Check a Start Capacitor

Start capacitors are common in home appliances and all sorts of HVAC equipment. If the motor on your washer makes a humming noise, but it won't start, check the start capacitor. You can perform a simple test to tell you if your capacitor is completely dead or if it still has some life left in it. See Step 1 for more information.

Steps

  1. Remove the start capacitor. The easiest and most convenient way to discharge the capacitor is to attach the terminals of a low wattage 120v light bulb (about 20 watts) to the capacitor terminals. This will safely discharge the electricity which may still be stored into it.
    • Be very careful that you do not short the terminals by connecting one to the other, until after the capacitor has been discharged. Doing so could injure or kill you. Use extreme caution when discharging the capacitor before proceeding.[1]
  2. Examine the capacitor for bulging or fluid. Signs that the top of the capacitor are bulging out slightly, as if expanding, is a sign that the capacitor might be dead. Likewise, check and look for any dark fluid that appear on the top of the capacitor.[2]
    • If you see either of these, it's still a good idea to run the check with a voltmeter, since it only takes a few seconds.
  3. Use an analog or digital volt meter. Both work in essentially the same way and are both appropriate for the job. Set the meter to 1k ohms to begin the test.
  4. Touch the two terminals with your ohms meter test leads. The basic check involves touching the test leads twice and comparing the reaction. Touch the test leads to the terminals and then reverse them.
    • The needle in your meter should swing to 0 ohms and swing back to infinity on an analog meter, and should display open line every time that you reverse on a digital meter. If it does, you've got a live capacitor and your problems are elsewhere. If there is no difference the capacitor is dead.
  5. Check for capacitance, if the capacitor is alive. If you have a multimeter, you can use the capacitance setting to perform a quick check. If the number is relatively close to the number listed on the capacitor, it's in good shape.[3]

Tips

  • Make sure that you discharge the start capacitor's electricity before doing any test on it. The way you do this is to take a metal object that has an insulated handle on it, such as a screwdriver. While holding the insulated end place the metal end across the contacts on the capacitor, and hold it for a few seconds. This will discharged the capacitor so you can handle it without being shocked.
  • The above tip to discharge the capacitor by shorting the terminals is a bad idea as the article warns against doing. Shorting the terminals of a fully charged high voltage capacitor will result in a very large current flow through the capacitor. That can result in the capacitor exploding. Imagine the results if you are holding the capacitor in your hands.

Warnings

  • Start capacitors store electricity on them; that is why you need to discharge them. If the capacitor is a large one meaning the larger the micro-farad number is the more electricity/energy is stored. So the more energy that is stored the more severe burn you will get!

Related Articles

Sources and Citations

  1. ↑ http://www.achrnews.com/articles/the-motor-doctor-diagnoses-motor-capacitors
  2. ↑ http://www.hvac-for-beginners.com/capacitor-testing.html
  3. ↑ http://www.electricaltechnology.org/2013/06/how-to-check-capacitor-with-digital.html
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