P1781

Circuit Description
The Engine Control Module (ECM) uses the Class-II serial data link to communicate with the Transmission Control Module (TCM). The ECM will continuously monitor the Class-II communications, which should be received from the TCM approximately every 25 milliseconds. If the ECM receives no Class-II communications from the TCM during a 20-second time interval, a loss of serial data communication between the TCM and ECM is assumed. The Malfunction Indicator Lamp (MIL) is illuminated and DTC P1781 is set.

Conditions for Setting the DTC
- No Class-II messages received from the TCM after a 20-second time interval.

Action Taken When the DTC Sets
- The Malfunction Indicator Lamp (MIL) will illuminate.
- The ECM will record operating conditions at the time the diagnostic fails. This information will be stored in the Freeze Frame and Failure Records Buffers.
- A history DTC is stored.

Conditions for Clearing the MIL/DTC
- The MIL will turn OFF after three consecutive ignition cycles in which the diagnostic runs without a fault.
- A history DTC will clear after 40 consecutive warm-up cycles without a fault.
- The DTC(s) can be cleared by using the scan tool.

Diagnostic Aids
An intermittent may be caused by a poor connection, rubbed-through wire insulation or a wire broken inside the insulation. Check for a poor connection or damaged ECM/TCM harness. Inspect the serial data link circuit for the following conditions:
- Improper mating
- Broken locks
- Improperly formed
- Damaged terminals
- Poor terminal-to-wire connection
- Damaged harness

Steps 1 - 9:

Test Description
The number(s) below refer to step(s) on the diagnostic table.
1. On-Board Diagnostic (OBD II) System Check prompts the technician to complete some basic checks and store the freeze frame and failure records data on the scan tool if applicable. This creates an electronic copy of the data taken when the malfunction occurred. The information is then stored on the scan tool for later reference.
6. The replacement ECM must be reprogrammed. Refer to the latest Techline procedure for ECM reprogramming.