Komatsu PC70 Excavator with Cummins Engine Faces Persistent Reliability Issues

[MikePhua]

The PC70 and Its Place in Komatsu’s Lineup
The Komatsu PC70 is a mid-size hydraulic excavator designed for urban construction, utility trenching, and light forestry work. Positioned between the compact PC60 and the larger PC78, the PC70 was intended to offer more digging power and reach while maintaining maneuverability in tight spaces. Komatsu, founded in 1921 in Japan, has long been a global leader in earthmoving equipment, with its excavators known for durability and precision.
Unlike many Komatsu machines that use in-house engines, the PC70 is equipped with a Cummins diesel powerplant. This pairing was meant to appeal to markets where Cummins support networks were stronger or where emissions compliance required alternative configurations. However, this combination has proven problematic in several field reports, with recurring issues in fuel delivery, electrical systems, and hydraulic coordination.
Engine Integration and Compatibility Challenges
The Cummins engine in the PC70, while robust in standalone applications, has shown integration mismatches with Komatsu’s hydraulic and electronic systems. Key concerns include:

• Inconsistent throttle response under load
  
• Difficulty syncing engine RPM with hydraulic demand
  
• Fault codes triggered by sensor miscommunication
  
• Fuel system air intrusion during startup cycles
  
• Premature wear of injector seals and lift pump diaphragms
  

A contractor in British Columbia noted that his PC70 would intermittently stall during trenching, despite clean filters and fresh fuel. Diagnostics revealed that the engine control module (ECM) was receiving erratic signals from the throttle position sensor, causing fuel cut-off during high hydraulic demand.
Hydraulic System Behavior and Load Sensitivity
The PC70 uses a variable displacement hydraulic pump controlled by load-sensing valves. When functioning properly, this system adjusts flow and pressure based on operator input and task requirements. However, when engine RPM fluctuates or sensor feedback is delayed, the pump may underperform, leading to:

• Slow boom and arm response
  
• Jerky swing motion
  
• Inconsistent bucket curl force
  
• Overheating of hydraulic fluid in prolonged cycles
  

Operators have reported that the machine performs well during light grading but struggles during deep digging or lifting, especially when multiple functions are engaged simultaneously.
Electrical and Diagnostic Complexity
The PC70’s electrical system includes:

• ECM for engine control
  
• VCU (vehicle control unit) for hydraulic coordination
  
• CAN-bus communication between modules
  
• Sensor arrays for temperature, pressure, and position
  

Failures in this network can be difficult to trace. Common issues include:

• Corroded connectors near the firewall
  
• Grounding faults under the cab
  
• Software mismatches after module replacement
  
• Inaccurate fault codes due to signal interference
  

One technician in Texas discovered that a persistent low oil pressure warning was caused by a damaged wire near the alternator, not an actual pressure drop. The ECM had interpreted the voltage fluctuation as a sensor failure.
Maintenance Recommendations and Field Solutions
To improve reliability:

• Replace fuel filters every 250 hours and inspect for air leaks
  
• Use OEM-rated sensors and avoid mixing brands
  
• Clean electrical connectors quarterly and apply dielectric grease
  
• Monitor hydraulic fluid temperature and change fluid every 1,000 hours
  
• Update ECM and VCU software to latest versions when available
  
• Install inline fuel pressure gauges for real-time monitoring
  
• Add auxiliary grounding straps to stabilize voltage
  

Some operators retrofit mechanical throttle linkages to bypass electronic lag. Others install aftermarket hydraulic coolers to reduce fluid temperature during summer operation.
Resale Value and Market Sentiment
Due to its reputation for erratic behavior, the PC70 has lower resale value compared to the PC60 or PC78. As of 2025, used units in fair condition range from $35,000 to $50,000, depending on hours and maintenance history. Buyers often request detailed service records and prefer machines with documented ECM updates and fuel system overhauls.
Despite its challenges, the PC70 remains in use across Asia, Africa, and South America, where parts availability and operator familiarity outweigh integration concerns. In North America, however, many contractors have shifted to models with fully Komatsu-built powertrains.
Conclusion
The Komatsu PC70 with Cummins engine represents a well-intentioned but flawed integration of two strong brands. While each component is capable on its own, their pairing has led to persistent reliability issues that frustrate operators and complicate diagnostics. With careful maintenance, targeted upgrades, and a deep understanding of its systems, the PC70 can still perform—but it demands more attention than its siblings. In the field, it’s not just about horsepower—it’s about harmony between systems.