P0787

DTC P0787 or P0788

Diagnostic Instructions

* Perform the Diagnostic System Check - Vehicle (Initial Inspection and Diagnostic Overview) prior to using this diagnostic procedure.
* Review Strategy Based Diagnosis (Initial Inspection and Diagnostic Overview) for an overview of the diagnostic approach.
* Diagnostic Procedure Instructions (Initial Inspection and Diagnostic Overview) provides an overview of each diagnostic category.

DTC Descriptor
DTC P0787
- 3-2 Shift Solenoid (SS) Control Circuit Low Voltage

DTC P0788
- 3-2 Shift Solenoid (SS) Control Circuit High Voltage

Circuit/System Description

The 3-2 shift solenoid (SS) valve is a normally-closed, 3-port, on/off device that controls the 3-2 downshift. The solenoid attaches to the control valve body within the transmission. The solenoid receives ignition voltage through the ignition 1 circuit. The transmission control module (TCM) controls the solenoid by providing a ground path on the 3-2 shift solenoid valve control circuit. During a 3-2 downshift, the 2-4 band applies as the 3-4 clutch releases. The TCM varies the timing between the 3-4 clutch release and the 2-4 band apply depending on the vehicle speed and the throttle position.

Conditions for Running the DTC

P0787
* The system voltage is 8-18 volts.
* The engine speed is greater than 475 RPM for 5 seconds.
* Vehicle speed is less than 200 km/h (124 mph).

P0788
* The system voltage is 8-18 volts
* The engine speed is greater than 475 RPM for 5 seconds.
* Vehicle speed is less than 200 km/h (124 mph).

Conditions for Setting the DTC

P0787

The following condition occurs for 5 seconds:

The TCM commands the solenoid OFF and the voltage feedback remains low, 0 volts.

P0788

The TCM commands the solenoid ON and the voltage feedback remains high, B+.

Action Taken When the DTC Sets

P0787
* DTC P0787 is a Type B DTC.
* The TCM inhibits 3-2 shift until vehicle speed is 20 km/h (12 mph) or less.
* The TCM freezes transmission adaptive functions.

P0788
* DTC P0788 is a Type B DTC.
* The TCM freezes transmission adaptive functions.

Conditions for Clearing the DTC

DTC P0787 and P0788 are Type B DTCs.

Reference Information
Schematic Reference

Automatic Transmission Controls Schematics (Electrical Diagrams)
Connector End View Reference

Component Connector End Views (Connector Views)
Description and Operation

Transmission General Information (Transmission General Information)
Electrical Information Reference

* Circuit Testing (Component Tests and General Diagnostics)
* Connector Repairs (Component Tests and General Diagnostics)
* Testing for Intermittent Conditions and Poor Connections (Component Tests and General Diagnostics)
* Wiring Repairs (Component Tests and General Diagnostics)

DTC Type Reference

Powertrain Diagnostic Trouble Code (DTC) Type Definitions (Diagnostic Trouble Code Descriptions)
Scan Tool Reference

Transmission Control Module Scan Tool Information (Transmission Control Module Scan Tool Information)
Special Tools

* J 35616 GM-Approved Terminal Test Kit
* J 44152 Jumper Harness (20 pins)

Circuit/System Verification

Verify that DTCs P0961, P0973, P0976, P2764, P2769 have not set, if so, diagnose first. With a scan tool, command the 3-2 downshift solenoid ON and OFF.

Circuit/System Testing

1. Inspect the TRANS fuse for an open.

• If the fuse is open, test the ignition 1 voltage circuit and the affected control circuit for a short to ground between the Fuse Block-Underhood and the AT inline 20-way connector.

2. Disconnect the AT inline 20-way connector. Connect the J 44152 jumper harness (20 pins), to the engine side of the connector. Using the J 35616 GM-approved terminal test kit, connect a test lamp between the ignition 1 voltage circuit and the affected control circuit. Turn ON the ignition with the engine OFF. Using the scan tool output controls command the affected solenoid ON and OFF. The test lamp should blink ON and OFF as commanded.

• If the test lamp is always OFF, test the circuit for an open.

3. Connect the J 44152 to the transmission side of the connector. Using the J 35616, measure the resistance between the ignition 1 voltage circuit and the affected control circuit. Resistance should be between 20-32 ohms for the 3-2 SS valve.

• If resistance is greater than the specified range, test the circuits for an open. If the resistance is less than the specified range, test for a faulty solenoid or a shorted together condition.

4. Measure the resistance between the ignition 1 voltage circuit and the transmission case. The resistance should be greater than 250K ohms.

• If the resistance is less than 250K ohms, test the ignition 1 voltage circuit for a short to ground.

5. Measure the resistance between the affected control circuit and ground. The resistance should be greater than 250K ohms.

• If the resistance is greater than 250K ohms, test the affected control circuit for a short to ground.

Component Testing

1. Using the J 44152 and the J 35616, measure the resistance between the ignition 1 voltage circuit and the 3-2 SS valve circuit. The resistance should be between 20-32 ohms for the 3-2 SS valve.

• If the resistance is not within the specified range, remove the affected solenoid.

2. Disconnect the wiring harness connector from the affected solenoid. Measure the resistance of the affected control circuit. The resistance should be between 20-32 ohms for the 3-2 SS valve.

• If the resistance is not within the specified range, replace the affected solenoid.

Repair Instructions

Perform the Diagnostic Repair Verification (Verification Tests) after completing the repair.

* Torque Converter Clutch Pulse Width Modulation Solenoid, Torque Converter Clutch Solenoid, and Wiring Harness (Service and Repair)
* Control and Shift Solenoids Replacement (Service and Repair)
* Refer to Control Module References (Programming and Relearning) for replacement, setup, and programming.