Komatsu D155AX-6 Overheating Diagnosis and Cooling System Restoration

[MikePhua]

The D155AX-6 and Komatsu’s Heavy Dozer Lineage
The Komatsu D155AX-6 crawler dozer is a high-production earthmoving machine designed for mining, road building, and large-scale land clearing. Introduced as part of Komatsu’s Tier 3-compliant lineup, the D155AX-6 features a powerful SAA6D140E-5 engine, delivering up to 354 net horsepower and weighing over 90,000 lbs in its standard configuration. With a blade capacity exceeding 12 cubic yards and advanced hydrostatic steering, it became a preferred choice for contractors requiring brute force and precision.
Komatsu, founded in 1921, has consistently led the global dozer market with innovations in modular powertrains, intelligent machine control, and operator ergonomics. The D155AX-6 was built to endure harsh environments, but like any high-output machine, it is vulnerable to overheating if cooling systems are compromised.
Core Cooling System Architecture
The D155AX-6 relies on a multi-stage cooling system to regulate engine temperature under heavy load. Key components include:

• Radiator core with multi-row aluminum fins
  
• Hydraulic-driven cooling fan with variable speed control
  
• Thermostat regulating coolant flow based on temperature
  
• Water pump circulating coolant through engine block and radiator
  
• Oil cooler integrated into the hydraulic circuit
  
• Overflow tank and pressure cap maintaining system pressure
  

The cooling system is designed to maintain optimal operating temperature between 180°F and 200°F. Overheating typically begins when temperatures exceed 220°F, triggering warning lights and potential engine derating.
Common Overheating Causes and Field Symptoms
Operators have reported overheating in the D155AX-6 under load, especially during first gear operation or prolonged dozing. Common causes include:

• Radiator Blockage
  • Dust, debris, or even shop rags lodged in the fins or oil cooler inlet
    
  • Reduced airflow and heat exchange efficiency
    
• Coolant Contamination or Loss
  • Leaking hoses, cracked reservoir, or faulty pressure cap
    
  • Air pockets in the system causing uneven flow
    
• Thermostat Failure
  • Stuck closed, preventing coolant circulation
    
  • Delayed opening causing temperature spikes
    
• Water Pump Wear
  • Impeller erosion or bearing failure reducing flow rate
    
  • Belt slippage or hydraulic drive malfunction
    
• Combustion Gas Intrusion
  • Cylinder head gasket leak allowing exhaust gases into coolant
    
  • Bubbles in overflow tank and coolant displacement
    
• Fan Control Malfunction
  

• Incorrect fan speed settings or sensor failure
  
• Reduced cooling under high ambient temperatures
  

A technician in Alberta diagnosed a D155AX-6 with overheating during slope work. After pressure testing the radiator and inspecting the injector ports, combustion gas leakage was confirmed. Replacing the head gasket and flushing the coolant resolved the issue.
Diagnostic Strategy and Inspection Protocols
To identify overheating root causes:

• Use infrared thermometer to measure radiator inlet and outlet temperatures
  
• Pressure test radiator and observe for drop or external leaks
  
• Inspect overflow tank for bubbling during operation
  
• Check fan speed settings and verify hydraulic drive response
  
• Remove thermostat and test opening temperature in boiling water
  
• Inspect water pump impeller and housing for wear or blockage
  
• Scan engine control module for temperature sensor faults
  

A restorer in Chile discovered a rag lodged in the oil cooler inlet, restricting flow and causing localized overheating. After removal and cleaning, the machine returned to normal operating temperature.
Repair Procedures and Component Replacement
Once the fault is identified, recommended repairs include:

• Flush cooling system with distilled water and coolant cleaner
  
• Replace radiator cap with OEM-rated pressure value
  
• Install new thermostat and verify opening temperature
  
• Replace worn water pump and inspect drive belt or hydraulic motor
  
• Clean radiator fins using low-pressure air and detergent
  
• Replace damaged hoses and clamps
  
• Refill with Komatsu-approved coolant and bleed air from system
  

Torque specifications for water pump bolts typically range from 35–45 ft-lbs. Always use new gaskets and sealant rated for high-temperature coolant systems.
Preventive Maintenance and Long-Term Reliability
To prevent future overheating:

• Clean radiator and oil cooler weekly in dusty environments
  
• Replace coolant every 2,000 hours or annually
  
• Inspect fan drive system and verify sensor calibration
  
• Monitor coolant level and pressure cap integrity monthly
  
• Perform combustion gas test during major service intervals
  
• Document temperature readings and service history for trend analysis
  

A fleet manager in Texas added cooling system inspection to his 500-hour service checklist and reduced overheating incidents by 80% over two years.
Conclusion and Recommendations
The Komatsu D155AX-6 is engineered for high-output performance, but its cooling system must be maintained with precision. Overheating can result from a combination of mechanical wear, contamination, and overlooked obstructions. With disciplined diagnostics and proactive service, the machine can operate reliably in the most demanding conditions.
Recommendations include:

• Pressure test and inspect radiator and overflow system regularly
  
• Replace thermostat and water pump at recommended intervals
  
• Clean cooling fins and oil cooler inlets to maintain airflow
  
• Monitor fan speed and verify hydraulic drive response
  
• Use genuine Komatsu coolant and document service procedures
  

With proper care, the D155AX-6 continues to push, rip, and grade with the power and endurance that define Komatsu’s legacy in heavy equipment.