How to test motor run capacitors

Written by g.k. bayne
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How to test motor run capacitors
(voltage image by Tadzio from

Motor run capacitors aid in providing more power to an electric motor. Typically, the run capacitor is employed on motors that are attached to blowers in heating systems and air compressors. The run capacitor is oblong in shape and has a thin metal case. There are generally two electrical connectors on top of the capacitor housing. These electrical connection points are isolated from the exterior metal case and provide power to the internal workings of the device. Any testing meter that is used for the capacitor will be attached to these connectors.

Skill level:

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Things you need

  • Needle-nose pliers
  • Volt ohmmeter (analogue or digital)

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  1. 1

    Remove all electrical power from the motor. This may entail shutting off the circuit breaker or pulling fuses that supply power to the electric motor.

  2. 2

    Use needle-nose pliers to pull the electrical connectors from the terminals on top of the run capacitor. Make a note as to which wires connect to the proper terminal. Remove the capacitor from the motor.

  3. 3

    Observe the outside of the capacitor housing. If the metal case is bulging outward from the smooth case, the capacitor is bad. Record the information on the capacitor’s label. Dispose of the capacitor in the proper method as per your local regulations (see the Warnings section). If the capacitor appears to be in good physical shape, continue to the next step.

  4. 4

    Set the capacitor on a flat work surface to be tested.

  5. 5

    Place the volt ohmmeter into the “volt” position. Insert the red lead into the connector labelled as “volts” on the front of the meter. Connect the black lead into the “com” or “common” connector on the meter.

  6. 6

    Touch the end of the leads to the terminals on the capacitor. Place the red lead to one terminal and the black lead to the other terminal. The meter should read “0 volts.” Hold the leads to the terminals until the capacitor fully discharges and the meter reads appropriately at “0 volts.”

  7. 7

    Remove the leads from the capacitor. Pull the red lead from the “volts” connector and insert the end into the “ohms” position. Switch the meter to the “ohm” position as well.

  8. 8

    Touch the two leads to the terminals on the capacitor as you did in Step 6. Observe the meter. The reading on the meter should slowly move from 0 ohms and rise in resistance. If the meter does not move, there is an open circuit in the capacitor and it needs replacing. If the meter quickly jumps to 0 zero ohms and never moves after that, there is a direct short in the capacitor. Replace the run capacitor with a new one of equivalent size.

Tips and warnings

  • Some digital meters may have a capacitor test position on the meter. Place the selector switch to the capacitor setting. There may be an icon on the meter that resembles this symbol -|(-. It represents a capacitor test setting. Touch the leads to the terminals as described in Step 6 above. The meter will read either “good” or “bad.”
  • Dispose of old capacitors in accordance to any local regulations for hazardous waste. Older capacitors may contain oil products that are deemed hazardous to landfills and other disposal methods.

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