How to Troubleshoot an A/C Contactor Switch
Contactor failure prevents an air-conditioning system from running correctly. The contactor, a type of electromagnetic relay, acts like a remote high-voltage switch in the condensing unit. When the system's thermostat detects a cooling demand, it energizes the low-voltage wire powering the contactor's relay coil. The coil forms an electromagnetic field that pulls on the contactor's high-voltage bus bar. When the bus bar closes, the bus bar's contact points let high-voltage electricity pass on to the condenser's equipment. Technicians troubleshoot the A/C contactor before replacing it. Does this Spark an idea?
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Things You'll Need
- Screwdriver set
- Multimeter
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Instructions
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Turn off the condensing unit's circuit breaker and switch the A/C system's thermostat to the "Off" position. Remove the condensing unit's electrical panel, using the correct screwdriver to remove the panel's mounting screws. The electrical panel mounts on the side of the condensing unit.
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Visually inspect the contactor. If the contactor contains burnt bus bar points or a damaged housing, discard the contactor. The high temperature caused when electricity arcs across the bus bar's points overheats the contactor points and its housing.
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Inspect the contactor's bus bar. If the bus bar remains in the closed position, replace the contactor. Turning off the thermostat disconnects the contactor coil's low-voltage signal, which should open the contactor's bus bar. If the bus bar stays in the closed position at all times, the condensing unit's equipment runs constantly.
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Depress the contactor's bus bar several times, using a screwdriver for leverage. If the bus bar does not move smoothly up and down, replace the contactor. The contactor uses a spring to hold the bus bar in the open position.
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Turn on the thermostat. Leave the condenser's circuit breaker off.
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Inspect the conatctor's bus bar. If the bus bar depresses, the contactor's coil works. If the bus bar remains in the open position, turn a multimeter to its VAC setting. Place a multimeter probe on each of the contactor coil's wire terminals. If the multimeter reads zero, troubleshoot the air-conditioning system's low-voltage circuit. If the multimeter reads 24 to 30 volts, turn off the thermostat and check the contactor's coil for continuity.
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Remove the low-voltage wires from the contactor's relay coil wire terminals. Turn the multimeter to its resistance or continuity setting. Place a multimeter lead on each relay coil wire terminal. Check the multimeter's readout screen for continuity across the relay coil. If the multimeter reads zero, replace the contactor. If the multimeter reads continuity, reconnect the low-voltage wires to the contactor's relay coil wire terminals.
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Turn on the thermostat and the condenser's circuit breaker.
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Turn the multimeter to its highest voltage setting. Touch the multimeter leads to the contactor's input high-voltage wire terminals, the wire terminals the circuit breaker's wires connect to. The multimeter should read between 210 and 240 volts. If the multimeter reads zero, troubleshoot the circuit breaker. Check the voltage across the contactor's output high-voltage terminals. The voltage reading at the output terminals should match the reading at the input terminals. If not, replace the contactor.
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