A recycling-facility operator reported that a Komatsu WA380-8 wheel loader was losing drive power under heavy load. When the operator loaded the bucket from a stockpile and attempted to drive forward, the engine RPM would surge but the machine would barely move - the torque converter appeared to be slipping excessively rather than multiplying torque and delivering it to the transmission. The transmission oil temperature warning also illuminated after 20 minutes of heavy loading cycles, forcing the operator to shut down and let the machine cool.
The technician first verified the transmission oil level (correct) and inspected the transmission cooler's external fins - packed with dust, plastic film, and recycling-yard debris. He cleaned the cooler thoroughly, but the drive-power problem persisted. He then installed a pressure gauge on the torque converter's outlet pressure test port and ran the loader under varying load conditions. Under no-load acceleration, the torque converter's outlet pressure read 14 bar (spec is roughly 18 bar). Under load, the pressure climbed to only 16 bar - far below the 25-to-28 bar expected during full torque multiplication. This pattern indicated the torque converter was not converting engine torque to output torque efficiently - consistent with an internal mechanical fault rather than an external hydraulic supply issue.
He understood the WA380-8's torque converter architecture. The converter uses three primary elements - the impeller (driven by the engine), the turbine (splined to the transmission's input shaft), and the stator (mounted on a one-way clutch between the impeller and turbine). The stator redirects the returning fluid from the turbine back toward the impeller in a way that multiplies torque during the high-slip phase of operation - when the loader is loading the bucket or launching from a standstill. The stator's one-way clutch allows the stator to rotate freely in one direction (during the coupling phase, when the loader is at travel speed) and locks it in the other direction (during the torque-multiplication phase, when the loader needs maximum torque). When the stator's one-way clutch fails and allows the stator to spin freely in both directions, the stator can no longer redirect the returning fluid - and the converter loses its torque-multiplication capability entirely, behaving as if it were slipping.
He removed the torque converter from the machine and bench-tested the stator's one-way clutch by attempting to rotate the stator by hand. The clutch should lock in one direction and rotate freely in the other. On this converter, the stator rotated freely in both directions - confirming the one-way clutch's sprag elements had worn and were no longer locking. He also inspected the converter's internal components - the impeller and turbine blades showed no damage, and the converter's bearings were within service limits.
The root cause was traced to stator one-way clutch wear. The sprag elements inside the one-way clutch are subjected to continuous load cycling every time the loader transitions between the torque-multiplication phase (bucket loading, launching) and the coupling phase (travel between load and dump points). Over extended service in a heavy-duty recycling environment with frequent high-load cycles, the sprags had worn to the point that they no longer locked in the intended direction - eliminating the converter's ability to multiply torque and producing the weak rimpull the operator described. The converter's excessive slip had also generated the heat that triggered the transmission temperature warning.
The repair involved replacing the complete torque converter with a remanufactured unit (the stator's one-way clutch is not field-repairable, and disassembling the converter to replace the clutch requires a hydraulic press and balancing equipment). The transmission cooler's fins were cleaned and the transmission oil and filter were replaced with fresh specified products. After reassembly, the converter's outlet pressure was restored to specification across the operating range - 18 bar at no-load and 26 bar under full load. The loader's rimpull was restored to specification during a loaded stockpile-penetration test, and the transmission temperature remained within the normal operating range across a full 8-hour loaded shift. Torque converter outlet-pressure checks were added to the machine's 500-hour service checklist.
