How to Check an Air Conditioner Capacitor with a Multimeter

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Capacitors are electrical devices, used in the compressors and fan motors of air conditioners, that store an electric charge. There are two common types used in HVAC units: The run capacitor, used in compressors and fan motors, maintain voltage supply leading to them and can increase torque when the device is started up. A start capacitor is used to provide extra torque to the compressor when a device starts up. Check either capacitor with a multimeter to see if they need to be replaced.

How to Check an Air Conditioner Capacitor with a Multimeter
(John Wiley/Demand Media)

Things You'll Need

  • Screwdriver
  • Needle-nose pliers
Step 1

Shut off all power to the air conditioner. Turn the air conditioner thermostat to the "off" position. Locate the breaker inside the breaker box that goes to the air conditioner and turn off the power there as well.

John Wiley/Demand Media
Step 2

Discharge the capacitor by running the metal tip of your screwdriver between the two terminals to rid the capacitor of any electrical charge. There may be a spark when this happens.

John Wiley/Demand Media
Step 3

Disconnect the wires to the capacitor using your needle nose pliers. Grip the wires with the pliers, pull them off the capacitor and place them to the side.

John Wiley/Demand Media
Step 4

Prepare your multimeter by placing the leads into the terminals to test for resistance. Place the black lead in the hole marked "com," and the red lead into the hole that says "cap" by it. Set your multimeter to read Ohms, or resistance, on the highest setting your style meter has. This is done by turning the dial on the multimeter to the setting labeled "cap" or that has the Ohms icon on it.

John Wiley/Demand Media
Step 5

Place one lead on each of the capacitor's terminals and see what reading you get. This reading should be within 6 percent of your particular capacitor's rating. If it is not, then your capacitor is faulty and must be replaced with a new one.

John Wiley/Demand Media

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Tips & Warnings

  • If your capacitor is leaking oil or is bulging out, then it needs replacement. There is no need to even test it.

References

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