GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Chevrolet 🢒 2006 🢒 Aveo 🢒 Restraints 🢒 SIR 🢒 DTC P1781

DTC Descriptor

This diagnostic procedure supports the following DTC:

DTC P1781 Malfunction of Engine Speed Signal

Conditions for Setting the DTC
    • The transmission control module (TCM) detects that malfunction of engine speed signal for 2 seconds.
    • No DTC U2105

Action Taken When the DTC Sets
    • The TCM will request the illumination of the malfunction indicator lamp (MIL) and store the DTC when the TCM detects a failure on the first ignition cycle.
    • No engine torque reduction control
    • No lock-up
    • No engagement pressure control
    • No 4th gear
    • No self-learning control
    • No timing solenoid control for N-D
    • The TCM judges the engine revolution is 0 RPM.

Conditions for Clearing the DTC
    • The TCM turns OFF the MIL when no further failures are detected on the next ignition cycle.
    • The scan tool can clear the DTC from the TCM history.
    • The TCM clears the DTC from the TCM history memory after 40 consecutive ignition cycles without fault.

Cause of Failure
    • The wiring harness or the connector between the engine control module (ECM) and the TCM
    • The ECM
    • The TCM

Step

Action

Values

Yes

No

1

Did you perform the Diagnostic System Check - Automatic Transmission?

--

Go to Step 2

Go to Diagnostic System Check - Automatic Transmission

2
  1. Turn OFF the ignition.
  2. Install the scan tool.
  3. Turn ON the ignition, with the engine OFF.
  4. Select Store Freeze Frame/Failure Records from the DTC Information menu.
  5. Store Freeze Frame/Failure Records.
  6. Select Clear DTC Information from the DTC Information menu.
  7. Clear the DTC information.
  8. Perform one vehicle drive cycle.

Is the malfunction indicator lamp (MIL) ON?

--

Go to Step 3

Go to Step 7

3
  1. Select Request DTC by Status from the DTC Information menu.
  2. Request DTC by Status.

Is DTC P1781 displayed?

--

Go to Step 4

Go to Step 7

4
  1. Turn OFF the ignition.
  2. Disconnect the transmission control module (TCM) wiring connector and the engine control module (ECM) wiring connector.
  3. Measure the resistance between terminal A17 of the TCM wiring connector and terminal K14 of the ECM wiring connector.
  4. Measure the resistance between terminal A7 of the TCM wiring connector and terminal K15 of the ECM wiring connector.

Is the resistance within the specified value?

0 Ω

Go to Step 5

Go to Step 6

5
  1. Turn ON the ignition, with the engine OFF.
  2. Measure the voltage of terminal A17.
  3. Measure the voltage of terminal A7.

Is the voltage within the specified range?

11-14 V

Go to Step 6

Go to Step 8

6

Repair the malfunctioning terminals, as necessary.

Did you complete the repair?

--

System OK

--

7

Repair the temporary connection failure of the connector. Refer to Wiring Harness and Connector Inspection Diagnosis .

Did you complete the repair?

--

System OK

--

8

Inspect the engine for proper operation.

Was a condition found?

--

Go to Diagnostic System Check - Vehicle

Go to Step 9

9
  1. Replace the TCM. Refer to Transmission Control Module Replacement .
  2. Turn OFF the ignition.
  3. Turn ON the ignition, with the engine OFF.
  4. Inspect if DTC P1781 is set.

Is the DTC set?

--

Go to Step 10

Go to Step 11

10

Replace the ECM. Refer to Engine Control Module Replacement for the 1.6L (L91) engine.

Did you complete the replacement?

--

Go to Step 11

--

11
  1. After the repair, use a scan tool Clear Info function and road test the vehicle.
  2. Review the DTC info.

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

--

Go to Step 2

System OK