How To Test A Central Air Conditioner Capacitor

Posted on: 3 October 2016

If your central air unit has trouble starting, or makes a humming sound, test the HVAC capacitor. The HVAC capacitor works like a battery storing power needed by the motor to start. The capacitor connects to two metal terminals on the motor.

It is easy for a novice to test a central HVAC capacitor, Here are tips to test an HVAC capacitor.

Prepare to Work

For this project, you need: 

  • work gloves
  • OHM meter (digital or analog) 
  •  needle-nose pliers
  • screwdriver 

Turn off power to the central air unit, then switch the thermostat to the lowest setting. For extra safety, turn the power off from the breaker box. Let the unit cool, if it has been running.

Locate the capacitor. The capacitor is commonly mounted on the cover of the outside unit on a central air unit. Refer to your brand's user's guide for the exact location.

Drain the Capacitor

If it essential to drain the capacitor of any electrical charge. It can still have an electrical charge , even when the power is off. Detach the panel on the outside unit or the sheet metal cover with a screwdriver.

Inspect the capacitor for leaks. If it is leaking, check disposal regulations for the proper way to dispose of it. and replace it. Otherwise, proceed with the test.

Take a picture, or make a note of how the wires connect. Disconnect the capacitor wires, then detach the metal retaining strap. Lay the capacitor with wires turned up on a flat work area with the wires attached.

Insert the tip of the screwdriver between two terminals, which should cause a spark. Grab the wires with the needle-nose pliers, and remove the capacitor from them. Set the terminal aside..

Test with an OHM Meter

An OHM meter tests for continuance. Set the OHM meter to the lowest setting, which is RX1. Digital OHM meters include a specific setting for capacitors, usually labeled "CAP". Connect the leads in the same manner as an analog OHM meter.

Connect the red lea to OHM connector, and connect the black lead to the "COM" connector. Bring the ends of the leads together. The display on the OHM meter should say "0". The display on a digital OHM meter will read "Good" or "Bad".

Connect the red lead to a terminal, and the black lead to the other terminal. The dial should gradually move to 0 , then to "INFINITE" on the left side. If the dial doesn't move, the capacitor is faulty.

Replace a faulty capacitor, and restore power to the unit. If the capacitor isn't faulty, or the unit still doesn't work after replacement, contact an HVAC service. 


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