How to Tell if a Fan Motor Is Broken on an Outside AC Unit

This AC unit is equiped with two fans.
This AC unit is equiped with two fans. (Image: Pompe à chaleur image by mattmatt73 from

An AC outside unit is designed to take heat energy from the outside air and move it to the inside living unit if the unit is in heating mode. If the unit is in cooling mode the process works in reverse, taking heat out of the living area and depositing it outside. This unit has a fan that circulates air through the unit's cooling fans. If that fan fails, the efficiency of the AC unit falls dramatically.

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Visually observe the fan over two or three operating cycles. One operating cycle is the time when the outside compressor is on. It sounds like a refrigeratior when it is on. The fan should be on during the same periods that the compressor is on. The exception to that is during winter time. Some AC units have a heating function designed to melt ice off of the unit. During those periods, the compressor will run but the fan will not.

Shut off the power to the AC unit. Inspect the motor and motor wiring for damage. Also check for rodents that may have gotten into the unit.

Unplug the motor and connect a voltage meter to the wiring harness feeding the motor. To do this, set the meter to measure alternating current. Check the wiring diagram of the motor to locate the common wire and the primary hot wire. Connect the black probe of the meter to the hot wire and connect the red probe of the wire to the common wire.

Turn the power to the unit back on, but do not touch the wires. Let the AC unit run until the compressor comes on. When it comes on, there should be a voltage reading on the voltage meter. If there is a high reading, the motor is bad. If there is no reading, the motor is fine and the problem is somewhere else.


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