How to Test An AC Capacitor

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Updated August 19, 2022

AC capacitor failure can happen from time to time, even to the best air conditioners. Because of this, capacitors need to be tested every few years or if you find your unit running weaker than normal. An indication of this can be the lights dimming when the air conditioner comes on. You don’t want to react and scrap your air conditioner, however. This article will show you how to test an air conditioner capacitor with a digital multimeter.

KEY TAKEAWAYS:

  • AC capacitors provide the initial power boost to an outdoor AC unit as it begins its cooling cycle.
  • You’ll need a multimeter and a basic tool kit to test a capacitor.
  • Capacitors need to be replaced around every twenty years.

Alternatively, if your unit is having other performance problems, check out our article on how to straighten air conditioner fins. Or to check whether your compressor is going bad, read up on how to test an air conditioner compressor.

Insider Tip

To check if your capacitor is blown, you can check it visually to see if it looks like a pop can with a pushed-up top.

What is an AC Capacitor?

The AC capacitor is a small silver cylindrical object with three wires connected to the top. It stores energy and delivers a large power boost to the motor, which kicks off the cooling cycle.

Testing Your AC Capacitor

STEP 1 Turn Off the AC System’s Power

Unplug the AC unit, or go to the breaker that powers the AC unit and turn it off.

STEP 2 Find and Inspect the Capacitor

  1. Unscrew the access panel.
  2. Locate the capacitor. Do not touch the capacitor at this point of the procedure.

STEP 3 Discharge the Power in the Capacitor

  1. To discharge the capacitor, locate these three wires: Herm, Fan, and Common. These should be labeled on your capacitor. Pull these wires loose from the wire posts. Once disconnected, make sure that the wires aren’t touching one another.
  2. Note down which wire attaches to which wire post.

STEP 4 Dismounting the Old Capacitor

  1. To dismount the old capacitor, unscrew the bracket on the capacitor terminal. Then, pull out the capacitor.

STEP 5 Use a Multimeter to Test the Capacitor

  1. Set your multimeter to the capacitor testing mode. Modes are explained in the device’s user manual, but it is usually located in the noon position. In test mode, the multimeter display should say “NF.”
  2. Check the capacitor label for the target numbers. For example, it might say 35uF/5uF.
  3. Place the red probe on the herm post (H) and the black probe on the common post (C). The number that shows up on the multimeter display should be the same as the first number (like 35uF) on the label.
  4. Then, keeping the black probe on the common post, switch the red probe over to the fan post (F). Then compare this number to the second one on the label (like 5uF).
  5. There is a rule that a capacitor has to read +/-6% to be considered in good condition. If your numbers fall outside the range, you’ll need to replace your capacitor.

STEP 6 Reassembly

  1. If your air conditioning capacitor tested fine, screw it back into the bracket.
  2. Reconnect the wires, then screw the access panel back on.
  3. Turn the AC power back on.

Warning

Make sure to disconnect the three wires before touching the capacitor with your bare hands to avoid injury.

F.A.Q.S

Will an AC run with a bad capacitor

If you have a bad AC capacitor, the unit will likely stop running, and you’ll hear a humming sound coming from it.


How long do AC capacitors usually last?

If kept under suitable conditions, AC capacitors can last for 20 years.


What causes AC capacitors to go bad?

AC capacitors often go bad from power surges, overheating, and short circuits.



STAT: The acceptable ohm range is usually labeled on the capacitor. If the capacitor tells you “50 ±5%,” the acceptable range is 47.5 to 52.5 F. (source)

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