GM Service Manual Online
For 1990-2009 cars only

DTC P0788 Hybrid


Object Number: 1817231  Size: MF
Master Electrical Component List
Automatic Transmission Controls Schematics
OBD II Symbol Description Notice
Control Module References

Circuit 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 43 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.

If the TCM detects a continuous short to voltage in the 4-3 downshift solenoid valve circuit, then DTC P0788 sets. DTC P0788 is a type C DTC.

DTC Descriptor

This diagnostic procedure supports the following DTC:

DTC P0788 Downshift Timing Solenoid Control Circuit High Voltage

Conditions for Running the DTC

The engine is running for 2 seconds.

Conditions for Setting the DTC

The 4-3 downshift solenoid valve feedback circuit indicates a continuous short to voltage for 150 milliseconds.

Action Taken When the DTC Sets

    • The TCM does not illuminate the malfunction indicator lamp (MIL).
    • The TCM freezes transmission adaptive functions.
    • The TCM records the operating conditions when the Conditions for Setting the DTC are met. The TCM stores this information as Failure Records.
    • The TCM stores DTC P0788 in TCM history.

Conditions for Clearing the DTC

    • A scan tool can clear the DTC.
    • The TCM clears the DTC from TCM history if the vehicle completes 40 warm-up cycles without an emission related diagnostic fault occurring.
    • The TCM cancels the DTC default actions when the ignition switch is OFF long enough in order to power down the TCM.

Test Description

The numbers below refer to the step numbers on the diagnostic table.

  1. This step tests the TCM and the wiring to the transmission.

  2. This step verifies that the circuit has the correct resistance.

DTC P0788

Step

Action

Value(s)

Yes

No

1

Did you perform the Diagnostic System Check - Vehicle?

--

Go to Step 2

Go to Diagnostic System Check - Vehicle

2

  1. Install a scan tool.
  2. Turn ON the ignition, with the engine OFF.
  3. Important: Before clearing the DTC, use the scan tool in order to record the Freeze Frame and Failure Records. Using the Clear Info function erases the Freeze Frame and Failure Records from the TCM.

  4. Record the DTC Freeze Frame and Failure Records.
  5. Clear the DTC.
  6. Start the engine.
  7. Select 4-3 downshift Sol. CKT Status parameter on the scan tool.

Does the scan tool display indicate a short to voltage?

--

Go to Step 3

Go to Testing for Intermittent Conditions and Poor Connections

3

  1. Turn OFF the ignition.
  2. Disconnect the AT inline 20-way connector. Additional DTCs may set.
  3. Install the J 44152 Jumper Harness (20 pins) on the engine side of the AT inline 20-way connector.
  4. Using the J 35616 GM-approved terminal test kit, connect a test lamp between the 4-3 downshift solenoid valve control circuit of the J 44152 and ground. Refer to Automatic Transmission Inline 20-Way Connector End View.
  5. Turn ON the ignition, with the engine OFF.

Is the test lamp ON?

--

Go to Step 4

Go to Step 5

4

Test the control circuit of the 4-3 downshift solenoid valve for a short to voltage between the TCM connector and the AT inline 20-way connector. Refer to Testing for a Short to Voltage and Wiring Repairs.

Did you find and correct the condition?

--

Go to Step 10

Go to Step 9

5

  1. Turn OFF the ignition.
  2. Disconnect the J 44152 from the engine side of the AT inline 20-way connector.
  3. Install the J 44152 on the transmission side of the AT inline 20-way connector.
  4. Using the DMM and the J 35616 GM-approved terminal test kit measure the resistance between the ignition 1 voltage and the 4-3 downshift solenoid valve control circuits of the J 44152 . Refer to Automatic Transmission Inline 20-Way Connector End View.

Is the resistance less than the specified value?

19 Ω

Go to Step 6

Go to Testing for Intermittent Conditions and Poor Connections

6

  1. Leave the DMM test leads connected.
  2. Disconnect the 4-3 downshift solenoid valve from the automatic transmission wiring harness.

Is the resistance less than the specified value?

19 Ω

Go to Step 7

Go to Step 8

7

Replace the automatic transmission wiring harness. Refer to Wiring Harness Replacement.

Did you complete the replacement?

--

Go to Step 10

--

8

Replace the 4-3 downshift solenoid valve. Refer to Control Valve Body Spacer Plate and 4-3 Downshift On/Off Solenoid Removal and Control Valve Body Spacer Plate and 4-3 Downshift On/Off Solenoid Installation.

Did you complete the replacement?

--

Go to Step 10

--

9

Replace the TCM. Refer to Control Module References for replacement, setup, and programming.

Did you complete the replacement?

--

Go to Step 10

--

10

Perform the following procedure in order to verify the repair:

  1. Select DTC.
  2. Select Clear Info.
  3. Start the engine.
  4. Select Specific DTC.
  5. Enter DTC P0788.

Has the test run and passed?

--

Go to Step 11

Go to Step 2

11

With the scan tool, observe the stored information, capture info. and DTC info.

Does the scan tool display any DTCs that you have not diagnosed?

--

Go to Diagnostic Trouble Code (DTC) List - Vehicle

System OK