Condenser Fan Contactor

Correct Control Wiring Problem or Replace the Thermostat

When checking control voltage, always use the common side of the circuit and not ground. Contactor control voltage generally originates from the AH/Furnace transformer and then to the thermostat (often on the (R) -red control wire).  On a call for cooling the thermostat closes a switch transferring voltage to the cooling control wire (often on the (Y) -Yellow wire).  The current is then sent to the outside condenser, and may pass through one or more pressure, temperature or other safety devices or time delays before finally reaching the contactor.

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AC low voltage wiring diagram