GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Saturn 🢒 2005 🢒 L300/LW300 🢒 DTC 🢒 DTC P1780

Circuit Description

To improve shift feel, the transmission control module (TCM) may request that the engine control module (ECM) reduce engine torque during shift events. When such a request is received, the ECM responds by delaying the base ignition timing and notifying the TCM that the request has succeeded. If the ECM is unable to comply with the request, the ECM sends the TCM a message that the request has failed.

The torque reduction request is sent to the ECM through a communication network called the controller area network (CAN). Two circuits are used to communicate CAN data between the ECM and TCM. A fault in the CAN will not cause DTC P1780 to set by itself. If a CAN fault occurs, other DTCs will set before DTC P1780.

When the TCM receives a torque reduction failure message from the ECM, DTC P1780 sets. DTC P1780 is a type C DTC.

DTC Descriptor

This diagnostic procedure supports the following DTC:

DTC P1780 Torque Reduction Signal

Conditions for Running the DTC
    • No other CAN errors are present.
    • The engine run time is greater than 5 seconds.

Conditions for Setting the DTC

The ECM notifies the TCM that a torque reduction request has failed for 1 second.

Action Taken When the DTC Sets
    • The TCM does not request the ECM to illuminate the malfunction indicator lamp (MIL).
    • The TCM requests the ECM to illuminate the service vehicle soon (SVS) lamp.
    • The TCM commands maximum line pressure.
    • 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 P1780 in TCM history.

Conditions for Clearing the SVS Lamp/DTC
    • The TCM clears the SVS lamp request when the condition no longer exists.
    • A scan tool can clear the SVS Lamp/DTC.
    • The TCM clears the DTC from TCM history if the vehicle completes 40 warm-up cycles without a non emission related diagnostic fault occurring.
    • The TCM cancels the DTC default actions when the ignition is OFF long enough in order to power down the TCM.

Step

Action

Yes

No

1

Did you perform the Diagnostic System Check - Vehicle?

Go to Step 2

Go to Diagnostic System Check - Vehicle in Vehicle DTC Information

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 ECM Freeze Frame and the TCM Failure Records. Using the Clear Info function erases the Freeze Frame and Failure Records from the ECM and the TCM.
       • Using the Clear Info function erases stored DTCs in both the ECM and TCM.

  3. Record the DTC Failure Records.

Did you record any ECM Failure Records?

Go to Diagnostic Trouble Code (DTC) List - Vehicle in Vehicle DTC Information

Go to Step 3

3
  1. Clear the DTC.
  2. Turn the ignition OFF for at least 30 seconds.
  3. Start the engine and allow it to idle.

Did DTC P1780 set again?

Go to Step 4

Go to Intermittent Conditions in Engine Controls - 3.0L (L81)

4

Did any other DTCs set?

Go to Diagnostic Trouble Code (DTC) List - Vehicle in Vehicle DTC Information

Go to Step 5

5

Replace the TCM.

Refer to Control Module References in Computer/Integrating Systems for replacement, set-up, and programming.

Is the action complete?

Go to Step 6

--

6

Perform the following operation to verify the repair:

  1. Select DTC on the scan tool.
  2. Select Clear Info.
  3. Start the engine and allow it to idle.
  4. Select Specific DTC.
  5. Enter DTC P1780.

Has the test run and passed?

Go to Step 7

Go to Step 2

7

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 in Vehicle DTC Information

System OK