How to Test an Air Compressor Motor Capacitor

Air compressor capacitors store electrical energy for later discharge.

A capacitor's electrical charge may be sufficient to cause physical harm.A capacitor's electrical charge may be sufficient to cause physical harm.
Capacitors contain two plates that build up and store electricity until the electricity is needed by the electrical system. An insulator, commonly in the form of a gel, separates the plates to prevent the electricity from arcing and discharging. Replace bad capacitors with visible physical defects, such as bulges, leaks, and corrosion build-up. If you suspect failure, even if there are no obvious signs, connect a multimeter to the capacitor leads. The multimeter measures the resistance and charging ability of the capacitor.

Turn off the air compressor. Disconnect the electrical plug. Unscrew the compressor housing to expose the electrical circuit board. Locate the capacitor to be tested.

Discharge the stored electricity contained in the capacitor. Find the two leads protruding from the edges or bottom of the capacitor. Strip enough plastic from an insulated wire to expose enough metal to touch both leads simultaneously. Grasp the wire by the insulated covering and touch the exposed wire to both leads protruding from the capacitor.

Hold the wire against the capacitor for 30 seconds or more. If the wire gets hot, remove it from the capacitor and allow the wire to cool. Reconnect the wire until a full 30 seconds can be maintained to drain the capacitor. The wire acts as a resistor to slowly discharge the capacitor's electrical charge.

Turn your multimeter to measure resistance, or "Ohms." Touch one of the multimeter leads to one side of the capacitor leads. Touch the other multimeter to the remaining capacitor lead. Monitor the resistance as measured on the multimeter.

Test the capacitor's ability to store energy. Failed capacitors will not store a charge, resulting in a "zero" reading on the multimeter. "Open" capacitors will not show any reading on the multimeter. Functional capacitors will initially display a "zero" reading on the multimeter and will slowly climb to infinity. Leaking capacitors will climb from zero, but will stop climbing. The climb results when the battery from the multimeter charges the capacitor. Discharge the capacitor between each test.

Replace capacitors with visual damage or failure. Replace capacitors that fail the resistance test.

Things You Will Need

  • Thick insulated wire
  • Wire strippers
  • Multimeter


  • Test known good capacitors to determine if the behavior of potentially leaking capacitors is normal.
  • Wrap bare wire with electrical tape to create an insulated barrier for gripping during testing.


  • Capacitors may contain an electrical charge strong enough to cause physical harm. Always discharge capacitors before handling.

About the Author

Skip Shelton has been writing since 2001, having authored and co-authored numerous articles for "Disclose Journal." He holds a Bachelor in Science in education and a Master of Business Administration with an emphasis in management from Northwest Nazarene University. Shelton also operates a small automotive maintenance and part-replacement shop.