Why Is the Torque Converter Running Hot on a D8N Dozer

[MikePhua]

Persistent Overheating and Pressure Anomalies
A Caterpillar D8N dozer, serial number 9TC02614, exhibited chronic transmission overheating despite recent replacement of both the torque converter and the cooler. The operator noted that the transmission temperature consistently climbed above 230°F, causing the engine temperature to rise to 210°F. Initial pressure readings showed zero inlet and outlet pressure at the torque converter, suggesting no flow to the cooler. After cleaning and reassembling the pressure and selector valve bodies, the issue persisted.
Pressure Testing and Gauge Accuracy
The first diagnostic attempt used a 3,500 psi gauge, which proved ineffective for low-pressure readings. Switching to a 100 psi gauge revealed more accurate data:

• Converter outlet pressure: 61 psi cold, 60.5 psi under load, dropping to 45 psi during stall test, and 10 psi at low idle.
  
• Converter inlet pressure: 75 psi at idle, rising to 145 psi at wide-open throttle.
  
• Pump pressure: Stable at 400 psi across throttle positions, dipping slightly to 375 psi at low idle.
  

These readings confirmed that flow existed, but pressure dropped significantly under load, indicating possible internal leakage or restriction.
Terminology Clarification

• Torque Converter: A hydraulic coupling between engine and transmission that multiplies torque and allows smooth power transfer.
  
• Stall Test: A diagnostic procedure where the machine is held stationary under full throttle to measure converter performance.
  
• Inlet/Outlet Pressure: Hydraulic pressure entering and exiting the torque converter, critical for cooling and function.
  
• Infrared Heat Gun: A non-contact tool used to measure surface temperatures for diagnostic comparison.
  

Cooling System Evaluation
The radiator had been cleaned and flushed two years prior using Caterpillar’s recommended cleaner. However, the operator had only relied on dashboard gauges to monitor temperature. A suggestion was made to use an infrared heat gun to measure:

• Top radiator hose temperature (coolant entering)
  
• Bottom radiator hose temperature (coolant exiting)
  

A healthy cooling system should show a temperature drop of approximately 18°F across the radiator. If the drop is less, it may indicate clogged cores or insufficient airflow.
Fan Efficiency and Installation Check
The fan and shroud were inspected, with all rubber seals intact. The fan was confirmed to blow air off the blades, but this alone does not verify correct installation. A reversed fan will still blow air but at reduced efficiency. A photo comparison or blade pitch inspection is recommended to confirm orientation.
Additional checks:

• Fan belts and pulleys: Ensure proper tension and alignment.
  
• Shroud sealing: Prevents air recirculation and improves cooling.
  
• Radiator fins: Must be free of debris and bent fins.
  

Recommendations and Next Steps

• Use a calibrated infrared gun to verify actual temperatures.
  
• Check converter outlet pressure under various loads to identify pressure drop patterns.
  
• Inspect valve body for wear or scoring that may cause internal leakage.
  
• Confirm fan blade orientation and airflow direction.
  
• Monitor engine and transmission temperature independently to isolate heat sources.
  

Conclusion
The D8N’s overheating issue appears to stem from a combination of pressure drop under load, possible valve body inefficiency, and cooling system limitations. While the torque converter and cooler were replaced, the underlying hydraulic behavior suggests further investigation is needed. Accurate pressure gauges and thermal diagnostics are essential tools in resolving this issue. With methodical testing and attention to airflow and fluid dynamics, the machine can be restored to reliable operating temperatures.