A masonry supply yard reported that a Manitou MT 1840 telescopic handler lost drive power when climbing a ramp with a 3.5-ton pallet of blocks. The telehandler would shudder, the wheels would slip, and the hydrostatic transmission temperature would spike to 110°C within 20 minutes. The boom and fork hydraulic functions operated normally.
The technician first checked the main hydrostatic pump and drive motor for external leaks-none were found. He installed a pressure gauge on the "Charge Pump Circuit." The charge pump supplies cool, filtered oil to the closed-loop hydrostatic drive to replenish internal leakage. The charge pressure was only 12 bar (spec is 25 bar). When the operator drove up the ramp, the charge pressure dropped to 5 bar, and the drive shuddered.
He removed the "Charge Pump Relief Valve" from the hydrostatic control block. The internal spring was 4 millimeters shorter than specification due to millions of load cycles and high-temperature fatigue. The fatigued spring allowed the relief valve to open prematurely at 12 bar instead of 25 bar.
Because the charge pressure was too low, the variable-displacement drive motor could not replenish the oil fast enough under heavy torque load. The drive motor pistons cavitated (the shudder and slip), and the lack of cooling oil flow caused the closed-loop fluid to overheat rapidly to 110°C. The heat thinned the oil, exacerbating the internal leakage and pressure loss.
The repair involved replacing the fatigued charge pump relief valve cartridge with a new unit. The entire hydrostatic system was drained and flushed to remove any heat-degraded oil. Fresh ISO 46 anti-wear hydraulic fluid was introduced. After reassembly and bleeding the closed-loop circuit through the high-point vents, the telehandler climbed the ramp smoothly with a full load, the charge pressure maintained 25 bar, and the hydrostatic temperature stabilized at 85°C.
