A road construction crew reported that a Doosan DL503 wheel loader's steering had become progressively heavier over a week. At the same time, the transmission temperature alarm triggered continuously, and the operator noticed the hydraulic oil tank level was dropping despite no visible external leaks.
The technician first checked the hydraulic oil tank and found the level significantly low. He then pulled the transmission dipstick and found the fluid level was above the "FULL" mark, and the oil had a thin, milky consistency. He suspected cross-contamination between the hydraulic and transmission systems.
Studying the machine's architecture, he found that the steering gear pump is mounted directly to the engine's front gear train via a drive shaft that passes into the torque converter housing. He removed the steering pump and inspected the "Front Shaft Oil Seal." The seal had completely worn through, and the pump's drive shaft was deeply grooved.
The failure mechanism was clear: pressurized hydraulic oil from the steering pump was leaking past the failed shaft seal directly into the torque converter housing. The oil flooded the converter, overfilling the transmission and diluting the TO-4 fluid. The diluted transmission fluid lost its lubricating film strength, causing the clutch packs to slip and generating the massive heat that triggered the alarm. Simultaneously, because a large volume of the steering pump's output was leaking internally into the engine, the steering circuit was starved of flow and pressure, causing the heavy steering.
The repair involved replacing the steering pump (the grooved shaft could not be reliably sleeved) and flushing the entire transmission system twice to remove the diluted, contaminated fluid. The torque converter was drained and refilled, and the transmission filter was replaced. After reassembly, the steering returned to normal, and the transmission temperature stabilized at 78°C under heavy load.
