A construction site reported that a Case 621F wheel loader had a "wandering" steering wheel. The operator had to constantly correct the wheel to drive straight, and the steering response felt jerky rather than smooth. The articulation joint was tight, and the steer cylinder mounts were solid.
The loader uses a priority flow control valve to ensure the steering gets hydraulic oil before the implement (bucket) circuit. The technician first tested the pilot pressure for the steering system. It was reading a fluctuating 400 psi, significantly lower than the required 600 psi. The flow was also unstable.
The priority flow control valve was removed and disassembled. The valve contains a feedback spool that moves to maintain constant flow to the steering regardless of engine speed. The spool was found to have deep score marks on its lands, caused by contamination in the hydraulic oil. The scoring allowed oil to bypass the steering circuit and return to the tank, causing the pressure drop.
When the engine RPM increased, the pump produced more flow, which temporarily overcame the leak, but at idle, the steering was almost non-existent. The jerky motion was caused by the spool vibrating inside the bore as it tried to compensate for the bypass leakage.
The repair involved honing the valve bore and installing a new oversized feedback spool kit. The hydraulic filter was changed, and the system was flushed. After reassembly, pilot pressure stabilized at a steady 600 psi, and the steering tracked straight without the constant need for correction.
