A quarry operator reported that a Volvo L150H wheel loader was drifting to the left while driving in a straight line. To keep the machine straight, the operator had to hold the steering wheel 30 degrees to the right. Additionally, the steering felt "jerky" or sticky, rather than smooth and progressive.
The Volvo L150H utilizes a load-sensing steering system with a priority valve and a steering metering unit (orbitrol). The technician first checked the steering hydraulic pressure and flow, which were within spec. The issue was isolated to the metering unit inside the steering wheel column. This unit meters the exact amount of oil to the steering cylinders based on the rotation of the wheel.
The steering wheel was removed, and the metering unit was disassembled. The internal gerotor (rotor and stator) set had deep gouge marks on the teeth. This was caused by fine contamination in the hydraulic oil, likely from a previous burst hose incident where metal shavings were not fully flushed. The gouges allowed internal bypass, meaning that when the wheel was centered, oil was still leaking through the metering unit to the left cylinder, causing the drift.
The "jerky" sensation was diagnosed separately. The steering cylinder lines were inspected. The fittings were tight, but the hoses were routed directly over the top of the front differential. The intense heat from the differential had caused the hydraulic fluid to boil slightly, creating microscopic air bubbles (aeration) in the lines. Air is compressible, so when the operator turned the wheel, the air compressed first, causing a delay (stickiness), followed by a sudden movement when the air pressure overcame the resistance.
The repair involved replacing the entire steering metering unit with a new OE component. The hydraulic lines were re-routed with heat shields to protect them from differential heat, and the system was purged of air by cycling the steering 20 times with the engine idling. The loader then tracked straight and steered smoothly.
