08-29-2025, 08:28 PM
The Role of the Pilot Pump in Excavator Control
In hydraulic excavators like the Hitachi EX120, the pilot pump plays a critical role in delivering low-pressure hydraulic fluid to the control valves. This fluid powers the pilot circuits that actuate the main valve spools, enabling precise control of boom, arm, bucket, and travel functions. Unlike the main pump, which operates at high pressure for heavy lifting, the pilot pump typically runs at 400–600 psi and supports joystick responsiveness and proportional control.
When the pilot pump exhibits abnormal behavior—such as rapid pulsing at the outlet hose—it can signal deeper issues in the hydraulic system, affecting machine responsiveness and safety.
Identifying Fast Pulsing in the Pilot Line
A technician observing the pilot pump outlet hose on an EX120 noted an unusually fast pulse, deviating from the expected smooth flow. This symptom may indicate:
Potential Causes of Pressure Instability
Several mechanical and hydraulic factors can contribute to erratic pilot pump behavior:
To isolate the issue:
Field Anecdotes and Practical Solutions
One operator in Southeast Asia reported similar symptoms on a Komatsu PC120. After replacing the pilot pump with a rebuilt unit, the pulsing persisted. The real culprit was a collapsed suction hose that looked fine externally but had delaminated internally. Replacing the hose resolved the issue instantly.
Another technician found that a pilot filter element had disintegrated, sending debris into the pressure-reducing valve. After cleaning the valve and replacing the filter, the pilot circuit stabilized.
Preventive Measures and Long-Term Reliability
To maintain pilot system health:
Conclusion
Fast pulsing at the pilot pump outlet on a Hitachi EX120 is more than a curiosity—it’s a warning sign of pressure instability, air ingress, or flow restriction. By methodically inspecting the pump, hoses, filters, and valves, technicians can restore smooth pilot operation and ensure precise control. In hydraulic systems, consistency is key—and the pilot circuit is where precision begins.
In hydraulic excavators like the Hitachi EX120, the pilot pump plays a critical role in delivering low-pressure hydraulic fluid to the control valves. This fluid powers the pilot circuits that actuate the main valve spools, enabling precise control of boom, arm, bucket, and travel functions. Unlike the main pump, which operates at high pressure for heavy lifting, the pilot pump typically runs at 400–600 psi and supports joystick responsiveness and proportional control.
When the pilot pump exhibits abnormal behavior—such as rapid pulsing at the outlet hose—it can signal deeper issues in the hydraulic system, affecting machine responsiveness and safety.
Identifying Fast Pulsing in the Pilot Line
A technician observing the pilot pump outlet hose on an EX120 noted an unusually fast pulse, deviating from the expected smooth flow. This symptom may indicate:
- Pressure instability
- Air entrainment in the pilot circuit
- Internal leakage in the pump or control valve
- Flow restriction downstream of the pump
- Cavitation due to suction-side issues
Potential Causes of Pressure Instability
Several mechanical and hydraulic factors can contribute to erratic pilot pump behavior:
- Worn Pump Components
If the pump’s internal gears or vanes are worn, it may struggle to maintain consistent output, especially at idle. This can cause surging or pulsing in the outlet line.
- Air in the System
A cracked suction hose or loose fitting can introduce air into the pilot circuit. Air bubbles compress and expand under pressure, creating a pulsing effect.
- Blocked Pilot Filter
A clogged pilot filter restricts flow and forces the pump to work harder, leading to pressure spikes and erratic delivery.
- Faulty Pressure Regulator
The pilot system includes a pressure-reducing valve to maintain safe operating pressure. If this valve sticks or malfunctions, pressure may fluctuate rapidly.
- Control Valve Leakage
Internal leakage in the pilot control valves can cause pressure drops and recovery cycles, resulting in visible pulsing at the hose.
To isolate the issue:
- Install a low-pressure gauge at the pilot pump outlet and monitor pressure stability
- Inspect the suction hose for cracks, soft spots, or loose clamps
- Replace the pilot filter and check for contamination
- Test the pressure-reducing valve for proper function and spring tension
- Bleed the pilot circuit to remove trapped air
- Check joystick response and control valve behavior under load
Field Anecdotes and Practical Solutions
One operator in Southeast Asia reported similar symptoms on a Komatsu PC120. After replacing the pilot pump with a rebuilt unit, the pulsing persisted. The real culprit was a collapsed suction hose that looked fine externally but had delaminated internally. Replacing the hose resolved the issue instantly.
Another technician found that a pilot filter element had disintegrated, sending debris into the pressure-reducing valve. After cleaning the valve and replacing the filter, the pilot circuit stabilized.
Preventive Measures and Long-Term Reliability
To maintain pilot system health:
- Replace pilot filters every 500–750 hours
- Use hydraulic fluid with proper anti-foaming additives
- Inspect hoses and fittings during every service interval
- Monitor pilot pressure during machine startup and warm-up
- Train operators to report sluggish controls or erratic joystick response
Conclusion
Fast pulsing at the pilot pump outlet on a Hitachi EX120 is more than a curiosity—it’s a warning sign of pressure instability, air ingress, or flow restriction. By methodically inspecting the pump, hoses, filters, and valves, technicians can restore smooth pilot operation and ensure precise control. In hydraulic systems, consistency is key—and the pilot circuit is where precision begins.
