A recycling facility reported that a Cat 950GC wheel loader was taking 5 to 7 seconds to engage forward gear after shifting from neutral. The reverse gear engaged instantly. When the machine finally moved, it would lurch forward violently. The transmission temperature was also running slightly higher than normal.
The technician first checked the transmission oil level using the sight glass. The level was correct with the engine running, but the oil appeared aerated and foamy. This indicated air was entering the system. The suction strainer was removed from the transmission sump. It was clean, but the technician noticed a dark, oily residue on the input shaft seal of the torque converter housing.
The transmission pump was removed to inspect the torque converter impeller hub. The hub, which drives the transmission pump, had deep grooves worn into the steel surface. The pump drive gear and bushing are located inside the torque converter housing. The worn hub had allowed the transmission pump to wobble slightly, creating a gap between the pump housing and the converter hub seal.
When the machine was in neutral, this gap allowed air to be drawn into the pump suction line (cavitation). When the operator shifted to forward, the air had to be compressed and purged from the control valve before the clutches could engage, causing the delay. The violent lurch was caused by the air compressing and then releasing suddenly.
The repair involved installing a new torque converter impeller hub (or a new torque converter assembly) and a new transmission pump. The flex plate was also inspected for cracks. After refilling with fresh Cat TO-4 transmission oil and bleeding the air from the system, the loader engaged forward gear instantly and smoothly, and the aeration bubbles in the sight glass disappeared.
