A large wheel loader was brought in with a complete loss of hydraulic function. The boom wouldn't lift, the steering was dead, and the bucket wouldn't curl. The incredibly strange symptom was that the diesel engine did not lug down or smoke when the operator pulled the joysticks. On a loader, if you dead-head a pump, the engine should instantly bog down. The fact that the engine kept revving freely told us the pump wasn't building any pressure at all.
We hooked a gauge to the main pressure test port and confirmed zero PSI. The shop had already condemned the main variable displacement piston pump and ordered a $9,000 replacement. We pulled the pump, but when we opened the back cover, the rotating group-the pistons, the swashplate, and the valve plate-looked pristine. There was zero scoring, zero metal in the oil. The pump wasn't broken; it just wasn't doing any work.
On a variable displacement pump, the angle of the swashplate dictates how much fluid the pump moves. When there is no system pressure, a spring holds the swashplate at maximum angle. As pressure builds, a hydraulic signal pushes against a small servo piston, which pivots the swashplate back to zero angle to prevent the pump from blowing a hose. We pulled the servo control housing and found the problem. A microscopic piece of contamination had scored the bore of the small servo piston. When the pump tried to build pressure, the pilot oil meant to push the servo piston was bypassing the scoring and draining back to the case. Because the servo piston couldn't hold pressure, the swashplate instantly flopped back to zero degrees the moment the engine started. The pump was essentially in "idle" mode permanently. We honed the servo bore, replaced the piston, and saved a $9,000 pump for the cost of a few O-rings and an afternoon of labor.
