A warehouse distribution center reported that a Clark C500-50 5,000-pound-capacity LPG forklift lost drive power whenever it attempted to accelerate with a full load. Without a load, the forklift drove normally up to full speed. With a 4-ton pallet on the forks, the engine revved freely but the forklift barely moved, and the transmission emitted a loud whining noise. After 15 minutes of loaded operation, the transmission fluid overheated and the relief valve vented hot fluid from the transmission breather.
The technician first verified the transmission fluid level (correct) and the charge pump pressure (22 bar at idle, within spec). He then installed a pressure gauge on the "Torque Converter Out" (turbine) pressure port and performed a stall test-with the forklift's wheels fully blocked and the throttle applied to full, the engine RPM reached only 2,300 RPM (spec is 1,800 RPM for a healthy converter). The elevated stall speed indicated the converter was slipping excessively-the impeller was spinning at full speed but not transferring torque to the turbine.
He unbolted the torque converter from the transmission and split it open on a workbench. The "Stator" is a bladed element between the impeller and turbine that redirects the oil flow during low-speed, high-torque conditions to multiply the torque. The stator is mounted on a one-way clutch (overrunning clutch) that locks the stator stationary during torque multiplication and freewheels during high-speed cruising. Upon inspection, the stator's one-way clutch sprag elements were worn and slipping in both directions-the clutch no longer locked the stator stationary.
With the stator freewheeling instead of providing torque multiplication, the converter operated as a simple fluid coupling, delivering no mechanical advantage. Under load, the forklift had no torque multiplication to accelerate, and the constant slip generated enormous heat in the transmission fluid. The loud whine was the sound of high-velocity oil cavitation inside the converter as the inefficient fluid coupling struggled to transfer power.
The repair involved replacing the complete torque converter assembly with a new unit, since the stator's one-way clutch is not field-repairable. The transmission cooler was flushed to remove any debris, the transmission filter was replaced, and fresh ATF was installed. The transmission was then run through a stall test-the stall speed settled to 1,750 RPM, indicating the new converter's stator was multiplying torque correctly. After reassembly, the forklift accelerated a full 4-ton load smoothly up a 10 percent grade, the transmission temperature stabilized at 85°C, and the whine disappeared.
