Circuit Description
While the vehicle is being driven, engine and transmission data accumulates in a Random Access Memory (RAM) device, which cannot store data reliably when the Transmission Control Module (TCM) powers down. Before the TCM powers down, the data is copied from RAM to an Electrically Erasable Programmable Read-Only Memory (EEPROM) device, sometimes referred to as Non-Volatile Memory (NVM).
Both the RAM and the EEPROM are inside the TCM. A normal function of the TCM programming is to test the EEPROM at TCM power-down by copying test data from RAM to the EEPROM and then comparing the two copies. If they do not match, DTC P1621 will set, which is a type 'A' DTC.
Conditions for Running the DTC
The ignition is ON.
Conditions for Setting the DTC
The TCM detects a checksum error when the RAM and EEPROM test data are compared.
Action Taken When the DTC Sets
| • | The TCM requests the Powertrain Interface Module (PIM) to illuminate the Check Powertrain Malfunction Indicator Lamp (MIL) when the 'Conditions for Setting the DTC' are met. |
| • | The TCM turns off the pressure control solenoid (causing maximum line pressure). |
| • | The TCM freezes the transmission adaptive learning function. |
| • | At the time of the first failure, the TCM records the operating conditions when the 'Conditions for Setting the DTC' have been met. The TCM stores this information as a failure record. |
| • | The TCM stores DTC P1621 into TCM history. |
Conditions for Clearing the MIL/DTC
| • | Tech 2 can 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 default actions when the ignition is off long enough to power down the TCM. |
| • | 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. |
Diagnostic Aids
TCM programming may cause a P1621 to set.
Test Description
The following number(s) refer to the step number(s) on the diagnostic table.
2. Performing this step helps verify that an external anomaly, such as Electromagnetic Interference (EMI), did not cause the DTC to set originally.
Step | Action | Values | 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 P1621 reset? | -- | Go to Step 3 | Go to Testing for Intermittent Conditions and Poor Connections | ||||
3 | Replace TCM. Refer to Transmission Control Module Replacement. Is the replacement complete? | -- | Go to Step 4 | -- | ||||
4 |
Has the test run and passed? | -- | System OK | Go to Step 2 | ||||
When all diagnosis and repairs are completed, clear all DTCs and verify correct operation. | ||||||||
