A sand and gravel yard reported that a Caterpillar 966H wheel loader suddenly lost drive power while climbing a stockpile. The engine ran normally, and the bucket hydraulics worked fine. The operator checked the transmission dipstick and found the oil level was correct, but the oil had a burnt smell.
The technician connected pressure gauges to the main pressure tap and the torque converter outlet. In neutral, main pressure was normal (280 PSI). When shifted into 1st gear, the pressure dropped to zero, and the engine stalled. This indicated a severe mechanical lockup in the transmission or torque converter.
The technician removed the transmission and torque converter assembly. Upon disassembling the torque converter, he found the "Stator Support Splines" had sheared off completely. The stator is the stationary element that redirects oil flow to multiply torque. The stator support is bolted to the transmission housing. If the splines shear, the stator spins freely, eliminating torque multiplication and effectively creating a direct fluid coupling with very low efficiency.
Further inspection revealed the "Torque Converter Outlet Filter" was completely plugged with fine black dust (clutch material). This blockage prevented oil from exiting the converter, causing the oil to overheat and expand, which increased the internal pressure and snapped the spline.
The repair involved replacing the torque converter assembly and the stator support shaft. The transmission valve body was also cleaned to remove any debris. The root cause was traced to the transmission clutch packs, which were worn due to high-cycle loading. The technician replaced the clutch seals and friction discs. After refilling with fresh Caterpillar TDTO fluid, the loader engaged gears smoothly and pulled the load without issue.
