A large quarry reported that a SEM 652 wheel loader (a Shandong Engineering Machinery brand) had a steering wheel that would not return to center (steering drift) and a boom that slowly dropped when the engine was idling. The machine had been used for heavy rock loading for over 5,000 hours.
The technician first tested the steering system. The SEM 652 uses a "Hydraulic Steering Unit" (orbitrol) with a priority valve. The steering wheel should center itself due to the internal valve geometry. He found that the "Steering Column Universal Joint" was seized. This mechanical binding prevented the steering wheel from returning to center, causing the machine to drift. He lubricated the joint, which fixed the steering feel, but the drift persisted.
He then checked the "Priority Flow Divider." This valve ensures steering gets oil before the implements. Upon removing the valve cartridge, he found the "Positioning Pin" (which holds the spool in neutral) was sheared. This allowed the spool to float, causing the steering system to starve for oil and the implement system to lose pressure.
The boom drop was a separate issue. He isolated the lift cylinders and found the left cylinder rod was covered in a thin film of oil. The "Piston Seals" (V-rings) had worn out, allowing high-pressure oil to leak into the rod chamber. This leakage caused the boom to drop under gravity.
The repair involved replacing the steering column U-joint and the priority valve spool. He also replaced the lift cylinder piston seals and honed the cylinder bore to remove scoring. The hydraulic tank was flushed. After reassembly, the steering returned to center instantly, and the boom held its position solidly.
