Circuit Description
To improve shift feel, the transmission control module (TCM) may request that the powertrain control module (PCM) reduce engine torque during shift events. When such a request is received, the PCM responds by retarding the base ignition timing and notifying the TCM that the request has succeeded. If the PCM is unable to comply with the request, the PCM sends the TCM a message that the request has failed.
The torque reduction request is sent to the PCM through a communication network called the controller area network (CAN). Two circuits are used to communicate CAN data between the PCM and TCM. A fault in the CAN will not cause DTC P1781 to set by itself. If a CAN fault occurs, other DTCs will set before DTC P1781.
When the TCM receives a torque reduction failure message from the PCM, then DTC P1781 will set. DTC P1781 is a type A DTC.
DTC Descriptor
This diagnostic procedure supports the following DTC:
DTC P1781 Engine Torque Signal
Conditions for Running the DTC
| • | The ignition voltage is 10.2-15.5 volts. |
| • | The engine speed is 400 RPM or greater. |
| • | No CAN DTC U2105. |
| • | Trans. Protection Mode is OFF. |
Conditions for Setting the DTC
The PCM notifies the TCM that a torque reduction request has failed for 4 seconds.
Action Taken When the DTC Sets
| • | The TCM requests the PCM to illuminate the malfunction indicator lamp (MIL). |
| • | The PCM records the operating conditions when the Conditions for Setting the DTC are met. The PCM stores this information as Freeze Frame and Failure Records. |
| • | The TCM records the operating conditions when the Conditions for Setting the DTC are met. The TCM stores this information as Failure Records. |
| • | The TCM turns the trans. protection mode ON following the shift, if failure occurs during shift. |
| • | The TCM sets engine torque to 270 N·m. |
| • | The TCM sets engine torque data to 300 N·m. |
| • | The TCM stores DTC P1781 in TCM history. |
Conditions for Clearing the MIL/DTC
| • | The PCM turns off the MIL after the third consecutive drive trip in which the TCM does not send a MIL illumination request. |
| • | A scan tool can clear the MIL/DTC. |
| • | The TCM clears the DTC from TCM history if the vehicle completes 40 warm-up cycles without an emission-related diagnostic fault occurring. |
| • | The TCM cancels the DTC default actions when the ignition is OFF long enough in order to power down the TCM. |
Step | Action | Values | Yes | No | ||||
|---|---|---|---|---|---|---|---|---|
1 | Did you perform the Diagnostic System Check - Vehicle in Vehicle DTC Information? | -- | Go to Step 2 | Go to Diagnostic System Check - Vehicle in Vehicle DTC Information | ||||
2 |
Important:
Did you record any PCM Failure Records? | -- | Go to Diagnostic Trouble Code (DTC) List - Vehicle in Vehicle DTC Information | Go to Step 3 | ||||
3 |
Did DTC P1781 reset? | -- | Go to Step 4 | Go to Intermittent Conditions in Engine Controls - 3.4L | ||||
4 | Did any other DTCs set? | -- | Go to Diagnostic Trouble Code (DTC) List - Vehicle in Vehicle DTC Information | Go to Step 5 | ||||
5 | Replace the PCM. Refer to Control Module References in Computer/Integrating Systems for replacement, setup, and programming. Is the action complete? | -- | Go to Step 6 | -- | ||||
6 | Perform the following operation to verify the repair:
Does the DTC reset? | -- | Go to Step 2 | Go to Step 7 | ||||
7 | With the scan tool, observe the stored information, capture info and DTC info. Does the scan tool display any DTCs that you have not diagnosed? | -- | Go to Diagnostic Trouble Code (DTC) List - Vehicle in Vehicle DTC Information | System OK |
