A construction site reported that a Liugong CLG856H wheel loader would not shift into "High Speed" (4th gear) when traveling on the road. The machine worked fine in 1st and 2nd gear for loading, but maxed out at 15 km/h on the haul road.
The technician connected a diagnostic tool to the "Transmission Controller." The fault code indicated "Speed Sensor Signal Loss." The transmission relies on an "Output Speed Sensor" on the gearbox tail shaft to determine when to shift up. He checked the sensor connector and found the "Signal Wire" was broken (rubbed through against the chassis frame due to vibration). Without the speed signal, the ECU could not calculate the vehicle speed, so it locked the transmission in "Low Range" (1st-2nd gear) as a safety default.
He repaired the wire, but the transmission still wouldn't shift. He then checked the "High-Speed Solenoid Valve" (K4 solenoid). He applied 24V power directly to the solenoid. It clicked but did not move. He removed the valve cartridge and found the "Spool" was stuck by varnish deposits. The transmission oil had overheated previously, causing the oil to degrade and form varnish. The stuck spool blocked the oil passage to the high-speed clutch pack.
The repair involved soldering the sensor wire and insulating it with heat-shrink tubing. The stuck solenoid valve was disassembled and cleaned with brake cleaner to remove the varnish. The transmission oil filter was replaced. After clearing the fault code, the loader shifted smoothly into 4th gear at 25 km/h.
