A Sany SY230C-8 excavator operating on a construction site suddenly experienced a complete loss of all hydraulic functions (boom, arm, bucket, swing, and travel) while the engine continued to run smoothly at normal speed. This case study outlines the diagnostic process that led to the identification of a failed hydraulic pump coupling and the subsequent repair.
The operator reported that the machine was functioning normally when it abruptly stopped responding to all control inputs. The engine started and idled correctly, but moving any joystick produced no movement from any hydraulic actuator. The absence of any unusual noises or warning lights initially complicated the diagnosis.
Electrical System Check: The first step was to rule out electrical issues, which are common causes of hydraulic system failures. The technician checked the main fuse box for any blown fuses; all fuses were intact. The pilot control solenoid valve was tested for voltage; a steady 24V DC supply was present, confirming the pilot switch, relays, and wiring were functional. The solenoid's resistance was measured at approximately 34 ohms, which is within the normal operating range, indicating the solenoid itself was not faulty.
Hydraulic Pressure Testing: With the electrical system verified, attention turned to the hydraulic system. Pressure gauges were installed at the main pump outlet and the pilot circuit. When the engine was started and controls were actuated:
Pilot circuit pressure: 0 psi (normal operating pressure is approximately 400 psi/28 bar).
Main system pressure: 0 psi (normal operating pressure is approximately 5,000 psi/350 bar).This confirmed a complete failure of the hydraulic pump to generate pressure.
Hydraulic Pump and Coupling Inspection: Since the pump was not producing pressure despite a running engine, the connection between the engine and the hydraulic pump was inspected. The main hydraulic pump is driven by the engine crankshaft via a flexible coupling (a splined shaft and hub assembly). The pump was removed, and the coupling was disassembled for inspection.
The inspection revealed that the internal splines of the coupling hub were completely stripped and worn away. The splines on the pump drive shaft were also severely damaged. This failure meant that although the engine was spinning the input side of the coupling, there was no mechanical connection to drive the hydraulic pump. Consequently, the pump remained stationary, unable to generate any hydraulic flow or pressure, resulting in the total loss of machine functions.
Component Replacement: The damaged coupling assembly (hub and shaft) and the worn pump drive shaft were replaced with genuine Sany OEM parts.
Pump Reinstallation: The hydraulic pump was carefully reinstalled, ensuring proper alignment with the engine drive flange. The new coupling was installed, and the mounting bolts were torqued to the manufacturer's specifications.
System Bleeding and Purge: Air was purged from the hydraulic system by cracking the main pump outlet lines and running the engine at low idle until a steady stream of oil (free of air bubbles) was observed. The lines were then tightened.
Function Testing: The engine was started, and all hydraulic functions were tested. The boom, arm, bucket, swing, and travel circuits all operated smoothly and with full power. System pressures were checked and confirmed to be within the manufacturer's recommended parameters.
This failure underscores the importance of regular inspection of the hydraulic pump coupling as part of routine maintenance. Technicians should include a check for excessive play, unusual noise, or signs of wear in the coupling during every major service. Additionally, ensuring the pump is properly aligned and the mounting bolts are correctly torqued can prevent premature failure of this critical component.
