Komatsu PC78US‑6: An In‑Depth Owner’s Guide to the Compact Midi Excavator

[MikePhua]

               

Overview and Design Philosophy
The Komatsu PC78US‑6 is a short-tail swing midi excavator introduced in 2001 as part of Komatsu’s GALEO series. It was engineered for tight jobsite environments where a reduced tail swing radius is critical. The US‑6 variant builds on the earlier PC78US model by offering improved operator comfort, increased fuel capacity, reduced noise, and easier maintenance access.
Key design objectives included:

• Enhanced cab size with lower operator fatigue
  
• Tail projection limited to approximately 80 mm beyond the tracks
  
• Larger fuel tank for extended runtime
  
• Simplified maintenance via accessible radiator and fuel drain
  

Technical Specifications

• Engine: Komatsu S4D95LE‑3, turbocharged four-cylinder diesel producing around 54–55 hp (40 kW)
  
• Operating Weight: Approximately 7,190 kg (15,850 lb), depending on configuration
  
• Hydraulic Capacity: ~125 liters fuel tank, ~110 liters hydraulic fluid, ~7 liters engine oil
  
• Bucket Sizes: Standard 0.28 m³; options from 0.2 to 0.37 m³ available
  
• Digging Reach & Depth: Maximum depth ~4,100 mm; reach ~6,380 mm
  
• Undercarriage: 39 shoes per side, 1 carrier roller, 5 track rollers, ground pressure around 0.31 kg/cm² (4.4 psi)
  
• Travel & Swing: Travel speed ~3 km/h (low) to ~4.5 km/h (high); swing speed ~10 rpm
  

Operational Strengths
Operators often highlight several benefits:

• Excellent maneuverability in confined spaces due to short tail swing
  
• Smooth and responsive hydraulic system with dual-variable pumps
  
• Comfortable cab with A/C, defroster, radio, and enhanced visibility
  
• Mode selection: A‑mode for productivity, E‑mode for fuel efficiency
  
• Simplified maintenance with quick-clean radiator and extended service intervals
  

Common Issues and Caveats
Field experience reveals a few known problems:

• Low power or engine stalling during heavy hydraulic use, such as deep digging or simultaneous multi-function operation
  
• Clogging in hydraulic filters or pilot control valves may cause sluggishness
  
• Gray-market units may include non-standard components, complicating part sourcing and servicing
  

Inspection and Pre‑Purchase Checklist

• Test travel gears for smooth engagement and response
  
• Operate bucket, boom, and swing under load; check for hydraulic lag
  
• Drain water separator; inspect fuel for contamination
  
• Examine undercarriage for wear—especially tension springs and shoes
  
• Ensure all cab electronics and HVAC are functioning
  
• Check radiator fins and debris screens
  
• Confirm the unit’s serial number matches the correct US‑6 parts catalog
  

Real‑World User Story
A landscaping contractor in Massachusetts bought a 2006 PC78US‑6 with 3,500 hours on the meter. Despite its age, the machine worked reliably in tight backyard spaces due to its compact swing radius. When performance dipped, the issue was traced to a clogged suction filter and minor valve leakage. After cleaning and resealing, full function was restored. Five years later, the machine is still in service with minimal repairs, thanks to preventive maintenance.
Case Study: Urban Utility Installation
A contractor in Seattle used the PC78US‑6 to install fiber-optic conduits in tight urban alleyways. The machine’s short tail swing eliminated the risk of striking nearby buildings or fences. The hydraulic strength was sufficient for trenching and lifting double conduit lines. Over two seasons, it only required hose replacement and routine service, proving its reliability in complex environments.
Terminology Clarified

• Short‑tail swing excavator: A machine with a rear counterweight that does not extend significantly beyond the track width, ideal for limited space
  
• Midi excavator: An excavator between mini (1–5 tons) and full-size (12+ tons), typically weighing 7–10 tons
  
• GALEO: Komatsu’s design line focused on advanced technology, operator comfort, and easy serviceability
  
• A‑mode / E‑mode: Selectable operating modes—A for high power, E for fuel economy
  
• Drawbar pull: The pulling force a machine can generate through its tracks
  
• Greasing interval: Time between lubrication of moving joints—up to 250 hours for this model
  

Maintenance Tips for Longevity

• Use the recommended greasing schedule with sealed bushings
  
• Replace hydraulic filters every 1,000 hours
  
• Keep radiator and oil coolers clean to avoid overheating
  
• Use manufacturer-approved hydraulic oil and fuel filters
  
• Inspect auxiliary circuits for leaks and solenoid valve response
  
• Check engine and fluid levels seasonally—especially in freezing or high-heat conditions
  

Conclusion
The Komatsu PC78US‑6 offers a well-balanced package of compact power, maneuverability, and operator comfort. Its short-tail swing makes it perfect for tight worksites, and its thoughtful design streamlines both performance and maintenance. While some early signs of wear may appear on older models, particularly related to hydraulics, these are usually manageable with routine care. For contractors needing versatility without sacrificing toughness, the PC78US‑6 remains a solid choice in the 7-ton range.