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 Instructions provides an overview of each diagnostic category.
Diagnostic Fault Information
Circuit/System Description
The primary source of heat in the 4ET50 transmission is the drive motors. The drive motors are cooled by the transmission fluid. Hot fluid exits the drive motors and flows to the transmission oil cooler through the transmission cooler supply line and thermal bypass valve. The thermal bypass valve assembly is located near the lower left side of the radiator. The cooled fluid returns to the transmission through the transmission fluid cooler pipe assembly and cools the control solenoid assembly. The transmission fluid temperature sensor is located in the control solenoid assembly and is not serviced separately. The Transmission Fluid Temperature parameter on the scan tool should rise steadily to a normal operating temperature, and then stabilize.
The thermal bypass valve is mechanically operated, based on the oil temperature of the transmission in order to maintain optimum transmission operating efficiency. The valve has an activating temperature of 82°C (176°F), with the range to move being 78-83°C (172-181°F). The full open temperature is 96°C (205°F). The thermal bypass valve assembly begins to open at 82°C (176°F) for optimum transmission operation and fuel economy. The valve has a typical oil flow rate of 9-10 lpm (2.4-2.6 gpm).
Cold Mode:
82°C (176°F) and below the thermal bypass valve bypasses the oil going to the cooler and sends the fluid back to the transmission.
Mixing Mode:
82-96°C (176-205°F) the thermal bypass valve is in mixing mode - some of the fluid flows back to the trans and some flows through the transmission oil cooler.
Hot Mode:
Above 96°C (205°F) The thermal bypass valve is fully open and transmission oil flows through the cooler then back to the transmission.
Reference Information
Schematic Reference
* Automatic Transmission Controls Schematics Electrical Diagrams
* Park Engine Off Park Engine Off
* Park Engine On Park Engine On
* 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 for scan tool information
Special Tools
* DT 24731 - Tempil Stick
* DT 45096 - Transmission Oil Cooler System Flush and Flow Test Tool
For equivalent regional tools, refer to Special Tools Tools and Equipment.
Component Testing
1. Perform the Fluid Leak Diagnosis Fluid Leak Diagnosis.
2. Verify airflow through the radiator is not obstructed. Refer to Radiator Cleaning Procedures.
3. Perform the Transmission Fluid Level and Condition Check Testing and Inspection procedure.
4. Perform the Line Pressure Check Line Pressure Check procedure.
5. Perform the Main Fluid Pump Pressure High or Low Main Fluid Pump Pressure High or Low procedure.
6. Perform the Transmission Fluid Cooler Flushing and Flow Test Transmission Fluid Cooler Flushing and Flow Test.
7. With a hand held or infra red thermometer or DT 24731 - tempil stick , test for the transmission cooler supply line thermal bypass valve assembly opening at 96°C (205°F). The transmission cooler supply line temperature should be higher on the transmission side of the thermal bypass valve assembly than the cooler side until 96°C (205°F) is reached.
• If the thermal bypass valve tests stuck closed, replace the thermal bypass valve assembly.
• If the thermal bypass valve tests normal inspect the transmission oil passages for a blockage.
Repair Instructions
Perform the Diagnostic Repair Verification Verification Tests after completing the diagnostic procedure.
* Transmission Fluid Auxiliary Cooler Replacement Transmission Fluid Auxiliary Cooler Replacement
* Transmission Fluid Cooler Thermal Bypass Valve Replacement Transmission Fluid Cooler Thermal Bypass Valve Replacement
* Transmission Fluid Cooler Inlet Pipe Replacement Transmission Fluid Cooler Inlet Pipe Replacement
* Transmission Fluid Cooler Outlet Pipe Replacement Transmission Fluid Cooler Outlet Pipe Replacement