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Motor or Compressor Will Not Run Properly? How To Test Your Capacitor

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A capacitor is a part that stores an electrical charge, somewhat like a battery; the charge is released upon start up for most motors and compressors. Capacitors can be found in most HVAC equipment, including: air conditioners and furnaces; you can also find capacitors in refrigerators and washing machines.

Many homeowners, who have experience with their furnace or A/C motor not working, often believe that the motor is defective, when in most cases it is usually the capacitor that is defective.

If you are experience a slow start up of your motor, this is a sign that your capacitor is starting to fail. If you notice any defects, for example: bulging, or leaking, this is a sign that your capacitor is defective and must be replaced right away.

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If you do not see any sign of bulging or leaking from the capacitor, you should have a qualified HVAC technician diagnose the problem. If you feel confident in your own skills, you may want to do the following (Please note: This advice should not be taken as a diagnosis):

To test a Single Capacitor:

Before testing your capacitor, be sure to ground any stored electrical charge by placing a screw driver directly over the terminals.

By taking a meter that measures 1000 ohms or higher, you can check to see if your capacitor is working or if it is defective.

To test a capacitor, you must place one of the prongs from your meter on one of the terminals. Place the other prong on the other terminal of the capacitor.

Your meter should jump up or should read numbers before falling back down to zero. If you change the prongs or reverse it, you should see the same results from the meter.

If you do not see a jump in the meter or it stays close to zero, then the capacitor is defective and should be replaced.

To test a Dual Capacitor:

You will test the command terminal (identified with the letter “C”) and the two other terminals (identified with “Fan” and “HERM”).

In order to test the “Fan” circuit, use one of the prongs from the meter and touch the terminal identified with a “C” and touch the prong to the “Fan” terminal on the capacitor.

Just like testing a Single Capacitor, you should see slight movement in the numbers from your meter.

You can also test to see if your capacitor has shorted out by placing one prong of your meter onto one of the terminals and by touching the other prong to the outer casing of the capacitor. You should not see any activity from your meter, if you do this means that the capacitor has shorted out and needs to be replaced.

When selecting a new capacitor, be sure to check that the uf/MED rating and the voltage rating is the same or higher than the old capacitor; for a dual capacitor, make sure the wires are reinstalled on the same terminals, as they were removed from (“HERM”, “C”, and “FAN”). Severe damage to the unit may occur if the capacitor is not wired properly. We suggest taking a picture of the top of the capacitor before you remove the wires, for reference, when reinstalling the capacitor.