Circuit Description
The flow of transmission fluid starts in the transmission pan. It is then drawn through the filter and transmission case into the oil pump assembly. The oil pump assembly pressurizes the fluid (line pressure), which becomes the main supply line of fluid. This fluid is directed to various components and hydraulic circuits within the transmission. The pressure regulator valve receives this fluid and directs it to the converter clutch shift valve. The converter clutch shift valve directs hot fluid leaving the torque converter, or regulated converter feed fluid, through the cooler line to the transmission oil cooler. The transmission oil cooler is located in the radiator. The vehicle may also be equipped with an auxiliary oil cooler. The cooled fluid (center lube) is returned to the transmission through the return cooler line and into center lube port of the transmission. The Automatic Transmission Fluid Temperature (TFT) Sensor, senses the fluid temperature in the transmission pan.
If the Transmission Control Module (TCM) detects a high TFT for a long period of time, then DTC 79 sets.
Conditions for Setting the DTC
| • | No TFT Sensor DTC 58. |
| • | The TFT is greater than 146°C (295°F). |
| • | All conditions are met for 27 minutes and 30 seconds. |
Action Taken When the DTC Sets
| • | The TCM does not illuminate the Malfunction Indicator Lamp (MIL). |
| • | The TCM freeze shift adapts from being updated. |
| • | Soft landing to default 2nd gear. |
| • | DTC 79 is stored in TCM history. |
Conditions for Clearing the DTC
| • | The TCM clears the DTC from Current to History when the fault condition(s) no longer exist and the ignition switch is cycled OFF and then ON. |
| • | A scan tool can clear the DTC from Current and History status. The TCM clears the DTC from the TCM history if the vehicle completes 40 ignition cycles without a failure reported. |
| • | The TCM cancels the DTC default actions when the fault no longer exists and the ignition is cycled OFF and then ON. |
Diagnostic Aids
| • | Inspect the cooling system fluid level and condition. |
| • | Verify the customer's driving habits, such as trailer towing, etc... |
| • | The scan tool Trans. Fluid Temp. (TFT) should rise steadily during warm-up cycles, then stabilize. |
| • | DTC 79 may set approximately 600 seconds after DTC 58 has set. Follow the diagnostic table for DTC 58 before proceeding to the diagnostic table for DTC 79. Repairing the condition that set DTC 58 will likely eliminate DTC 79. |
Test Description
The numbers below refer to the step numbers on the diagnostic table.
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DTC 58 may also set a DTC 79. Go to the DTC 58 tables for diagnosis.
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This step inspects for air restrictions and loss of transmission fluid flow, causing an extremely high TFT.
Step | Action | Value(s) | Yes | No | ||||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
1 | Was the Powertrain On-Board Diagnostic (OBD) System Check performed? | -- | ||||||||||||||||||||
2 |
Important:: Before clearing the DTC's, record the Current and History DTC's information for reference. The Clear Info function erases the stored Current and History DTC from the TCM. Was the fluid checking procedure performed? | -- | Go to 4L80-E Transmission Fluid Check | |||||||||||||||||||
Is DTC 58 also set? | -- | Go to Diagnostic Aids | ||||||||||||||||||||
Was a condition found? | -- | |||||||||||||||||||||
5 | Perform the line pressure check procedure. Refer to 4L80-E Line Pressure Check . Was a condition found? | -- | ||||||||||||||||||||
6 | Inspect the torque converter stator for damage. Refer to Torque Converter Clutch Diagnosis . Was a condition found? | -- | Go to System Diagnosis Table Transmission Overheating | |||||||||||||||||||
7 | In order to verify your repair, perform the following procedure:
Is the TFT less than 145° C (293° F) for the test drive? | -- | System OK | Begin Diagnosis again. |
