A highway rehabilitation contractor reported that a Komatsu PC200-8 excavator's air conditioning system had progressively lost cooling capacity over a two-week period. Initially the A/C would blow cool air for the first 20 minutes of operation and then gradually warm up; by the end of the fault's development, the system produced only ambient-temperature air. The operator also reported an intermittent grinding noise from the engine bay whenever the A/C was switched on.
The technician first connected manifold gauges to the A/C system's high- and low-side service ports. With the engine at idle and the A/C commanded on, the low-side pressure read 2.1 bar (spec is roughly 2.0 to 2.5 bar), but the high-side pressure climbed to only 6 bar (spec is roughly 12 to 18 bar under these conditions). This pressure profile indicated the compressor was not pumping - it was spinning without developing pressure differential, suggesting either a failed compressor clutch (the compressor not actually being engaged) or a failed compressor internal mechanism.
He inspected the A/C compressor clutch. The clutch is an electromagnetic device: when the A/C is commanded on, the ECU energizes the clutch coil, which pulls the clutch plate into contact with the compressor's drive pulley, mechanically coupling the engine's belt drive to the compressor's internal mechanism. When he tested the clutch coil with 12 volts applied directly from a battery, the clutch did not engage - the coil showed an open circuit when measured with an ohmmeter, confirming the clutch coil had burned out.
However, the technician noticed the clutch assembly had also developed significant radial play at its bearing. The "Clutch Pulley Bearing" - a sealed double-row bearing supporting the pulley's rotation on the compressor's nose - had lost its grease and was producing the grinding noise the operator had reported. This bearing failure had allowed the pulley to wobble, which had over-stressed and fatigued the clutch coil's wiring, eventually burning it open. A secondary finding explained the earlier "gradually warming" symptom: he discovered a corroded fitting at the A/C condenser's inlet connection, with visible refrigerant-oil residue around the fitting - a slow refrigerant leak that had progressively reduced system charge, reducing cooling capacity before the clutch failure eliminated cooling entirely.
The repair involved replacing the complete A/C compressor with a remanufactured unit (replacing only the clutch would have left the worn internal compressor mechanism in service), replacing the corroded condenser inlet fitting and O-rings, and evacuating the system with a vacuum pump to remove moisture and non-condensable gases. The system was charged with the specified R134a refrigerant to the correct weight using a charging scale, and UV dye was added to the system for future leak detection. After reassembly, the A/C produced 4°C air at the cab vent, the compressor engaged without grinding noise, and a follow-up inspection confirmed the UV dye had not revealed any further leaks.
