Automatic Transmission Overheating
Automatic Transmission Overheating
Diagnostic Instructions
* Perform the Diagnostic System Check - Vehicle Diagnostic System Check - Vehicle prior to using this diagnostic procedure.
* Review Strategy Based Diagnosis Strategy Based Diagnosis for an overview of the diagnostic approach.
* Diagnostic Procedure Instructions Diagnostic Procedure Instructionsprovides an overview of each diagnostic category.
Diagnostic Fault Information
Circuit/System Description
The primary source of heat in the transmission is the drive motor generators. The drive motor generators are cooled by the transmission fluid. Hot fluid exits the drive motor generators and flows to the transmission cooler supply line. The supply line connects to the cooler, which is in front of the radiator. The transmission fluid cooler pipe assembly is equipped with an internal thermostat that opens at 80°C (176°F) for optimum transmission operation and fuel economy. The transmission cooler pipe thermostat is part of the transmission fluid cooler pipe assembly, located near the left side of the radiator and is not serviced separately from the cooler lines, the fluid returns through the transmission fluid cooler pipe assembly and enters the lubrication circuits. After lubricating the internal components, the fluid returns to the oil pan. The transmission fluid temperature (TFT) sensor is located in the control solenoid (w/body and TCM) valve assembly and is not serviced separately. The driver information center (DIC), if equipped, will display "TRANSMISSION HOT - Idle Engine" when the TFT reaches 135°C (275°F). The TFT parameter on the scan tool should rise steadily to a normal operating temperature, and then stabilize. Ask about the customer's driving habits, trailer towing, etc. Ensure the engine cooling system is functioning normally.
Reference Information
Schematic Reference
* Automatic Transmission Controls Schematics 2ML70 - Automatic Transmission
* Park - Engine Running Park - Engine Running
* Fluid Passages Fluid Passages
Description and Operation
* Transmission General Description Transmission General Description
* Transmission Indicators and Messages Transmission Indicators and Messages
Scan Tool Reference
Control Module References Programming and Relearning
Special Tools
J 24731 - Tempil Stick
Component Testing
* Perform the Fluid Leak Diagnosis Fluid Leak Diagnosis.
* Perform the Transmission Fluid Level and Condition Check Transmission Fluid Level and Condition Check procedure.
* Perform the Line Pressure Check Line Pressure Check procedure.
* Perform the Main Fluid Pump Pressure High or Low Main Fluid Pump Pressure High or Low procedure.
* With a hand held infra red thermometer or J 24731 - tempil stick , test for the transmission cooler supply line thermostat opening at 80°C (176°F). The transmission cooler supply line temperature should be higher on the transmission side of the thermostat than the cooler side until 80°C (176°F) is reached.
If the thermostat tests stuck open or closed, replace the transmission cooler supply line assembly.
Repair Instructions
Perform the Diagnostic Repair Verification Verification Tests after completing the diagnostic procedure.
Repair or replace the automatic transmission cooler or lines as necessary. Refer to Transmission Fluid Cooler Pipe Replacement Transmission Fluid Cooler Pipe Replacement.