A road construction crew reported that a Volvo EC220D excavator suffered from "Boom Nod"-a jerky, nodding motion when the boom was lowered. Additionally, the arm (stick) function was slow when curling in, especially when the boom was raised high. The engine RPM was stable, and the hydraulic oil temperature was normal.
The technician first checked the main pump flow, which was within specs. The "Boom Nod" symptom typically indicates a regeneration circuit failure. In a standard excavator, when the boom is lowered, the oil from the piston side is routed back to the rod side to prevent cavitation and increase speed. This is controlled by a regeneration check valve. The technician removed the boom control valve block. The regeneration check valve was found to have a broken return spring. The valve was stuck open, allowing oil to flow uncontrolled, creating a pressure imbalance that caused the boom to nod.
The slow arm function required a deeper diagnostic. The technician tested the line pressure for the arm cylinders. The pressure would rise to a set point and then drop suddenly, indicating internal leakage. The main spool valve for the arm circuit was removed. The spool lands were found to be heavily scored with deep grooves. This scoring allowed pressurized oil to leak past the spool and return to the tank, rather than flowing to the cylinder. This is a common failure mode in machines that perform constant "truck loading" cycles, as the high-cycle shifting of the spool causes wear over time.
The repair involved replacing the broken regeneration spring and installing a new arm control spool. The hydraulic oil was also changed, as the score marks on the old spool suggested the presence of metallic fines. A magnetic filter was installed on the return line to catch any remaining debris. After reassembly, the boom lowering was smooth, and the arm curl speed matched the manufacturer's specifications.
