A heavy construction crew reported that a Hitachi Zaxis 210-3 excavator's boom would drift down when the engine was idling. Additionally, when the operator lowered the boom at full speed, the hydraulic system made a loud "chattering" noise, and the movement was jerky.
The technician diagnosed the hydraulic control valve. The symptoms pointed to two distinct valve failures within the main control block. First, the "Boom Drift" was caused by the "Load Check Valve" (located in the boom inlet port). He removed the check valve cartridge. The valve seat was scored, and a small piece of rubber seal was lodged in the seat. This prevented the valve from fully closing, allowing oil to leak back into the tank when the boom was held up.
Second, the "Chattering" during boom lowering was caused by the "Anti-Rebound Valve." This valve is designed to prevent the cylinder pressure from dropping too fast (cavitation) when the boom is lowered rapidly. He removed the valve and found the spool was stuck in the closed position by varnish (oxidized hydraulic oil). The stuck valve blocked the "regeneration circuit," forcing the boom cylinder to pull oil from the tank against resistance, causing the jerky motion and noise.
The repair involved lapping the load check valve seat to restore the seal (using a fine grinding compound). The Anti-Rebound valve was cleaned with a solvent to dissolve the varnish, and the spool was freed up to move smoothly. The hydraulic oil was tested and found to be contaminated. The technician flushed the system and replaced the return filter. After reassembly, the boom held steady, and the lowering motion was smooth and quiet.
