To the top of the document
Volt
   
GMDE Start Page Load static TOC Load dynamic TOC Help?

Automatic Transmission Overheating

Diagnostic Instructions
    • Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure.
    • Review Strategy Based Diagnosis for an overview of the diagnostic approach.
    • Diagnostic Procedure Instructions provides an overview of each diagnostic category.

Diagnostic Fault Information

Cause

DTCs

Engine:

    • Engine coolant level low
    •  Engine Overheating
    • Incorrect engine coolant
    • Old coolant or incorrect concentration level
    • Cooling system leak
    • Radiator hose kinked or pinched
    • Loose, missing, or damaged radiator air seals or deflector -- restricted air flow
    • The radiator and air conditioning condenser has air flow obstructions to include large amount of bent fins
    • The cooling fans do not operate
    • Inspect the engine mechanical for worn, leaking, cracked cylinder heads, or engine block

P1258

Transmission:

Vehicle Overload or Beyond Capacity Towing:

    • Vehicle is overloaded
    • Vehicle is towing beyond capacity
    • Vehicle modifications

P0634

Heat Transfer:

    • Engine overheating
    • Cooler restrictions -- transmission fluid auxiliary cooler restricted flow or blockage
    • Restricted air flow
    • Vehicle modifications
    • Aftermarket auxiliary cooler installed incorrectly, incorrect flow direction - reversed flow
    • Restricted, blocked, or pinched transmission cooler lines
    • Stuck closed transmission thermal bypass valve

P0634

Transmission Abuse:

    • Vehicle overloaded
    • Vehicle is towing beyond capacity
    • Vehicle modifications

P0634

Vehicle Damage:

    • Transmission oil cooler pipe kinked, pinched or restricted
    • Restricted air flow

P0634

Transmission Fluid:

    • Fluid leaks
    • Low fluid level
    • Contaminated or poor transmission fluid quality

P0634

Low Fluid Pump Output:

    • Low fluid level
    • Blocked, damaged or loose filter
    • Pinched or restricted cooler lines
    • Restricted cooler - damage or debris
    • Inefficient pump - worn

P0634

Clutch Slippage:

    • Low fluid level
    • Low fluid pump output
    • Damaged or leaking clutch seals
    • Internal hydraulic leak
    • Worn clutch plate assembly

P079A

P079B

P079C

P079D

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 stabilise.

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
    •  Park Engine Off
    •  Park Engine On
    •  Fluid Passages

Description and Operation

    •  Transmission General Description
    •  Transmission Indicators and Messages

Scan Tool Reference

Control Module References 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 .

Component Testing
  1. Perform the Fluid Leak Diagnosis .
  2. Verify airflow through the radiator is not obstructed. Refer to Radiator Cleaning .
  3. Perform the Transmission Fluid Level and Condition Check procedure.
  4. Perform the Line Pressure Check procedure.
  5. Perform the Main Fluid Pump Pressure High or Low procedure.
  6. Perform the Transmission Fluid Cooler Flushing and Flow Test .
  7. With a hand held or infrared 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.
    8. ⇒If the thermal bypass valve test indicates that the valve has stuck closed, replace the thermal bypass valve assembly.
    ⇒If the thermal bypass valve functions correctly, inspect the transmission oil passages for a blockage.

Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

    •  Transmission Fluid Auxiliary Cooler Replacement
    •  Transmission Fluid Cooler Thermal Bypass Valve Replacement
    •  Transmission Fluid Cooler Inlet Pipe Replacement
    •  Transmission Fluid Cooler Outlet Pipe Replacement
   


&#169© Copyright Chevrolet. All rights reserved