A wheel loader was brought in with a classic complaint: it would drive down the road at 20 mph perfectly fine, but the moment you put the bucket into a dirt pile, the engine would lug down and the machine couldn't push hard enough to fill the bucket. The shop had already checked the transmission pressures, which were perfect, and the engine ran strong on a dyno test.
Because the transmission pressures were good, the transmission shop cleared the job, assuming it was an engine issue. The engine shop cleared it, assuming it was a transmission issue. The problem was hidden inside the torque converter.
Inside the torque converter, there are three main components: the impeller (driven by the engine), the turbine (driving the transmission), and the stator. The stator is a small, stationary wheel of blades sitting in the middle, mounted to the transmission case via a one-way roller clutch. Its job is to redirect the fluid coming off the turbine back into the impeller at the exact right angle to multiply torque when the machine is working at low speeds.
When the loader was driving down the road, the torque converter's lock-up clutch engaged, bypassing the fluid coupling entirely, which is why it drove fine. But in the dirt pile, the lock-up clutch disengaged, relying on fluid multiplication. We pulled the torque converter and tested the stator's one-way clutch. It had failed. Instead of locking stationary to multiply torque, the stator was spinning freely in both directions. The fluid inside the converter was just churning in a circle without being redirected, providing zero torque multiplication. The machine was trying to push into a dirt pile using only the raw engine power, with no gear reduction assist from the converter. We replaced the stator assembly, and the machine instantly regained its digging power. When a machine loses low-speed pushing power but drives fine on the road, always test the torque converter stator before tearing into the engine or transmission.
