Diagnosing Boom and Stick Hydraulic Issues on Mid-Size Excavators

[MikePhua]

The Evolution of the 490E, 690E, and Hitachi EX Series
John Deere’s 490E and 690E excavators, along with Hitachi’s EX120 and EX200, represent a generation of mid-size hydraulic machines that dominated the construction market in the 1990s. Built during the Deere-Hitachi joint venture era, these models shared core hydraulic architecture, control logic, and component sourcing. The 490E and 690E were powered by reliable diesel engines and featured pilot-operated hydraulic systems, while the Hitachi EX series offered similar performance with slightly different control valve configurations and boom geometry.
These machines were widely adopted across North America, Asia, and Europe, with tens of thousands sold. Their reputation for durability and mechanical simplicity made them popular in rental fleets, municipal work, and owner-operator businesses. However, as these units age, hydraulic issues—especially in the boom, stick, and crowd functions—begin to surface.
Terminology Notes

• Crowd Function: The hydraulic movement that extends or retracts the stick (also called dipper arm)
  
• Boom Cylinder: The hydraulic actuator that raises or lowers the boom
  
• Stick Cylinder: The cylinder responsible for crowd movement
  
• Pilot Pressure: Low-pressure hydraulic signal used to control high-pressure valves
  
• Main Control Valve: The central hydraulic manifold that directs flow to various actuators
  

Symptoms of Hydraulic Dysfunction
Operators of aging 490E, 690E, EX120, and EX200 units often report:

• Boom lifts slowly or not at all under load
  
• Stick retracts but hesitates or stalls during extension
  
• Crowd function lacks power or moves erratically
  
• Audible hydraulic whine or cavitation during operation
  
• No improvement after filter or fluid change
  

These symptoms typically point to internal leakage, valve spool sticking, pilot signal loss, or pump wear. In one case, a contractor in Alberta noticed his EX200’s boom would lift fine when cold but stall after 30 minutes of operation. The issue was traced to a worn spool in the main valve block that expanded under heat and began to bind.
Diagnostic Strategy and Pressure Testing
To isolate the fault:

• Check hydraulic fluid level and condition (should be clean, amber, and free of foam)
  
• Inspect pilot lines for cracks, pinholes, or loose fittings
  
• Use a pressure gauge to test pilot pressure (typically 400–600 PSI)
  
• Test main pump output at full throttle under load (should exceed 4,000 PSI)
  
• Remove and inspect control valve spools for scoring or contamination
  
• Verify that the relief valves are not stuck open or misadjusted
  

If pilot pressure is low, the control valves may not fully open, resulting in weak or delayed actuator movement. If main pressure is adequate but movement is slow, internal leakage in the cylinders or valve block is likely.
Common Failure Points and Field Solutions
Frequent culprits include:

• Worn boom or stick cylinder seals causing internal bypass
  
• Sticky valve spools due to varnish or debris
  
• Cracked pilot hoses reducing signal strength
  
• Weak pump output from worn swash plate or drive shaft
  
• Misadjusted or failed relief valves bleeding off pressure
  

Solutions:

• Rebuild cylinders with new seals and inspect rods for scoring
  
• Clean valve spools and bores with solvent and polish lightly
  
• Replace pilot hoses with reinforced hydraulic-rated lines
  
• Test and rebuild the main pump if output is below spec
  
• Adjust relief valves to factory settings and replace if leaking
  

A technician in Georgia shared that after replacing the pilot filter and cleaning the pilot manifold, his 690E regained full stick power and operated smoothly under load.
Preventive Maintenance Recommendations
To maintain hydraulic performance:

• Change hydraulic fluid every 1,000 hours or annually
  
• Replace pilot and return filters every 500 hours
  
• Grease all pivot points weekly to reduce cylinder side load
  
• Inspect hoses and fittings monthly for wear or leaks
  
• Perform pressure tests during scheduled service intervals
  

For machines operating in dusty or humid environments, consider installing breather filters and desiccant caps on the hydraulic reservoir to prevent moisture ingress.
Stories from the Field
A grading crew in Oregon used a 490E to dig utility trenches. After noticing sluggish boom response, they discovered that the pilot pressure line had rubbed against the frame and developed a pinhole leak. Replacing the line restored full responsiveness and reduced fuel consumption by 8%.
In another case, a demolition operator in South Africa retrofitted his EX200 with a pressure monitoring system and alarm. The system alerted him to low pilot pressure before failure, allowing him to shut down and replace a clogged pilot filter without damaging the valve block.
Conclusion
Hydraulic issues in mid-size excavators like the John Deere 490E, 690E, and Hitachi EX120/EX200 are often rooted in pilot signal loss, valve wear, or internal leakage. With methodical diagnostics and targeted repairs, most problems can be resolved without major overhaul. These machines, though aging, remain mechanically sound and capable of productive service when maintained with care and precision.