A cement plant reported a peculiar issue: a Liugong CLG856III wheel loader would start and drive forward perfectly, but the moment the operator shifted into Reverse, the engine would lug down and stall immediately, as if a brake was locked on. The transmission temperature gauge was reading normal.
The technician first verified that the service brakes and parking brake were fully released, which they were. He then checked the transmission pressure. In Forward gears, the pressure was a steady 1.8 MPa. When shifted into Reverse, the pressure spiked rapidly to 4.0 MPa (relief pressure) before the engine stalled. This symptom-a rapid pressure spike-indicates a hydraulic blockage or a seized component downstream of the pump.
The torque converter and transmission were removed for inspection. The technician focused on the Reverse clutch pack. Upon disassembling the transmission valve body, he found that the "Reverse Modulating Valve" spool was seized in the closed position. This valve is designed to soften the shift into reverse. Furthermore, the piston seals on the Reverse clutch housing were found to be shredded.
The root cause of the stall was the blocked modulating valve. Because it couldn't open to bleed off initial pressure, the pump tried to force oil into a sealed system instantly. The shock load stalled the engine. The shredded seals were a secondary issue caused by the pressure spikes. The technician also discovered the transmission oil cooler return line was kinked, which had caused the initial pressure build-up that damaged the valve. The repair involved freeing the seized valve spool, replacing the piston seals, and straightening the cooler return hose. The loader now shifted smoothly into Reverse.
