GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Chevrolet 🢒 2005 🢒 Optra 🢒 Transmission/Transaxle 🢒 Automatic Transmission - ZF 4HP 16 🢒 DTC P1779

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

Conditions for Setting the DTC
    • System voltage is too high or too low
    • 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.
    • Adjustable value, calculated over the substitute map engine torque - throttle position, engine speed.

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 1779

Step

Action

Values

Yes

No

1

Perform the Diagnostic System Check.

Is the check complete?

--

Go to Step 2

Go to Diagnostic System Check - Automatic Transmission

2
  1. Install the scan tool.
  2. Turn the ignition ON, with the engine OFF.
  3. Observe the throttle position value on the scan tool.

Is the TPS within the specified value?

0-100%

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
  1. Replace the TCM. Refer to Transmission Control Module Replacement .
  2. Turn the ignition ON, with the engine OFF.
  3. Verify if DTC P1779 is set.

Is The DTC set?

--

Go to Step 8

Go to Step 9

8

Replace the ECM. Refer to Engine Control Module Replacement in Engine Controls - 1.8L or Engine Control Module Replacement in Engine Controls - 2.0L.

Is the action complete?

--

Go to Step 9

--

9
  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