High Compressor Amps With a Weak Condenser Fan Capacitor
A 3-ton R-410A split system with a PSC condenser fan motor is called out for no cooling on a 98°F day. The compressor and outdoor fan both run. After 10 minutes: suction 128 psig, suction line 57°F, liquid 390 psig, liquid line 101°F, outdoor ambient 98°F. Compressor nameplate RLA is 16.7 A; measured 20.1 A. Condenser fan motor nameplate is 1.4 A; measured 1.9 A. Supply voltage at the contactor is 230 V, and at the compressor 227 V. The dual run capacitor is rated 35/5 µF; your meter reads 34 µF on the compressor section and 2.8 µF on the fan section. The condenser coil is dirty, and the fan blade is clean and secure. Indoor airflow is correct, the filter is clean, and the evaporator coil is clean. Subcooling is 13°F and superheat is 12°F. The compressor overload has not tripped yet, but the owner reports it shut off yesterday and reset after 20 minutes.
1. Determine the likely cause of the high compressor amps. Include the capacitor readings and how they affect condenser performance.
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The fan section of the dual capacitor is rated 5 µF but measures 2.8 µF, roughly 44% low. That weakens the condenser fan motor, which is drawing 1.9 A against a 1.4 A nameplate. A slow fan cannot reject heat, so head pressure rises to 390 psig (about 114°F saturation) even though outdoor air is 98°F. The dirty condenser coil adds to that high condensing pressure. High condensing pressure means the compressor must work against a higher pressure ratio, so amperage climbs from 16.7 RLA to 20.1 A. The compressor capacitor section (34 µF versus 35 µF) is acceptable. The compressor is not the primary fault; the weak fan capacitor and dirty coil are driving the high amps. The compressor overload trip yesterday is consistent with this.
2. What is the correct repair sequence and what readings confirm the repair?
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Disconnect power, discharge the capacitor safely, and replace it with the correct 35/5 µF rating and voltage. Clean the condenser coil, verify the fan blade is at the correct depth and rotation, and confirm the fan motor turns freely. Restore power and check fan motor amps against nameplate. Head pressure should drop and subcooling should move toward the manufacturer’s target. Then check compressor amps; they should fall below RLA. Do not add or remove refrigerant until condenser airflow is correct, because charge readings are invalid with a hot condenser. If amps remain high after airflow and capacitor repair, check voltage drop, start components, and compressor windings. Document before-and-after readings.