A logistics warehouse reported that a Clark C25 forklift's forks would slowly tilt forward when the engine was idling. The mast also made a "clunking" sound when tilting back.
The technician inspected the "Tilt Cylinders." He checked the rod seals, which were dry. He then focused on the "Main Control Valve." The tilt circuit uses a "Load Holding Check Valve" to prevent drift. He removed the tilt spool cartridge. The check valve seat was contaminated with metal shavings. The shavings prevented the check valve from fully closing, allowing oil to leak back to the tank, causing the forward drift.
The "Clunking" sound was traced to the "Tilt Cylinder Pivot Pins." The bolts securing the cylinder mount to the mast frame were loose. The oversized bolt holes allowed the cylinder to rock back and forth under the load of tilting. The "clunk" was the cylinder eye hitting the bolt.
The repair involved cleaning the check valve seat with a soft brass pick (to avoid scratching) and flushing the valve body. The loose mount bolts were replaced with new high-grade bolts (Grade 10.9) and thread-locking compound. The tilt cylinder air was purged. The forks then held their position, and the clunking stopped.
