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 P1780 to set by itself. If a CAN fault occurs, other DTCs will set before DTC P1780.
When the TCM receives a torque reduction failure message from the PCM, then DTC P1780 will set. DTC P1780 is a type C DTC.
DTC Descriptor
This diagnostic procedure supports the following DTC:
DTC P1780 Torque Reduction Signal
Conditions for Running the DTC
| • | No CAN DTC U2105. |
| • | The ignition voltage is 10.2-15.5 volts. |
| • | The engine speed is 400 RPM or greater. |
| • | Trans. Protection Mode is OFF. |
Conditions for Setting the DTC
The PCM notifies the TCM that a torque reduction request has failed for 0.080 second.
Action Taken When the DTC Sets
| • | The TCM does not request the PCM to illuminate the malfunction indicator lamp (MIL). |
| • | The TCM increases line pressure control solenoid valve (SLT). |
| • | The line pressure is at maximum, and freezes the transmission adapt functions. |
| • | The PCM records the operating conditions when the Conditions for Setting the DTC are met. The PCM stores this information as a Freeze Frame. |
| • | The TCM records the operating conditions when the Conditions for Setting the DTC are met. The TCM stores this information as a Failure Record. |
| • | The TCM stores DTC P1780 in TCM history. |
| • | The TCM limits shift lines to 4000 RPM. |
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 | Yes | No | ||||
|---|---|---|---|---|---|---|---|
1 | Did you perform the Diagnostic System Check - Vehicle? | Go to Step 2 | |||||
2 |
Important:
Did you record any PCM Failure Records? | Go to Step 3 | |||||
3 |
Did DTC P1780 reset? | Go to Step 4 | Go to Testing for Intermittent Conditions and Poor Connections | ||||
4 | Did any other DTCs set? | Go to Step 5 | |||||
5 | Replace the PCM. Refer to Control Module References 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? | System OK |
