A mining aggregate site reported that a Komatsu WA380-8 wheel loader would drift to the left while driving straight, and the steering wheel required excessive force to turn to the right. The operator noted that the issue was worse when the machine was cold in the morning but improved slightly as the hydraulic oil warmed up.
The technician began by testing the pilot pressure system. The WA380-8 uses a pilot-operated steering system. A separate pilot pump supplies low-pressure oil (approx. 3-4 MPa) to the steering control valve. The gauge showed that pilot pressure would drop to 1.5 MPa when the steering wheel was turned to the right, but remained steady at 3.5 MPa when turned to the left. This pressure asymmetry pointed to a blockage in the steering control unit (SCU).
The SCU was removed and disassembled. Inside, the rotary spool valve-which directs pilot oil based on the steering wheel angle-was found to have deep score marks on the "right turn" lands. Microscopic debris from a previously failed hydraulic hose had lodged in the spool bore, causing it to bind. This restricted the flow of pilot oil to the main steering valve, making the steering feel "hard" in one direction.
However, the "drift" issue was separate. The technician jacked up the front axle and inspected the steering cylinders. The left steering cylinder was found to have a "spongy" feel. When the cylinder was extended, air bubbles were visible in the hydraulic oil at the bleed screw. The cylinder had been replaced recently, but the technician found that the breather port on the cylinder housing was blocked by paint overspray, preventing the cylinder from "breathing" during thermal expansion. This trapped air pocket was causing the steering to drift as the pressure compressed the air bubble. The repair involved honing the SCU spool bore, replacing the scored spool, clearing the cylinder breather, and bleeding the steering system thoroughly. The drift and hard steering were fully resolved.
