A bulk landscape supply yard reported that a John Deere 644K wheel loader developed a severe "nodding" action when carrying a loaded bucket. The boom would visibly bounce up and down 15 centimeters while driving. Additionally, when tilting the bucket back to hold a load of gravel, the bucket would slowly droop forward, spilling material over the tires.
The technician first checked the hydraulic oil for contamination and found it clean. He then installed a pressure gauge on the boom raise circuit. When the lever was released to neutral, the pressure fluctuated wildly between 50 bar and 200 bar (spec is a steady 200 bar). This fluctuation matched the "nodding" rhythm of the boom.
He removed the main control valve and isolated the "Boom Lock Valve" (a hydraulic check valve that traps oil in the boom cylinder to prevent drifting). The lock valve's internal piston was deeply scored, and the spring was broken. The damaged lock valve was allowing high-pressure oil to leak back to the tank, causing the boom to drop until the pressure switch caught it and re-fired the pump, creating the nodding effect.
To find the cause of the bucket droop, he inspected the "Load-Sensing (LS) Relief Valve" cartridge on the main valve block. The LS relief valve is critical for signaling the variable-displacement piston pump to destroke. The valve cartridge had completely shattered internally-the poppet had snapped off its stem. The broken poppet was bypassing all load-sensing pressure to the tank. Because the pump couldn't sense the load, it was fluctuating between 0 bar and maximum pressure erratically, starving the bucket tilt cylinder of holding pressure and causing the droop.
The repair involved replacing the boom lock valve assembly and the LS relief valve cartridge. The valve body bores were thoroughly cleaned to remove all metal shavings from the shattered poppet. The hydraulic tank was drained and refilled with fresh John Deere Hy-Gard oil (15W-40) with a new return filter. After reassembly and bleeding, the boom held firm with zero nodding, and the bucket maintained its holding pressure under full load.
