Circuit Description
The engine control module (ECM) sends accelerator pedal position data to the transmission control module (TCM), which then uses this information to modify the transmission shift speeds. The data from the ECM to the TCM is through a communication network called the GM Local Area Network (GMLAN). Two circuits are used to communicate GMLAN data between the ECM/TCM and other modules.
A fault in the GMLAN will not cause DTC P0120 to set by itself. If a GMLAN fault occurs, other DTCs will set before DTC P0120.
When the TCM receives invalid accelerator pedal data from the ECM, then DTC P0120 will set. DTC P0120 is a type 'B' DTC.
Conditions from the DTC
| • | Ignition voltage is between 8-18 volts. |
| • | Engine speed is between 450-6,800 RPM. |
| • | The engine run time is greater than 5 seconds. |
| • | No other GMLAN errors are present. |
Conditions for Setting the DTC
The ECM GMLAN message received by the TCM does not contain a valid throttle position value for 2 seconds.
Action taken when the DTC Sets
| • | The TCM requests the powertrain interface module (PIM) to illuminate the check powertrain malfunction indicator lamp (MIL) in the instrument cluster during the second consecutive trip in which the 'Conditions for Setting the DTC' are met. The TCM stores this information as a Freeze Frame. |
| • | At the time of the first failure, the TCM records the operations when the 'Conditions for Setting the DTC' are met. The TCM stores this information as a Failure Record. |
| • | At the time of the second failure, The TCM records the operating conditions the 'Conditions for Setting the DTC' are met. The TCM stores this information as a Freeze Frame. |
| • | The TCM uses the ECM default accelerator pedal value for determining shift speeds. |
| • | The TCM stores the DTC information into memory when the diagnostic runs and fails. |
| • | The TCM turns off the pressure control solenoid (PCS), causing maximum line pressure. |
| • | The TCM commands the TCC off and TCC solenoid is disabled. |
| • | The TCM freezes the transmission adaptive learning function. |
| • | The TCM stores DTC P0120 into TCM history. |
Conditions for Clearing the MIL/DTC
| • | The PIM turns off the Check Powertrain MIL after the sixth consecutive drive trip in which the TCM does not send a Check Powertrain MIL illumination request. |
| • | Use Tech 2 to clear the DTC. |
| • | The TCM clears the DTC from its memory (history DTC) if the vehicle completes 40 warm-up cycles without a non-emission related DTC occurring. |
| • | The TCM cancels the DTC default actions when the ignition is off long enough to power down the TCM. |
Step | Action | Yes | No |
|---|---|---|---|
Schematic Reference: Automatic Transmission Controls Schematics For information on Testing Wiring Faults, refer to Wiring Systems. | |||
1 | Has the Diagnostic System Check - Automatic Transmission been completed? | Go to Step 2 | |
2 |
Did you record any ECM Failure Records? | Go to Step 3 | |
3 |
Did DTC P0120 reset? | Go to Step 4 | Go to Testing for Intermittent Conditions and Poor Connections |
4 | Did other DTCs set? | Go to Step 5 | |
5 | Replace the TCM. Refer to Transmission Control Module Replacement. Is the replacement complete? | Go to Step 6 | -- |
6 | Perform the following operation to verify the repair:
Does the DTC reset? | Go to Step 7 | Go to Step 2 |
7 | Use Tech 2 to read DTC information. Does the Tech 2 display any DTCs that have not been diagnosed and rectified? | System OK | |
When all diagnosis and repairs are completed, clear all DTCs and verify correct operation. | |||
