A heavy civil contractor reported that a John Deere 470G LC excavator had a specific issue: the arm (stick) was slow when curling inward (arm-in), but fast when extending. Additionally, when the arm was held horizontally in the air, it would slowly drift downward over 5 minutes.
The technician began with the "Arm Cylinder By-Pass Test." He raised the arm, placed the bucket on the ground, and loosened the hydraulic lines at the cylinder. The drift was isolated to the cylinder seals leaking internally. However, the "slow arm-in" speed was not just a seal issue.
He inspected the main control valve for the arm circuit. The arm function uses a "regeneration circuit" to speed up the arm-in motion. When the operator pulls the joystick, the oil from the rod side of the cylinder is pushed out. Instead of going to the tank, a "Regeneration Check Valve" opens, directing this oil back to the piston side to add flow. The technician removed the valve block and found that the regeneration check valve spool was stuck in the closed position by a small burr of metal. This blocked the regeneration flow path, forcing all the oil through the main relief valve, which caused the slow speed and generated excess heat.
The repair involved replacing the arm cylinder piston seals (which were worn flat) and honing the regeneration valve bore to remove the burr. A magnetic plug was installed in the hydraulic line to catch any remaining debris. After reassembly, the arm curled in rapidly, and the drift was eliminated.
