A masonry supply yard reported that a Mitsubishi FG25N 5,000-pound LPG counterbalance forklift had developed a progressive travel-performance fault over a two-month period. When new, the truck accelerated briskly from a standstill and maintained 20 km/h top speed. Over the past two months, acceleration had become progressively sluggish, and top speed had dropped to roughly 14 km/h. When carrying a full 5,000-pound load of brick pallets across the yard, the truck would slow noticeably and the transmission oil temperature warning would illuminate after 30 minutes of continuous load-and-carry operation. The operator reported a faint "burning oil" smell from under the truck's hood during hot-weather operation.
The technician first verified the transmission oil level (correct) and inspected the transmission cooler's external fins - heavily packed with brick dust, mortar residue, and yard debris, forming an insulating layer that restricted airflow through the cooler. He cleaned the cooler thoroughly with compressed air and low-pressure water, restoring airflow. However, despite the cleaned cooler and fresh transmission oil, the drive-power problem persisted.
He installed a pressure gauge on the torque converter's outlet pressure test port and ran the truck under varying load conditions. Under no-load acceleration, the torque converter's outlet pressure read 12 bar (spec is roughly 16 bar). Under load, the pressure climbed to only 14 bar - far below the 22-to-24 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 FG25N's powertrain architecture. The truck uses a torque converter transmission - a three-element torque converter (impeller, turbine, stator) coupled to a powershift transmission. The stator is mounted on a one-way clutch between the impeller and turbine, and it 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 truck is carrying heavy loads or launching from a standstill. 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.
He removed the torque converter from the truck 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 compounded by cooler blockage. The sprag elements inside the one-way clutch are subjected to continuous load cycling every time the truck transitions between the torque-multiplication phase (bucket or load launching) and the coupling phase (travel between load points). Over extended service in a heavy-load masonry yard environment, 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 blocked cooler had compounded the heat problem.
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), cleaning the transmission cooler's external fins, and replacing the transmission oil and filter with fresh specified products. After reassembly, the converter's outlet pressure was restored to specification across the operating range - 16 bar at no-load and 23 bar under full load. The truck's rimpull was restored to specification during a loaded yard test, and the transmission temperature remained within the normal operating range across a full 8-hour loaded shift. Torque converter outlet-pressure checks and transmission cooler cleaning were added to the truck's 500-hour service checklist.
