/high resistance. If the circuit tests normal and the ignition circuit fuse is open, test all components connected to the ignition circuit and replace as necessary.
4. Connect a test lamp between the control circuit terminal F and the ignition circuit terminal E.
5. Command the 4-3 downshift solenoid ON and OFF with a scan tool. The test lamp should turn ON and OFF when changing between the commanded states.
• If the test lamp is always ON, test the control circuit for a short to ground. If the circuit tests normal, replace the TCM.
• If the test lamp is always OFF, test the control circuit for a short to voltage or an open/high resistance. If the circuit tests normal, replace the TCM.
6. If all circuits test normal, test the internal transmission harness and the 4-3 downshift solenoid.
Component Testing
1. Ignition OFF, disconnect the automatic transmission (AT) inline 20-way connector at the transmission.
2. Install J 44152 on the transmission side of the AT inline 20-way connector.
3. Test for 15-25 ohms of resistance between the control terminal F and the ignition terminal E of the transmission.
• If not within the specified range, test the circuits for a short to ground or an open/high resistance. If a circuit is shorted or open replace the internal transmission harness, if the circuits test normal, replace the 4-3 downshift solenoid.
4. Test for infinite resistance between each terminal and the transmission housing/case.
• If not the specified value, test the circuits for a short to ground or an open/high resistance. If a circuit is shorted replace the internal transmission harness, if the circuits test normal, replace the 4-3 downshift solenoid.
Repair Instructions
Perform the Diagnostic Repair Verification (Verification Tests) after completing the repair.
* Control Module References (Programming and Relearning) for TCM replacement, setup, and programming.
* Control Valve Body Spacer Plate and 4-3 Downshift On/Off Solenoid Removal (18. Control Valve Body Spacer Plate and 4-3 Downshift On/Off Solenoid Removal)
* Control Valve Body Spacer Plate and 4-3 Downshift On/Off Solenoid Installation (96. Control Valve Body Spacer Plate and 4-3 Downshift On/Off Solenoid Installation)