Troubleshooting Hydraulic Issues on the Kobelco SK70 Excavator

[MikePhua]

Introduction to the Hydraulic Problem
Owners of the Kobelco SK70SR excavator report a troubling issue: the engine vibrates under light load, the hydraulic fluid overheats quickly, and the machine loses hydraulic power. The dozer blade circuit still operates (lifting and holding), but all other hydraulic functions—boom, arm, bucket, track travel, and swing—fail .
Possible Causes and Diagnostics

• Main Pump or Coupling Wear
  Vibration and overheating often point to a failing hydraulic pump or pump coupling. Worn engine mounts can also contribute to felt vibrations .
  
• Control Valve or Relief Valve Issues
  The dozer blade operates independently via a separate relief or control valve. If other functions stop working but the blade still responds, it suggests that the main control valve manifold may be malfunctioning. Swapping dozer relief valves OR9 and OR10 or inspecting valve spools may reveal the fault .
  
• Low Pilot Pressure or Faulty Solenoid
  A collapsed or non-functioning pilot pressure circuit can prevent hydraulic valves from activating. Inspecting and testing the solenoid lock‑out switch, especially after bypassing safety interlocks, is recommended .
  

Diagnostic Steps to Take

• Check Hydraulic Pressures
  Connect pressure test gauges to hydraulic test points, particularly targeting dozer raise/lower circuits, main relief, and pilot lines .
  
• Inspect Control Valves
  Locate and assess the main control valve assembly. Ensure solenoids and spools actuate correctly. Manifold-mounted pressure or pilot sensors must be tested .
  
• Verify Solenoid Operation
  Measure resistance in solenoid coils; zero ohms indicates a short and coil failure. Additionally, check for correct activation voltage (should not exceed ~28 V) without engine overspeeding the ECM .
  

Common Hydraulic System Issues to Evaluate

• Contamination: Dirt or debris in fluid can clog valves, causing overheating and pressure loss .
  
• Leaks: Seal failure or hose damage can reduce system pressure and cause heat build-up .
  
• Air in System: Air entrainment can lead to spongy operation and overheating .
  
• Component Wear: Pump internal wear or control valve degradation can reduce flow efficiency and fluid cooling ability .
  

Maintenance and Resolution Overview

• Replace or rebuild the main hydraulic pump or coupling if pressures remain low or inconsistent.
  
• Clean hydraulic fluid, replace filters, and flush the system if contamination is confirmed.
  
• Replace faulty pilot solenoids or control valve assemblies if resistance testing or voltage checks fail.
  
• Sandblasting, cleaning, or replacing valve spools and cartridge components may restore function to the dozer lift circuit and restore full hydraulic operation.
  

Terms Glossary

• Pilot Pressure: Low-pressure hydraulic line used to activate valve spools.
  
• Control Valve Manifold: Central hydraulic valve assembly directing flow to different circuits.
  
• Solenoid Lock‑Out: Electronic switch controlling activation of hydraulic circuits.
  
• Relief Valve OR9/OR10: Preset hydraulic valves managing dozer blade flow independently.
  
• Valve Spool: Internal movable component that directs fluid when actuated.
  

Case Example
An operator bypassed the safety switch expecting full function but had only dozer lift power remaining. Testing revealed zero pilot activation on all other circuits—a clear sign that the solenoid responsible for unlocking the main valve pack was faulty. After replacing the solenoid and recalibrating, full hydraulic control returned with normalized fluid temperature and no shaker engine behavior reported.
Key Takeaways

• Investigate both mechanical (pump, coupling) and electrical (solenoid, pilot pressure) causes.
  
• Pressure testing across different circuits is crucial for accurate diagnosis.
  
• Leaks, air entrainment, and fluid contaminants often exacerbate symptoms.
  
• Swift troubleshooting can avoid major replacements and restore safe operation.
  

If you’d like, I can build you a step-by-step pressure test plan or guide on solenoid replacement and valve manifold service.