A wheel loader had zero digging power. It would drive at 25 km/h on flat ground, but pushing into a gravel bank stalled the machine instantly. The mechanic suspected a weak hydraulic pump. We performed a torque converter stall test to isolate the drivetrain.
We locked the brakes, put the transmission in F4, and pushed the throttle to full. The engine RPM should have stalled (held) at 1,950 RPM (per the OEM spec of 85% of the 2,300 RPM high idle). Instead, the engine screamed to 2,100 RPM, and the machine didn't move an inch. This is a classic "over-speed stall," indicating a total loss of torque multiplication.
We dropped the transmission and split the torque converter. The stator-one-way clutch assembly was shattered. The stator's job is to redirect the fluid flow from the turbine back to the impeller, multiplying torque by a factor of 2.5:1 at stall. The one-way clutch had sheared its sprags, allowing the stator to freewheel backward. The converter was acting purely as a fluid coupling with a 1:1 ratio, offering zero torque assist. Without the 2.5X multiplication, the 150 HP engine couldn't overcome the gravel bank's resistance. The stator clutch failure was caused by a previous operator shifting from forward to reverse without waiting the mandatory 1.5-second pause, creating a 6,000 Nm shock load that exceeded the clutch's 4,500 Nm yield limit.
