A sand and gravel yard reported that a Liugong CLG855N wheel loader would drive forward normally, but when shifted into Reverse, the engine would stall immediately. There was also a loud "whining" noise coming from the transmission pump area.
The technician checked the transmission oil level, which was correct but the oil looked milky, indicating potential water contamination. He removed the transmission suction filter (located inside the hydraulic tank). The filter was completely plugged with a grey, metallic paste. This paste is a byproduct of clutch plate wear. The plugged filter was starving the transmission pump, causing the cavitation noise. The pump was sucking air and generating inadequate flow.
However, the "Stall in Reverse" symptom was specific. The technician suspected that the Reverse clutch required more pressure than the Forward clutch, and the low flow caused it to slip. He removed the transmission valve body. The "Reverse Modulating Valve" (which softens the shift) was stuck in the "closed" position by debris. This meant that when the operator shifted into Reverse, the pressure spike hit the clutch instantly without the softening effect, shocking the engine. Furthermore, the piston rings on the Reverse clutch drum were worn, requiring higher pressure to engage.
The repair involved flushing the hydraulic tank and lines with diesel to remove the metallic paste. The transmission pump was replaced (it was damaged by cavitation). The Reverse Modulating Valve spool was cleaned and freed up. The Reverse clutch seals were also replaced. After reassembly and refilling with fresh 8# hydraulic transmission oil, the machine engaged Reverse smoothly, and the whining noise disappeared.
