An aggregate operation reported a highly uncomfortable and potentially damaging issue with a Liebherr L 566 wheel loader. During heavy acceleration from a standstill, particularly with a loaded bucket, the drivetrain would violently oscillate, causing the machine to lurch back and forth in rapid succession. The engine RPM remained steady during these events, ruling out a fuel or engine governor issue, but the jerking was so severe that operators refused to drive the machine.
The L 566 utilizes a hydrostatic travel drive, where a variable displacement axial piston pump drives a variable displacement motor. Hydrostatic systems are generally smooth, but they are highly sensitive to internal leakage and pressure fluctuations. The technician connected a high-frequency pressure transducer to the high-pressure drive line. Upon acceleration, the pressure graph showed severe, rhythmic spikes and drops, oscillating between 1,000 and 4,500 psi in less than a second. This rapid pressure cycling was physically slamming the drive motor and axles, creating the lurching sensation.
The first component suspected was the travel drive flushing valve. In a closed hydrostatic loop, a small amount of oil is constantly bled off through a flushing valve to be cooled and filtered, replaced by fresh oil from the charge pump. If this valve sticks, the closed loop overheats, and the oil rapidly degrades, losing viscosity. The flushing valve was removed and found to be stuck in the open position due to a broken centering spring. This allowed too much oil to bleed off, causing the charge pump to struggle to maintain loop pressure, leading to momentary cavitation and the resulting pressure oscillations.
However, fixing the flushing valve did not entirely cure the jerkiness. The charge pump relies on a nitrogen-charged accumulator to absorb shock and provide instant makeup oil when the drive direction is reversed or rapidly accelerated. The technician checked the accumulator's nitrogen pre-charge using a specialized gauge. The pre-charge should have been 15 bar, but it read zero. The accumulator bladder had ruptured, rendering it completely useless. Without the accumulator to buffer the pressure spikes, the charge pump could not react fast enough to the sudden demand for oil during acceleration, causing the drive motor to starve and slip momentarily before grabbing again.
The repair involved replacing the broken flushing valve spring and installing a new drive accumulator bladder, charging it to the exact 15 bar nitrogen specification. The entire hydrostatic loop was flushed to remove any degraded oil, and the high-pressure filters were replaced. Upon testing, the loader accelerated smoothly and powerfully under a full bucket load, completely free of drivetrain oscillation.
