CAT D5M Overheating Under Load and Pressure Diagnostics

[MikePhua]

The CAT D5M dozer may exhibit intermittent overheating under load due to pressure anomalies in the cooling system, degraded thermostat performance, or transmission heat transfer inefficiencies. Even after basic maintenance, persistent heat buildup suggests deeper hydraulic or thermal imbalances.
CAT D5M Overview and Cooling System Design
The Caterpillar D5M was introduced in the late 1990s as part of the M-series dozer lineup, designed for grading, land clearing, and construction site preparation. Powered by a CAT 3116 turbocharged diesel engine, the D5M features a direct-drive transmission, hydraulic fan, and a pressurized cooling system. Caterpillar Inc., founded in 1925, has sold tens of thousands of D5M units globally, with strong adoption in North America and Southeast Asia.
The cooling system includes a radiator, thermostat, water pump, transmission cooler, and overflow reservoir. The system is designed to maintain optimal engine temperature under heavy load, with pressure regulation critical to preventing boil-over and coolant loss.
Terminology Note

• Thermostat: A temperature-sensitive valve that regulates coolant flow between the engine and radiator.
  
• Coolant Back Pressure: Pressure buildup behind the thermostat, often caused by flow restriction or combustion gas intrusion.
  
• Transmission Cooler: A heat exchanger that transfers heat from transmission oil to engine coolant.
  
• Infrared Heat Gun: A non-contact tool used to measure surface temperatures on engine and cooling components.
  
• Sensor Code 673.02: Indicates a fault in the transmission output speed sensor.
  

Initial Maintenance and Observations
The operator performed standard cooling system maintenance:

• Flushed and cleaned the radiator
  
• Replaced the thermostat and gasket
  
• Checked fan belt tension
  
• Verified coolant cleanliness
  

After 30 hours of operation, the engine overheated again. Upon removing the thermostat, a strong burst of coolant shot several feet into the air, indicating excessive pressure on the engine side. No bubbles were observed in the radiator during startup, and oil leaks were later located and repaired.
Persistent Overheating Under Load
Despite repairs, the dozer continued to run hot during pushing operations. At idle or mid-range RPMs, temperature remained normal. However, sustained load caused rapid heat buildup. Cooling improved when the machine was stationary, suggesting insufficient heat dissipation during high hydraulic demand.
Advanced Diagnostics and Recommendations

• Use an infrared heat gun to measure temperatures at key points:
  • Radiator inlet and outlet
    
  • Transmission cooler inlet and outlet (coolant side)
    
  • Transmission cooler inlet and outlet (oil side)
    
• Compare temperature differentials to identify heat transfer inefficiencies. A small delta between radiator inlet and outlet may indicate poor coolant flow or radiator blockage.
  
• Inspect transmission cooler for internal restriction or oil contamination. If transmission oil overheats, it can transfer excess heat to the coolant.
  
• Check for combustion gas intrusion using a block test kit. A blown head gasket can pressurize the cooling system and cause thermostat ejection.
  
• Replace temperature sensors if fault codes persist. Sensor 673.02 and 693.07 suggest transmission speed and clutch solenoid issues, which may affect cooling fan logic or load response.
  

Preventive Maintenance Tips

• Replace coolant every 2,000 hours or two years, using CAT-approved formulations.
  
• Inspect radiator fins monthly for debris and clean with low-pressure water.
  
• Torque thermostat housing bolts to spec to prevent leaks and warping.
  
• Monitor fault codes and address electrical sensor issues promptly.
  
• Keep a log of operating temperature under different loads to identify patterns.
  

Field Experience and Anecdotes
In Mississippi, a D5M operator observed coolant ejection during thermostat removal after a week of downtime. The radiator cap had been removed beforehand, ruling out residual pressure. This pointed to combustion gas buildup or blocked coolant flow. After replacing the thermostat and switching to CAT coolant, the machine ran cooler but still overheated under load.
In Alberta, a forestry contractor found that a clogged transmission cooler caused similar symptoms. Replacing the cooler restored normal temperature even during sustained pushing.
Conclusion
Overheating in the CAT D5M dozer, especially under load, often stems from pressure anomalies, transmission heat transfer issues, or hidden flow restrictions. While basic maintenance may restore temporary balance, persistent heat buildup requires deeper diagnostics using infrared tools and pressure testing. With methodical inspection and targeted repairs, the D5M can return to reliable service even in demanding conditions.