A landscaping supply yard reported that a Doosan Mega 300-V wheel loader's boom chattered violently when raising a full load of gravel. The boom took 8 seconds to raise (spec is 4 seconds), and when the operator pulled the bucket back rapidly, a loud clunking noise was heard from the boom cylinder. The engine and steering were unaffected.
The technician first installed a pressure gauge on the boom raise circuit. When the lever was moved to raise, the pressure fluctuated wildly between 80 bar and 200 bar, matching the physical chatter of the boom. He checked the main relief valve and the load-sensing pump, both of which tested within specification.
He then focused on the "Boom Cylinder Make-Up Check Valve." In a loader, when the boom is lowered or the arm is pulled in rapidly, the geometry can physically move the boom cylinder faster than the pump can supply oil, creating a vacuum (cavitation) in the cylinder. A make-up check valve opens to draw oil from the tank to fill this void. The technician removed the check valve cartridge from the cylinder head. Its internal precision wire mesh filter was completely packed with hard, black varnish and degraded O-ring fragments.
Because the make-up check valve was blocked, it could not supply oil to the cylinder during rapid movements. The severe vacuum on the rod side of the cylinder physically pulled the piston backward, slamming it against the cylinder head (the clunking noise). This vacuum also prevented full pressure from building on the head side during the raise cycle, causing the pump to starve and the pressure to fluctuate (the boom chatter).
The repair involved extracting and cleaning the make-up check valve cartridge, replacing the degraded O-ring, and flushing the valve housing. The hydraulic tank was drained, and a 3-micron flushing cart was used to remove all varnish and debris. Fresh ISO 68 anti-wear hydraulic oil and a new return filter were installed. After reassembly and bleeding, the boom raised smoothly in 4 seconds, the pressure held steady at 220 bar, and the rapid arm-in clunking disappeared.
