How to Troubleshoot an Indoor AC Fan Motor


Technicians use their eyes, ears, nose and a multimeter to troubleshoot an indoor air-conditioning unit's fan motor before replacing it. An indoor air-conditioning unit, called an air handler, uses a capacitor-operated electric motor. The electric motor pulls warm air through the air-conditioning system's return vent and pushes the air through the evaporator coil and into the building's duct system. If the electric motor fails and the outside unit continues to run, ice will form on the evaporator coil. Always replace a defective motor with a compatible model.

Things You'll Need

  • Screwdriver set
  • Electrical tape
  • Wire ties
  • Lightweight oil
  • Multimeter
  • Turn off the air handler's circuit breaker.

  • Remove the screws holding the air handler's access panel in place with the correct screwdriver, usually either a 1/4- or 5/15-inch hex-head screwdriver. Pull the panel off the air handler.

  • Visually inspect the fan motor's wiring. Look for burn marks and worn or cracked insulation. If the motor's insulation contains wear marks, repair the wiring with electrical tape and secure the wires to the motor's mounting brackets with wire ties. If the wiring has burn marks or cracked insulation, replace the motor.

  • Smell the motor. If the motor has a burnt smell, replace the motor. A burnt smell indicates the motor has shorted windings or worn bearings.

  • Spin the motor's squirrel cage, sometimes called the blower wheel, with a hand. If the squirrel cage will not spin, replace the motor. If the squirrel cage moves side-to-side, replace the motor. If the motor squeaks, pull the cap off the motor's oil port and lubricate the motor with a lightweight oil. If the motor continues to squeak after applying oil, replace the motor. Many motors are maintenance-free and do not have an oil port. If a maintenance-free motor squeaks, replace the motor.

  • Pull the motor's brown-colored leads off the motor's capacitor, the galvanized cylinder mounted to the squirrel cage's housing. Turn a multimeter to its capacitance setting. Hold the multimeter's probes on the capacitor's terminals and check the multimeter's readout. Compare the readout to the capacitor's rating, as stated on the capacitor's label. If the readout shows the capacitor has lost more than 10 percent of its capacitance, replace the capacitor. Reconnect the motor's brown-colored leads to the capacitor.

  • Turn on the air handler's circuit breaker. Use extreme care while continuing to troubleshoot the motor.

  • Turn the multimeter to its volts alternating current (VAC) setting. Place the multimeter's probes on the air handler's terminal block, where the wires from the circuit breaker enter the unit. The meter should read about 220 volts.

  • Turn the thermostat's fan switch to the "On" position. The fan should turn on. If not, place the multimeter's probes on the fan relay's motor terminals and compare the readout to the terminal block's readout. The motor's wires connect to the fan relay's motor terminals. If the fan relay has the appropriate voltage at the motor's terminals, replace the motor. If the multimeter does not show voltage at the motor's terminals, troubleshoot the fan relay and the air handler's low-voltage control system.

  • Turn the multimeter to its amperage setting. Clamp the multimeter's amp-probe, the multimeter's clamp-on jaws, over the motor's black-colored wire. Read the multimeter. Turn off the thermostat's fan switch. Find the motor's amperage rating on the motor's identification tag. Compare the multimeter's amperage readout to the motor's amperage rating. If the multimeter's amperage readout exceeds the motor's amperage rating, replace the motor. Normally a motor will run between 50 and 75 percent of the stated amperage rating.

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