A busy lumber yard reported that a Hyster H80FT pneumatic tire forklift had developed extremely heavy, unresponsive steering, requiring the operator to muscle the wheel to execute turns. The power-assist function seemed entirely absent, drastically slowing loading operations and creating a serious safety hazard when maneuvering in tight aisles. Because the forklift utilized a load-sensing hydraulic system, the troubleshooting process required a systematic evaluation of both the priority valve and the steering control unit.
The first step was to verify basic pump output and system relief pressure. A flow meter was connected to the auxiliary test port on the main gear pump. At wide-open throttle, the pump delivered the specified 22 GPM, and the main relief valve cracked at the correct 2,800 psi. This confirmed the issue was isolated to the steering sub-circuit, not a failing main pump.
In a load-sensing system, the priority flow valve is responsible for ensuring the steering circuit receives hydraulic flow before the lift or tilt circuits. A gauge was installed at the "P" (pressure) port of the steering orbital unit (the steering control valve beneath the steering wheel). With the engine idling and the steering wheel held at full lock, the priority valve should have been sending full system pressure to the steering unit. However, the gauge only registered 1,200 psi, far below the required pressure to overcome the steering cylinder's mechanical resistance.
The priority valve was removed from the main control valve bank. Inside, the metering spool was found to be heavily scored with metallic debris, and the load-sensing signal passage was completely blocked by a compressed wad of friction material from a previously failing lift cylinder. This blockage prevented the priority valve from detecting the steering unit's demand for flow, leaving the steering circuit starved of both volume and pressure.
However, replacing the priority valve spool and cleaning the signal lines did not entirely resolve the heavy steering. While the assist was restored, the wheel still felt physically "notchy" and stiff to turn. This pointed to mechanical wear inside the steering orbital unit. The rotor and stator were pulled, and a visual inspection revealed severe scoring and edge rounding on the internal gerotor elements. This wear allowed pressurized fluid to bypass internally inside the orbital unit instead of being directed to the steering cylinder, reducing the displacement efficiency of the steering motor.
The final repair involved installing a new priority spool kit, flushing the load-sensing lines, and replacing the complete steering orbital unit with a factory remanufactured assembly. The steering column U-joints were also greased to eliminate any mechanical binding. Upon start-up, the steering wheel spun freely with just two fingers of pressure, and a pressure test at the steering cylinders confirmed full system pressure was being achieved during turns. This case highlights how a single piece of contamination can cascade through a load-sensing system, starving priority circuits and accelerating wear in downstream components.
