A quarry operator reported that a Caterpillar 966M wheel loader would stall the engine immediately when trying to pull away in First gear or Reverse with a full bucket. However, if the operator started in Second gear, the machine would move, and once up to speed, First gear could be selected and worked normally. Engine diagnostic codes showed no fuel or electronic issues.
The technician suspected a transmission or torque converter (T/C) mechanical issue. He performed a "Stall Speed Test." He chocked the wheels, applied the service brake, selected First gear, and fully depressed the accelerator. The engine RPM stalled at 1300 RPM, significantly lower than the specified stall speed of 1750 RPM. A low stall speed indicates that the torque converter is creating excessive drag (fluid coupling resistance) rather than multiplying torque.
He removed the transmission and disassembled the torque converter. The "Stator One-Way Clutch" (sprag clutch) was seized in the "Locked" position. The stator is designed to remain stationary during low-speed, high-torque operations, redirecting the fluid flow from the turbine back to the impeller to multiply torque. However, as the machine accelerates, the fluid flow reverses, and the stator must freewheel (unlock) to allow the fluid to pass.
Because the stator was seized, it could not freewheel. In First gear and Reverse (where ground speed is low), the locked stator fought the fluid flow, creating massive drag that lugged the engine down to a stall. Once the machine was moving (in Second gear), the impeller speed was high enough to overcome the stator drag, allowing it to function poorly but move. The seizure was caused by a broken inner race bearing roller that had wedged the clutch cams.
The repair involved replacing the torque converter stator assembly and the one-way clutch. The transmission oil cooler was flushed to remove any metal debris. After reassembly, the stall speed test yielded 1740 RPM, and the loader pulled away in First gear with full buckets effortlessly.
