A foundation contractor reported that a Sany SY215C excavator's hydraulics functioned normally for five or six minutes after a cold start, then progressively slowed and eventually stopped entirely. When the operator shut down the machine and restarted it, the hydraulics would resume for another five or six minutes before failing again. No unusual noises or external leaks were present during the failure.
The technician recognized the pattern of a load-sensing system collapse. Modern excavators use a closed-center, load-sensing variable-displacement piston pump. The pump regulator continuously adjusts the swashplate angle based on the Load-Sensing (LS) signal pressure from the main control valve. If the LS signal pressure decays or becomes blocked, the pump regulator destrokes the pump to zero displacement, and all hydraulic functions stop. Shutting down and restarting the engine momentarily resets the regulator, but if the underlying blockage persists, the failure recurs.
He first removed the "LS Signal Line Filter" (a small mesh screen filter located between the main control valve and the pump regulator). The screen was completely packed with a black, gelatinous sludge-the product of thermally degraded hydraulic oil that had been run hot for an extended period. The sludge was progressively restricting the LS signal pressure from reaching the pump regulator. In the first five minutes of operation, the cold, viscous sludge was compacted but still allowed marginal signal pressure through. As the system warmed and the oil thinned, the sludge packed tighter against the screen, eventually blocking the signal entirely and causing the pump to destroke to zero flow.
To verify the diagnosis, he removed the filter screen entirely and cleared the LS passage. The excavator operated continuously for 30 minutes without failure. However, he suspected a secondary contributor, because the sludge had clearly been forming for a long time. He disassembled the "Pump Regulator PC (Pump Control) Valve." The PC valve spool was partially stuck in the maximum-displacement position by the same sludge, which explained why the pump initially worked but then could not destroke properly when the LS signal pressure finally collapsed. The stuck PC valve was masking the LS filter blockage for the first five minutes; when the LS signal finally dropped to zero, the stuck PC valve could not compensate, and the pump collapsed.
The repair involved replacing the LS signal line filter screen and the pump regulator PC valve cartridge. The entire hydraulic system was flushed twice with a solvent flush to remove the varnish and sludge. Fresh ISO 68 anti-wear hydraulic fluid and new main return and pilot filters were installed. The root cause of the sludge was traced to a degraded suction strainer that had been shedding fiber into the system; the strainer was replaced, and a post-repair 50-hour oil sample came back clean with no contamination flags. The site added a quarterly hydraulic oil sample analysis to catch thermal degradation before it produces the sludge that starves the pump regulator.
