Sumitomo SH120 Excavator Troubleshooting and Performance Insights

[MikePhua]

The SH120 and Sumitomo’s Excavator Lineage
The Sumitomo SH120 is a 12-ton class hydraulic excavator developed for urban construction, utility trenching, and light demolition. Manufactured by Sumitomo Construction Machinery, a division of Sumitomo Heavy Industries founded in 1963, the SH series represents decades of Japanese engineering focused on fuel efficiency, hydraulic precision, and operator comfort. The SH120 was introduced in the late 1990s and continued through multiple revisions, including the SH120-3 and SH120-5, each incorporating improved electronics and emissions compliance.
Sumitomo’s partnership with Case and Link-Belt allowed its machines to reach global markets under different branding, but the core hydraulic architecture and engine configurations remained consistent. The SH120 typically features an Isuzu 4-cylinder diesel engine, open-center hydraulic system, and pilot-operated control valves.
Hydraulic System Architecture and Common Faults
The SH120 uses a variable displacement axial piston pump to supply hydraulic flow to the boom, arm, bucket, swing, and travel motors. Pilot pressure from a gear pump actuates control valves, allowing precise modulation of movement. The system operates at pressures up to 4,500 psi and includes:

• Main pump with load-sensing control
  
• Pilot pump for joystick and travel lever input
  
• Control valve block with proportional solenoids
  
• Relief valves and accumulator for pressure regulation
  
• Return filters and suction strainers
  

Common hydraulic issues include:

• Slow boom or arm movement due to clogged pilot filters
  
• Swing hesitation from low pilot pressure
  
• Travel motor stalling from worn pump swash plate
  
• Bucket curl delay from solenoid valve sticking
  

In Alberta, a contractor noticed his SH120’s swing function lagged during cold starts. After testing pilot pressure, he found the gear pump output was below spec. Replacing the pump restored full responsiveness.
Electrical System and Sensor Integration
Later SH120 models include electronic monitoring of engine RPM, hydraulic pressure, and fuel consumption. The machine’s controller interfaces with:

• Pressure sensors on main and pilot circuits
  
• Solenoid valves for auxiliary hydraulics
  
• Engine ECU for throttle control
  
• Travel speed selector and auto-idle function
  

Electrical faults often manifest as:

• Unresponsive travel levers
  
• Erratic throttle behavior
  
• Auxiliary hydraulics failing to engage
  
• Warning lights without diagnostic codes
  

Solutions include:

• Checking ground continuity at the controller
  
• Inspecting harness connectors for corrosion
  
• Testing solenoid coil resistance
  
• Resetting the ECU via battery disconnect
  

In Finland, a technician diagnosed a travel fault on an SH120-3. The issue traced to a broken wire inside the joystick harness. After splicing and sealing the wire, the machine resumed normal operation.
Engine Performance and Fuel Delivery
The Isuzu engine in the SH120 is known for reliability but requires clean fuel and regular maintenance. Fuel system components include:

• Mechanical injection pump
  
• Inline fuel filter and water separator
  
• Lift pump for priming
  
• Glow plugs for cold start assistance
  

Symptoms of fuel system issues:

• Hard starting in cold weather
  
• Black smoke under load
  
• Engine stalling at idle
  
• Loss of power during travel
  

Recommended actions:

• Replace fuel filters every 250 hours
  
• Bleed air from lines after filter change
  
• Inspect lift pump diaphragm for cracks
  
• Test glow plug resistance and relay function
  

In Texas, a contractor experienced poor cold starts. After replacing the glow plugs and upgrading to winter-grade diesel, the SH120 started reliably even at –5°C.
Preventive Maintenance and Long-Term Reliability
To extend the life of the SH120:

• Change hydraulic fluid every 2,000 hours
  
• Inspect track tension monthly
  
• Grease pivot points daily
  
• Replace air filters every 500 hours
  
• Monitor swing bearing play annually
  

A fleet manager in Poland implemented a preventive schedule using QR-coded service logs. His SH120 units averaged over 8,000 hours before requiring major hydraulic repairs.
Field Stories and Adaptations
In Alaska, an SH120 was used for pipeline trenching in permafrost. The operator added hydraulic oil heaters and insulated the cab, allowing full operation in –20°C conditions.
In Shanghai, a demolition crew retrofitted their SH120 with a quick coupler and breaker circuit. The machine handled concrete removal efficiently, and the operator praised its smooth controls and low fuel consumption.
Conclusion
The Sumitomo SH120 is a compact yet capable excavator that delivers consistent performance when properly maintained. Its hydraulic and electrical systems are straightforward to troubleshoot, and its engine offers reliable power across diverse conditions. With attention to pilot pressure, sensor integrity, and fuel cleanliness, the SH120 remains a trusted tool in the field. In the world of mid-size excavators, precision and simplicity go hand in hand—and the SH120 proves it daily.