GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 2002 🢒 GMC C Sierra - 2WD 🢒 Engine 🢒 Engine Controls - 6.6L 🢒 DTC P1781

Circuit Description

The transmission control module (TCM) malfunction indicator lamp (MIL) request circuit signals the engine control module (ECM) that the TCM is requesting MIL illumination.

Conditions for Running the DTC

The ignition is ON.

Conditions for Setting the DTC

The ECM is detecting an improper voltage level on the TCM MIL request circuit.

Action Taken When the DTC Sets
    • The control module illuminates the malfunction indicator lamp (MIL) on the second consecutive ignition cycle that the diagnostic runs and fails.
    • The control module records the operating conditions at the time the diagnostic fails. The first time the diagnostic fails, the control module stores this information in the Failure Records. If the diagnostic reports a failure on the second consecutive ignition cycle, the control module records the operating conditions at the time of the failure. The control module writes the operating conditions to the Freeze Frame and updates the Failure Records.

Conditions for Clearing the MIL/DTC
    • The control module turns OFF the malfunction indicator lamp (MIL) after 3 consecutive ignition cycles that the diagnostic runs and does not fail.
    • A current DTC, Last Test Failed, clears when the diagnostic runs and passes.
    • A history DTC clears after 40 consecutive warm-up cycles, if no failures are reported by this or any other emission related diagnostic.
    • Clear the MIL and the DTC with a scan tool.

Step

Action

Values

Yes

No

Schematic Reference: Engine Controls Schematics

1

Did you perform the Diagnostic System Check-Engine Controls?

--

Go to Step 2

Go to Diagnostic System Check - Engine Controls

2

Connect a scan tool.

Did DTC P1781 fail this ignition?

--

Go to Step 4

Go to Step 3

3
  1. Observe the Freeze Frame/Failure Records data for this DTC.
  2. Turn OFF the ignition for 30 seconds.
  3. Start the engine.
  4. Operate the vehicle within the Conditions for Running the DTC as specified in the supporting text or as close to the Freeze Frame/Failure Records data that you observed.

Does the DTC fail this ignition?

--

Go to Step 4

Go to Intermittent Conditions

4
  1. Turn OFF the ignition.
  2. Disconnect the TCM. Refer to Transmission Control Module Replacement in Automatic Transmission-Allison.
  3. Connect a DMM from the TCM MIL request circuit at the TCM harness connector to a good ground.
  4. Turn ON the ignition, with the engine OFF.

Does the voltage measure near the specified value?

B +

Go to Step 5

Go to Step 6

5

Test the TCM MIL request circuit for a short to voltage and repair as necessary. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct a condition?

--

Go to Step 12

Go to Step 7

6

Test the TCM MIL request circuit for an open and repair as necessary. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct a condition?

--

Go to Step 12

Go to Step 7

7
  1. Leave the TCM disconnected.
  2. Connect a fused jumper wire from the TCM MIL request circuit in the TCM harness connector to a good ground.
  3. Clear the DTCs with a scan tool.

Does DTC P0700 set, but not DTC P1781?

--

Go to Step 8

Go to Step 10

8

Perform the following procedures:

    • Check the battery and powers and grounds of the TCM for high resistance or corrosion. Refer to Circuit Testing and Connector Repairs in Wiring Systems.
    • Test the TCM connectors for a poor connection. Refer to Circuit Testing and Connector Repairs in Wiring Systems.

Did you find and correct a condition?

--

Go to Step 12

Go to Step 9

9

Replace the TCM. Refer to Transmission Control Module Replacement in Automatic Transmission - Allison.

Did you complete the action?

--

Go to Step 12

--

10

Test the ECM connectors for a poor connection. Refer to Circuit Testing and Connector Repairs in Wiring Systems.

Did you find and correct a condition?

--

Go to Step 12

Go to Step 11

11

Replace the ECM. Refer to Engine Control Module Replacement .

Did you complete the replacement?

--

Go to Step 12

--

12
  1. Use a scan tool to clear the DTCs.
  2. Turn OFF the ignition for 30 seconds.
  3. Start the engine.
  4. Operate the vehicle within the Conditions for Running the DTC as specified in the supporting text.

Does the DTC run and pass?

--

Go to Step 13

Go to Step 2

13

With a scan tool, observe the stored information, Capture Info.

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

--

Go to Diagnostic Trouble Code (DTC) List

System OK