GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Holden 🢒 2008 🢒 Viva 🢒 Safety and Security 🢒 Supplemental Inflatable Restraints 🢒 DTC P1780

Circuit Description

The transmission control module (TCM) is an electronic device which monitors inputs to control various transaxle functions, including shift quality and transaxle sensors, switches, and components to process for use within the control program. Based on this input information, the TCM controls various transaxle output functions and devices.

Information is transmitted between the TCM and engine control module (ECM) through the controller area network (CAN). Information is as follows:

    • Engine malfunction signal
    • Engine speed
    • Engine output torque reduction signal
    • Engine coolant temperature

DTC Descriptor

This diagnostic procedure supports the following DTC:

DTC P1780 Torque Reduction Signal

Conditions for Setting the DTC
    • The engine speed is greater than 7,000 RPM.
    • System voltage is too high or too low.
    • CAN transmitting wiring harness shorted or open
    • The TCM or ECM is defective.
    • Immediately after the above condition occurs.

Action Taken When the DTC Sets
    • No lamp control required, but diagnostic information should be stored immediately when the malfunction is detected.
    • Adopt Emergency/Substitute mode and constant 4th gear.
    • Open lock up clutch.

Conditions for Clearing the Hold Lamp/DTC

Using a scan tool can clear history DTCs.

Diagnostic Aids
    • Inspect the poor wiring harness connection for TCM connectors and transaxle wiring connectors.
    • Inspect the improperly formed or damaged terminals.

DTC 1780

Step

Action

Values

Yes

No

1

Did you perform the Diagnostic System Check - Vehicle?

--

Go to Step 2

Go to Diagnostic System Check - Vehicle

2
  1. Install the scan tool.
  2. Turn the ignition ON, with the engine OFF.
  3. Record, then clear the DTCs
  4. Turn the ignition OFF.
  5. Observe the engine speed on the scan tool.

Is the speed within the specified range?

0-7,000 RPM

Go to Diagnostic Aids

Go to Step 3

3
  1. Turn the ignition OFF.
  2. Disconnect the transmission control module (TCM) wiring connector and the engine control module (ECM) wiring connector.
  3. Measure the resistance between terminal A8 of the TCM wiring connector and terminal B14 of the ECM wiring connector.
  4. Measure the resistance between terminal A16 of the TCM wiring connector and terminal B30 of the ECM wiring connector.

Is the resistance approximately equal to the specified value?

0 ohms

Go to Step 5

Go to Step 4

4

Repair the malfunctioning terminals, as necessary.

Is the action complete?

--

System OK

--

5
  1. Turn the ignition ON, with the engine OFF.
  2. Measure the voltage of terminal A8.
  3. Measure the voltage of terminal A16.

Is the voltage within the specified range?

11-14 V

Go to Step 6

Go to Step 7

6

Repair the short to power.

Is the action complete?

--

System OK

--

7

Inspect the Engine Speed.

Refer to .

Did you find and correct the condition?

--

Go to Step 10

Go to Step 8

8
  1. Replace the TCM. Refer to Transmission Control Module Replacement .
  2. Turn the ignition ON, with the engine OFF.
  3. Verify if DTC P1780 is set.

Is The DTC set?

--

Go to Step 9

Go to Step 10

9

Replace the ECM. Refer to Engine Control Module Replacement for the 1.8L (L79) engine or Engine Control Module Replacement for the 2.0L (L34 GMDAT) engine.

Is the action complete?

--

Go to Step 10

--

10
  1. After the repair, use a scan tool "clear info" function.
  2. Road test the vehicle.
  3. Review the DTC info.

Has the last test failed or is the current DTC displayed?

--

Go to Step 2

System OK