A mining contractor reported that a Komatsu WA380-6 wheel loader would drive perfectly in 1st and 2nd gears, but when shifted into 3rd or 4th gear, the machine would lose all propulsion. The engine would rev freely, but the machine would coast to a stop. There were no strange noises or warning lights.
The technician first checked the transmission oil level and the filter condition. The oil was clean. He then connected pressure gauges to the forward and reverse clutch test ports. In 1st and 2nd gears, the clutch pressure was a steady 350 PSI. However, when shifted to 3rd gear, the pressure dropped rapidly to 50 PSI. This indicated a massive internal leak or a mechanical disconnect.
The WA380-6 uses a lock-up torque converter in 3rd and 4th gears to improve efficiency. The technician suspected the lock-up clutch was failing. He removed the transmission and torque converter assembly. Upon inspection, the connection between the torque converter turbine and the transmission input shaft was examined. This connection is made via a "Splined Adapter Sleeve."
The inspection revealed that the internal splines of the adapter sleeve had been completely stripped (worn smooth). In 1st and 2nd gears, the torque converter uses fluid coupling to transmit power, which creates less torque stress on the splines. However, in 3rd and 4th gears, the lock-up clutch engages, creating a direct mechanical connection. The worn splines could not handle the direct torque load, causing the shaft to spin freely inside the sleeve.
The root cause was traced to a slight misalignment of the transmission mounting bell housing, likely from a previous repair. This misalignment caused uneven wear on the splines over time. The repair involved replacing the splined adapter sleeve, the torque converter, and the transmission input shaft. The bell housing alignment was checked with a dial indicator and corrected using shims. After reassembly, the loader engaged all gears smoothly.
