Intermittent Hydraulic Pump Disengagement on Hitachi EX220 Excavators

Hydraulic pump disengagement—often described as the pump “kicking off”—is a frustrating and potentially dangerous issue in excavator operation. On the Hitachi EX220, a once-powerful machine may suddenly lose hydraulic function during movement or operation. The pump disengages momentarily, then re-engages after a few seconds. This behavior occurs across all directions and functions, including track movement, boom lift, and swing.

Operators typically report that when the machine does move, it still has full power, suggesting that the issue is not related to engine output or hydraulic pump capacity, but rather to a control or protection mechanism that interrupts hydraulic flow under certain conditions.

Possible Causes of Hydraulic Pump Disengagement

This type of intermittent disengagement can be caused by several factors:

• Pressure relief valve malfunction
• Hydraulic pump control logic error
• Scored valve plates within the pump
• Sensor misreadings triggering safety cutoffs
• Electrical faults in the pump solenoid circuit

In particular, scoring on the valve plates of the hydraulic pump can cause oil to bypass or force components out of alignment under pressure. When pressure builds, the damaged surfaces may allow oil to escape into unintended paths, triggering a protective disengagement to prevent further damage.

Valve Plate Scoring and Internal Pump Damage

Valve plates are critical components in axial piston pumps, which are commonly used in excavators. These plates regulate the flow of hydraulic fluid between the pistons and the pump housing. Over time, contaminants, cavitation, or excessive heat can cause scoring—grooves or scratches—on the valve plate surface.

Consequences of valve plate scoring include:

• Loss of sealing between piston and barrel
• Reduced volumetric efficiency
• Pressure spikes due to uneven flow
• Mechanical instability causing disengagement

In one documented case, a Hitachi EX220 exhibited pump kick-off behavior after years of reliable service. Upon teardown, technicians discovered deep scoring on the valve plate and barrel assembly. The pump was rebuilt with new components, and the issue was resolved.

High-Pressure Cutoff and Safety Logic

Modern hydraulic systems often include high-pressure cutoff mechanisms to protect components from overload. These systems rely on pressure sensors and control valves to monitor real-time conditions. If pressure exceeds a threshold—due to a jammed cylinder, blocked line, or internal leakage—the system may disengage the pump to prevent catastrophic failure.

Key components involved:

• Pressure relief valve
• Load-sensing control valve
• Pump control solenoid
• Electronic control unit (ECU)

If any of these components malfunction or misinterpret system conditions, they may trigger unnecessary pump disengagement. For example, a faulty pressure sensor may report a false spike, causing the ECU to shut down the pump momentarily.

Electrical and Sensor-Driven Faults

Electrical faults can mimic mechanical failures. A loose wire, corroded connector, or intermittent signal from a pressure sensor can cause the pump control solenoid to deactivate. These faults are especially common in older machines exposed to moisture, vibration, and thermal cycling.

Recommended diagnostic steps:

• Inspect all wiring and connectors to the pump solenoid
• Test pressure sensors for accuracy and stability
• Use a multimeter to verify voltage at the solenoid during operation
• Check for fault codes in the ECU (if equipped)

In one field case, a Hitachi excavator experienced pump kick-off during trenching. After ruling out mechanical issues, technicians discovered a corroded connector at the pump control solenoid. Cleaning and resealing the connector restored normal operation.

Preventive Maintenance and Long-Term Solutions

To prevent pump disengagement and extend hydraulic system life, operators and technicians should implement a proactive maintenance strategy:

• Replace hydraulic filters at recommended intervals
• Monitor fluid cleanliness using ISO particle count standards
• Inspect pump components during major service intervals
• Use high-quality hydraulic oil with proper viscosity and additive package
• Perform regular electrical system checks, especially in high-humidity environments

Additionally, consider installing a bypass indicator on the hydraulic filter housing. This allows operators to detect when the filter is clogged and redirecting flow—often a precursor to pressure instability.

Conclusion

Pump kick-off in Hitachi EX220 excavators is a multifaceted issue that may stem from mechanical wear, hydraulic instability, or electrical faults. Valve plate scoring is a common root cause, but pressure sensors, solenoids, and control logic must also be evaluated. Through systematic diagnostics and preventive care, operators can restore full hydraulic performance and avoid costly downtime. In the world of heavy equipment, understanding the interplay between fluid dynamics and control systems is key to keeping machines moving—and working hard.