A landscaping company reported that a Sany SY75C mini excavator's boom would slowly drift down when the safety lever was engaged, and there was a strong smell of hot hydraulic oil after just one hour of operation.
The technician first checked for external leaks, finding none. He touched the hydraulic tank and cooler return line; the tank was extremely hot, but the cooler return line was lukewarm. This indicated that the oil was bypassing the cooler. The SY75C has a "Priority Valve" that sends oil to the steering and pilot systems. The pilot system includes a relief valve that maintains low pressure for the control sticks.
The technician removed the pilot relief valve. The valve seat was scored, and the spring was weak. This caused the pilot pump to constantly pump oil into the relief valve, generating heat. Furthermore, the pilot pressure was lower than spec. The boom drift was linked to this. The main control valve uses "pilot pressure" to assist in moving the spools. With low pilot pressure, the "Load Check Valve" in the boom circuit was not fully closing, allowing oil to leak from the boom cylinder back to the tank.
The repair involved replacing the pilot relief valve cartridge. The hydraulic cooler was also flushed to remove any debris. The technician also found that the suction strainer in the hydraulic tank was clogged with mud, restricting flow and causing cavitation. The suction strainer was cleaned, and the hydraulic oil was replaced. After the repair, the cooler return line became hot (indicating flow), the smell stopped, and the boom held its position.
