A quarry reported that a Volvo L120H wheel loader's boom shuddered violently when raised past the halfway point. The shudder only happened under a heavy load, and the main hydraulic pump made a groaning noise.
The technician first checked the "Boom Cylinder Cushion Valves" (designed to slow the boom at the top of the stroke), which were functioning. He then focused on the "Hydraulic Pulse Control Valve" (PCV) on the main pump. The L120H uses a load-sensing system with a nitrogen-charged accumulator to stabilize the pump's pressure feedback. He pressed the Schrader valve on the accumulator; no gas escaped, but hydraulic oil squirted out.
This indicated the "Accumulator Bladder" had ruptured. The nitrogen charge is what dampens pressure spikes in the load-sensing line. With the bladder broken, the load-sensing signal was hydraulically locked. When the boom passed the midpoint, the lever control valve opened rapidly, causing a sudden pressure drop. Without the accumulator to dampen the signal, the pump's "Pulse Control Valve" spool oscillated rapidly (chattered) trying to keep up with the pressure changes. This pump oscillation caused the violent mechanical shudder in the boom.
The repair involved replacing the hydraulic accumulator (the internal bladder cannot be serviced separately). The pulse control valve spool on the main pump was also cleaned with brake cleaner, as it had stuck slightly due to the lack of dampening. The system was bled of air. Afterward, the boom lifted smoothly under full load, and the pump groan disappeared.
