A wheel loader was performing weakly. It would lift a light pallet just fine, but as soon as it tried to curl a heavy bucket, the bucket would slow to a crawl. Curiously, the steering remained perfectly responsive and fast throughout the cycle. The mechanic replaced the lift cylinders and the main pump, but the weakness persisted.
The problem was in the "Steering Priority Valve." In many loaders, the steering system is given priority over the hydraulic implement system. This valve ensures that even if the operator is lifting a full bucket and jerks the wheel, there is enough oil to keep the machine steering without stalling.
The valve is spring-loaded to block flow to the implements until the steering demand is satisfied. Over time, the spring inside the valve fatigued, or the spool scored. The valve began to favor the steering circuit too aggressively, sending almost all the pump flow to the steering circuit and starving the lift circuit. The operator would turn the wheel slightly, and the machine would steer effortlessly, but the lift hydraulics would bleed out. We rebuilt the steering priority valve block, restoring the flow balance to the implement spool.
