A mining site reported that a Doosan DL550-5 wheel loader suddenly lost drive power in all gears while loading a haul truck. The engine ran perfectly, and the bucket hydraulic functioned normally, but the machine would not move. The transmission oil level was correct, and the filter had been changed recently.
The technician connected diagnostic gauges to the transmission pump and the torque converter outlet. The pump pressure was normal, but the torque converter outlet pressure was zero. This indicated that the oil was not circulating through the converter. The transmission was removed and the torque converter was cut open for inspection. The root cause was found inside the torque converter housing.
The torque converter consists of three main elements: the Pump (impeller), the Turbine, and the Stator. The Stator is the stationary element that redirects fluid flow. The Stator is mounted on a "Stator Support Shaft," which is splined to the transmission housing. The inspection revealed that the internal spline teeth on the stator support shaft had sheared off completely. This allowed the stator to spin freely on its shaft.
When the stator spins freely, it cannot redirect the oil flow from the turbine back to the pump. This breaks the "vortex flow" of the torque converter, effectively turning it into a simple fluid coupling with very low torque multiplication. The engine power was slipping past the stator without transferring to the output shaft.
The repair involved replacing the torque converter assembly and the damaged stator support shaft. The transmission flex plate (flywheel) was also inspected for cracks, as the sudden load shift could have fatigued the metal. After reassembly, the loader engaged gears smoothly and pulled the load up the ramp without issue.
