A sand and gravel quarry reported that a CAT 966H wheel loader had very weak bucket tilt power. The boom would rise slowly, and the bucket would not curl enough to hold a full load of gravel. The transmission and steering worked perfectly. The operator noted that the hydraulic oil temperature was running about 10°C higher than normal.
The technician first checked the pilot pressure, which was normal at 400 psi. The main relief valve was tested and found to be cracking at the correct 3,500 psi. However, when the joystick was moved to tilt the bucket, the pressure would rise slowly, and the engine felt like it was under very little load.
The CAT 966H uses a load-sensing piston pump. The pump has a compensator valve that senses the pressure load demand. When a function is activated, the pressure signal tells the compensator to increase the pump swash plate angle, increasing flow. The technician removed the compensator valve cover. The spool was found stuck in the "destroked" position (minimum flow) due to a tiny piece of rubber from a degraded O-ring in the hydraulic tank.
Because the spool was stuck, the pump remained at its standby flow setting (about 10 GPM) instead of ramping up to the full 75 GPM demanded by the tilt cylinder. This low flow caused the slow response and the high heat, as the system was forcing fluid through the relief valve at a constant rate.
The technician cleaned the compensator spool with solvent, replaced the pilot filter (which was also clogged), and flushed the hydraulic lines. The pump swash plate mechanism was exercised manually to ensure it moved freely. After reassembly, the pump responded instantly to load demand, and the bucket tilt power was restored to full capacity.
