Testing a Run Capacitor

If your central air condensing unit fan motor or furnace blower motor aren’t working, or your microwave oven won’t heat, a defective run capacitor may be the culprit. A run capacitor is a powerful component. So powerful that it retains a potent electrical charge even when the appliance or product it’s installed in has been unplugged or the power supply has been shut off.

What does a run capacitor do?

Simply put, a single or dual run capacitor stores an electrical charge and then releases it. The component is most often used to power motors and compressors, and can be found in central air condensing units, furnaces, and other heating and cooling applications. Capacitors can also be found in appliances such as microwave ovens, refrigerators, even washing machines. A single run capacitor supports a single electric motor and is commonly used in small air conditioners and microwave ovens; a dual run capacitor will support two motors, the first being a compressor and the second being a fan motor. Products that have both a compressor and a fan motor, such as furnaces and central air condensing units, will invariably rely on a dual run capacitor.

How to tell if a capacitor is bad

Over time, a run capacitor can weaken and lose its ability to hold a full charge. It can also fail completely. The motor or compressor being powered by the capacitor will have trouble running smoothly or may not run at all. You can often tell that a capacitor has failed because the casing will bulge or leak. If you observe any bulging or leaking, the capacitor will require immediate replacement. If there are no visible signs of damage, the capacitor can be tested to determine if its working properly.

Testing the run capacitor with a multimeter

You can use an analog Ohm meter to test the capacitor’s ability to store and release an electrical charge. You can also test the component to determine if it has the proper capacitance rating by measuring the microfarads present in the capacitor. This second test can be done using a capacitor tester or a multimeter with a capacitor testing function. IMPORTANT: To prevent shock before handling a capacitor, you should release the potentially stored electrical charge by placing a screwdriver blade across each set of terminals. Avoid touching the screwdriver blade when doing this.

Here’s how you can use an analog Ohm meter to confirm a single run capacitor can properly store and release a charge:

  • Rotate the meter’s range selection dial to a setting of 1000 Ohms or higher.
  • Calibrate the meter, if necessary, by touching the red and black probes together as you set the needle to “0”.
  • Use one probe to touch one of the capacitor’s terminals and the other probe to touch the other terminal. The meter needle should deflect toward zero Ohms and return to infinite resistance.
  • Reverse the probes and you should get the same result.
  • If the needle does not move or stays near zero Ohms, the capacitor has failed.

When checking a dual run capacitor, you will test between the common terminal and each of the other terminals:

  • To test the fan circuit, touch one probe to the common terminal (often identified by the letter “C”) and the other probe to the fan terminal (usually identified as “FAN”). As noted above, the needle should deflect toward zero Ohms then return to infinite resistance.
  • You can repeat the test to determine if the third terminal, usually identified as “HERM” or “COMP”, is functioning properly.

A standard Ohm meter can also be used to determine if the capacitor has shorted to ground:

  • Place one probe on each of the terminals while touching the second probe to the side of the casing. No terminal should display continuity – a continuous electrical path.
  • If the meter display does indicate electrical continuity, the capacitor has shorted and will need to be replaced.

It’s also important to confirm the capacitor actually has a capacitance rating that matches the one on the component’s label. You will need to use a dedicated capacitor tester for this test or a multimeter that has a capacitor testing function. To identify the capacitance rating on a single run capacitor:

  • Note the microfarad rating on the capacitor’s label (Example: 7.5%).
  • Choose the comparable setting on the tester or meter.
  • Connect the probes to the terminals and press the tester or meter button to display the microfarad rating. The rating should be close to the one identified on the label. If the display shows the microfarads are lower, the capacitor has weakened and should be replaced.

Dual run capacitors have two microfarad ratings: a higher rating is typical for the HERM or COMP circuit (Example: +6%) and a lower rating is typical for the fan circuit (Example: -6%). You should test each circuit separately to determine if the display readings closely match the ratings on the label.

Find the right run capacitor with Repair Clinic

Since there are literally thousands of capacitors available, you’ll want to make sure you’re purchasing the right replacement. This is where Repair Clinic can help. Start by entering the full model number of your appliance parts or heating and cooling product in the Repair Clinic website search bar. Next, select “Capacitor” from the part category filter followed by the appropriate part title, if necessary, to identify the specific capacitor you need for your application. Repair Clinic stocks genuine manufacturer capacitors from top brands like Goodman, Lennox, Carrier, GE, Samsung, LG, Frigidaire, and Whirlpool, so you’re sure to find what you need.

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