Hitachi EX200-2 Main Pump Fails to Reach Full Stroke Under Load

[MikePhua]

EX200-2 Excavator Background and Pump Control System
The Hitachi EX200-2 hydraulic excavator was introduced in the early 1990s as part of Hitachi’s second-generation lineup, designed for mid-size earthmoving and general construction. With an operating weight of approximately 20 metric tons and powered by an Isuzu 6BG1T engine, the EX200-2 featured a dual-pump hydraulic system controlled by electronic sensors and solenoids. The machine’s main pumps are variable displacement axial piston units, regulated by a swash plate mechanism that adjusts stroke length based on load demand and control signals.
The pump control system includes angle sensors, solenoids, and a pump control computer that modulates flow and pressure. Under normal conditions, disabling the pump solenoids should allow the swash plate to move to full stroke, delivering maximum flow. However, in some cases, the pump stalls at partial stroke, causing engine load and black smoke from the exhaust.
Symptoms of Incomplete Pump Stroke
Operators have reported the following behavior:

• Swash plate only reaches approximately 80% of full stroke
  
• Engine bogs down and emits black smoke under load
  
• Disabling solenoids does not allow full stroke
  
• Voltage readings from the angle sensor are incorrect or unstable
  
• Unload valve adjustments have no effect
  

These symptoms suggest a combination of electrical signal loss and internal hydraulic restriction.
Terminology Clarification

• Swash Plate: A tilting plate inside the pump that controls piston stroke length and thus flow rate.
  
• Solenoid Valve: An electrically actuated valve that modulates hydraulic control pressure.
  
• Angle Sensor: A potentiometer or Hall-effect device that measures swash plate position.
  
• Unload Valve: A valve that relieves pressure in the pump circuit during startup or idle.
  

Electrical and Sensor Diagnostics
When the pump fails to reach full stroke, the first suspects are the angle sensor and its wiring. A faulty sensor may send incorrect feedback to the pump controller, causing it to limit stroke length. Similarly, damaged wiring or poor grounding can disrupt voltage signals.
Recommended checks include:

• Inspecting angle sensor wiring for continuity and corrosion
  
• Measuring voltage output during pump actuation (typically 0.5–4.5V range)
  
• Verifying pump control relay function and solenoid coil resistance
  
• Checking DP (differential pressure) sensor for signal integrity
  

If the sensor is functional but the swash plate still stalls, the issue may be mechanical.
Internal Hydraulic Restrictions and Mechanical Blockage
In rare cases, the swash plate may be physically restricted due to internal wear, contamination, or a failed control piston. Hydraulic passageways inside the pump body can become blocked by debris or varnish, especially in machines with poor fluid maintenance.
Mechanical causes include:

• Scored control piston bore
  
• Stuck swash plate pivot
  
• Debris in pilot pressure lines
  
• Worn or misaligned pump barrel
  

To diagnose, the pump must be removed and bench-tested. A teardown may reveal scoring, broken springs, or hardened seals that prevent full actuation.
Field Anecdote and Practical Insight
A technician in Southeast Asia encountered a similar issue on an EX200-2 used in quarry work. After replacing the angle sensor and solenoids without success, he discovered that the pump’s control piston had seized due to varnish buildup. The machine had been running on degraded hydraulic oil for years. After cleaning the piston bore and replacing seals, the pump returned to full stroke and engine load normalized.
Another operator in Australia noted that his pump would only reach full stroke after warming up for 30 minutes. This pointed to thermal expansion compensating for internal binding—a temporary fix that masked deeper mechanical wear.
Preventive Measures and Recommendations
To maintain pump performance:

• Replace hydraulic fluid every 1,000 hours or annually
  
• Use OEM-grade filters and monitor for contamination
  
• Inspect electrical connectors quarterly and apply dielectric grease
  
• Keep a record of sensor voltages and solenoid resistance values
  
• Bench-test pumps during major service intervals
  

Conclusion
When the EX200-2 main pump fails to reach full stroke, the cause may lie in a complex interplay of electrical signals, sensor feedback, and internal hydraulic mechanics. While solenoid disabling should allow full stroke, mechanical restrictions or faulty sensors can override expected behavior. By combining electrical diagnostics with careful inspection of pump internals, technicians can restore full flow and prevent engine overload. In hydraulic systems, precision and cleanliness are everything—and the EX200-2 is no exception.