A concrete batching plant reported that a Komatsu WA470-8 wheel loader's boom would slowly drop to the ground when the engine was idling. The hydraulic tank temperature was constantly exceeding 110°C, causing the machine to derate.
The technician first checked the "Boom Cylinder Rod Seals," which were dry. He then focused on the "Main Control Valve." The WA470-8 features a "Boom Float" function, which allows the boom to freely follow the ground contour. He removed the "Boom Float Solenoid Valve Cartridge." The spool was seized in the "Open" position by hard debris. This created a permanent bypass from the cylinder to the tank, explaining the drifting boom.
To find the source of the debris, he removed the "Hydraulic Tank Return Filter Cover." The internal filter element had burst at the seam. The burst filter allowed unfiltered oil to wash back through the system. The debris (likely from a previously failed pump) jammed the float spool. The constant high-pressure bypass through the jammed spool generated the extreme heat.
The repair involved flushing the entire hydraulic tank, installing a new return filter element, and replacing the damaged boom float valve cartridge. The system was refilled with clean ISO 68 oil. After bleeding the air, the boom held its position firmly, and the operating temperature stabilized at 78°C.
