A quarry site reported that a Caterpillar 950GC wheel loader would drive normally in 1st and 2nd gear, but the engine would race (overspeed) and the machine would lose all drive when shifting into 3rd gear. The transmission temperature was also noted to be slightly higher than normal.
The technician first checked the transmission oil level and condition. The oil was dark and smelled burnt, indicating clutch slippage. Pressure gauges were installed at the main transmission pump and the clutch pack ports. In 1st and 2nd gears, the clutch pressures were within the 280-320 psi range. However, when shifted into 3rd gear, the pressure dropped rapidly to near zero.
The 950GC uses a countershaft transmission where 3rd gear is a direct drive. The technician dropped the transmission oil pan to inspect the control valve body. The filter screen was clean, ruling out a blocked solenoid. The issue was suspected to be internal. The transmission was removed and disassembled. Upon inspection, the clutch plates for the 3rd speed clutch pack were found to be blackened and glazed, confirming the slippage.
However, the root cause was traced to the torque converter. The impeller hub, which drives the transmission oil pump, is held by a woodruff key. This key had sheared partially, allowing the hub to slip under high torque loads. In 3rd gear, the torque multiplication demand is lowest, and the pump speed fluctuates if the hub slips, causing pressure drops. The worn 3rd gear clutch plates were a symptom of the pressure instability. The repair involved replacing the torque converter, the transmission oil pump, and the complete 3rd gear clutch pack. The torque converter was also sent out for re-sealing. After reassembly, the loader shifted smoothly through all gears.
