Mitsubishi MS40-8 Excavator Performance and Hydraulic Troubleshooting

[MikePhua]

Mitsubishi’s Compact Excavator Lineage
Mitsubishi Heavy Industries, a diversified Japanese conglomerate founded in 1884, entered the construction equipment market with a focus on compact and mid-size excavators tailored for urban infrastructure, agriculture, and utility work. The MS40-8 was part of a generation of hydraulic excavators produced in the late 1980s and early 1990s, designed to offer reliable performance in confined spaces while maintaining mechanical simplicity. Though Mitsubishi eventually phased out its construction equipment division, machines like the MS40-8 remain in use across Asia, Eastern Europe, and parts of South America.
The MS40-8 is a zero-tail swing compact excavator with a conventional boom and dipper arm layout. It was built to compete with models from Komatsu, Hitachi, and Kobelco in the 4-ton class, offering a balance of digging power, transportability, and serviceability.
Core Specifications and Operating Profile

• Operating weight: approximately 4,000 to 4,500 kg
  
• Engine: Mitsubishi diesel, typically 40–50 hp
  
• Hydraulic system: open-center with gear-type pump
  
• Maximum digging depth: around 3.5 meters
  
• Bucket breakout force: approximately 25 kN
  
• Track width: 400 mm
  
• Swing radius: reduced tail swing for tight maneuvering
  

The machine features dual travel motors, a pilot-operated control system, and a mechanical swing gear. Its undercarriage is rigid with rubber or steel track options depending on market.
Terminology Notes

• Open-Center Hydraulics: A system where fluid flows continuously through control valves until actuated, common in older excavators.
  
• Pilot Control: A low-pressure hydraulic system used to operate main control valves with minimal effort.
  
• Travel Motor: A hydraulic motor that drives the tracks, allowing forward and reverse movement.
  
• Spool Valve: A cylindrical valve component that directs hydraulic flow based on joystick input.
  

Common Hydraulic Issues and Track Behavior
Operators have reported a recurring issue with the MS40-8 where one track—typically the right—fails to engage independently at low engine speeds. This behavior often improves at higher RPMs but may still present intermittent failure. The symptoms include:

• Right track not responding unless left track is also engaged
  
• Audible clunking from undercarriage during attempted movement
  
• No engine load increase during failure, suggesting fluid bypass
  
• Improved response after seal replacement but not full resolution
  

This issue is often linked to internal leakage or pressure loss in the spool valve assembly, particularly in the section that controls travel motor flow. A worn seal or misaligned spool can divert fluid away from the motor, causing partial or no engagement.
Field Anecdote and Improvisation
In 2017, a small contractor in Romania used an MS40-8 for trenching in hilly terrain. After noticing the right track stalling on inclines, he replaced the travel motor seals and flushed the hydraulic lines. The issue persisted until he discovered a worn spool in the travel control valve. Unable to source a replacement, he machined a custom brass sleeve to restore alignment. The fix held for over 600 hours of operation.
Diagnostic Strategy and Service Tips
To isolate the cause of track failure:

• Test hydraulic pressure at both travel motor ports during operation
  
• Inspect spool valve seals and check for scoring or misalignment
  
• Swap left and right travel motor hoses to verify motor integrity
  
• Check pilot pressure and joystick response for lag or dead zones
  
• Inspect relief valve settings and bypass flow under load
  

If the issue is intermittent and RPM-dependent, it may indicate marginal pressure loss or internal leakage that only resolves at higher pump output.
Preventive Maintenance and Recommendations

• Replace hydraulic fluid every 1,000 hours or annually
  
• Clean pilot filters and inspect control valve seals quarterly
  
• Use high-quality hydraulic oil with anti-foaming additives
  
• Monitor track tension and travel motor temperature during operation
  
• Flush system after any major seal replacement to remove debris
  

For machines operating in cold climates, consider installing a hydraulic fluid heater or using low-viscosity oil to improve responsiveness during startup.
Design Simplicity and Operator Experience
The MS40-8 offers a straightforward cab layout with analog gauges and mechanical levers. Visibility is good, and the control response is predictable. While it lacks modern diagnostics, its modular hydraulic system allows for field repairs and component swaps without specialized tools.
Operators appreciate its stability and digging power, but note that hydraulic quirks—especially in aging units—require patience and mechanical intuition. The machine’s compact size makes it ideal for urban jobs, but its performance depends heavily on fluid cleanliness and valve integrity.
Conclusion
The Mitsubishi MS40-8 remains a capable compact excavator despite its age and limited parts availability. Hydraulic issues such as one-track failure are often solvable with careful diagnosis and seal replacement. With proper maintenance and occasional improvisation, the MS40-8 continues to serve as a reliable tool in tight spaces and tough conditions. In the world of compact excavators, it stands as a reminder that mechanical simplicity can still deliver powerful results.