A mining site reported that a Lonking LG855D wheel loader's transmission temperature gauge consistently read above 120°C (red zone) after one hour of work. The machine also felt weak when climbing hills, and the transmission oil had a burnt smell.
The technician checked the "Transmission Oil Cooler" (a heat exchanger located near the hydraulic tank). The cooler fins were completely blocked by a layer of dry mud and cement dust, which acted as an insulator, preventing heat dissipation. He pressure-washed the cooler fins.
However, the temperature remained high. He then tested the clutch pressures. The "Forward Clutch" pressure was 0.8 MPa (spec: 1.2 MPa). He removed the transmission valve body. The "Pressure Regulating Valve" (which maintains system pressure) was functioning, but the "Forward Clutch Modulating Valve" was stuck.
He disassembled the valve and found a small piece of "Friction Material" (from a worn clutch disc) lodged in the spool groove. This debris held the valve partially open, allowing pressure to leak. The low pressure caused the forward clutch to slip slightly under load. The slipping clutch generated massive friction heat (more than the cooler could handle). The combination of the blocked cooler and the slipping clutch caused the thermal runaway.
The repair involved flushing the transmission with diesel, replacing the transmission oil filter, and cleaning the modulating valve spool. The transmission oil was replaced with new YC-8 transmission oil. After reassembly, the pressure returned to 1.2 MPa, and the temperature stabilized at 85°C under load.
