Troubleshooting Slow Hydraulic Cylinder Movements (Boom, Arm, and Bucket) on the EL300 Excavator During Operation

 **Cause Analysis** In the absence of a specific hydraulic system schematic or detailed test data, we conducted a comparative analysis using the schematic for the Hitachi UH171 excavator. We identified the following potential causes: severe internal or external leakage in the hydraulic cylinders; a malfunction in the combined control valve assembly (including the relief valve); a damaged relief valve causing most of the pressurized oil to return to the tank; a main pump failure; or a pilot pump failure. Since the movements of the boom, arm, and bucket were all sluggish, we began by testing the main pump.

 

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 **3. Testing Procedure** Loosen the hydraulic tank's plug to vent compressed air. Connect the inlet of the PFM2B hydraulic tester to the main pump outlet, and route the tester's outlet directly back to the hydraulic tank. Run the pump until the hydraulic oil temperature reaches 53°C.

 Gradually increase the engine speed from idle while simultaneously tightening the tester's load valve. We observed the pressure rising steadily; at an engine speed of 1800 ± 30 rpm, the pressure was 27 MPa. Upon momentarily increasing and then decreasing the engine speed, the peak system pressure measured was 29.5–30.0 MPa.

 

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Based on these test results, we concluded that the main pump was functioning correctly. We then shifted our focus to testing the pilot pump pressure. A pressure gauge with a 7.5 MPa range was connected directly to the hydraulic line between the pilot relief valve and the pilot control valve. To prevent overloading the pilot pump during testing, a mechanic stood by to adjust the pilot relief valve's setting screw as needed. We gradually increased the engine speed and adjusted the pilot relief valve, finding the pressure to be within the 1.0 ± 0.3 MPa range. However, adjusting the screw travel resulted in little change in pressure, indicating that the fault lay in this component. Upon opening the pilot safety valve, three silvery-white aluminum chips (1–2 mm in size) were found on the sealing cone surface of the valve spool. Maintenance personnel reported that the pilot pump had recently been replaced. We changed the hydraulic oil, cleaned the oil lines and filters, reassembled the safety valve, and adjusted the pilot pressure to a measured 2.45 MPa. Subsequent machine operation confirmed that the operating speeds of the various cylinders met the specified requirements.

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