A heavy earthmoving contractor reported that a Komatsu PC300LC-8 excavator's upper structure violently jittered (rapidly starting and stopping) whenever the operator tried to make a slow, fine swing movement. Additionally, the engine would not return to idle (auto-deceleration) when the joysticks were released; the engine stayed at high idle, and the main pump whined loudly.
The technician first tested the pilot pressure at the swing joystick. The pressure was fluctuating wildly between 15 bar and 40 bar (spec is a steady 40 bar) even when the lever was held perfectly still. He traced the pilot circuit to the "Pilot Pump and Accumulator" assembly. He disconnected the pilot accumulator gas charging valve and found zero nitrogen pressure.
The "Pilot Accumulator" bladder had ruptured. The accumulator's job is to dampen pressure pulsations from the pilot gear pump and provide a steady flow of control oil for precise joystick modulation. Without the accumulator, the pilot pump delivered a pulsating flow, causing the main swing control valve spool to rapidly open and close, resulting in the violent swing jitter.
The loss of auto-decelel was a related symptom. The Komatsu system relies on a steady pilot pressure drop to signal the pump regulator to destroke (reduce displacement) to zero flow when the levers are released. The pulsating pressure confused the pump regulator, keeping the main pump partially stroked and preventing the engine ECM from dropping RPMs. Furthermore, when he removed the pilot filter, he found the mesh screen completely packed with a black, tar-like sludge, restricting flow and exacerbating the pressure fluctuations.
The repair involved replacing the pilot accumulator (as a complete pre-charged assembly), replacing the pilot filter element, and flushing the pilot circuit. The swing control valve spool was inspected and found undamaged. Fresh ISO 46 anti-wear hydraulic oil was introduced. After bleeding the system, the pilot pressure stabilized at 40 bar, the swing operation became smooth, and the auto-decel function engaged properly when the levers were centered.
