How to Tell if a Capacitor is Bad: AC Capacitor Test

How to Tell if a Capacitor is Bad: AC Capacitor Test 

 A functional AC capacitor is essential if you want an air conditioner to run correctly when you need it to. HVAC technicians often find themselves working with these components. Like any other component of a complex system, AC capacitors can wear down and break with time. Being able to tell if they are operating correctly is an important step when troubleshooting an air conditioning unit. In this article, we will break down how to test an AC capacitor.

Types of Capacitors

To know the right way to approach your testing process, you will want to be familiar with the different types of AC capacitors. There are a few common types that you are likely to find on the units that you work with for your clients.


Start capacitors are a very basic type of capacitor that serves one simple purpose: booting up an air conditioning unit. These simple AC capacitors maintain just enough energy for the startup process so that the system can begin to run.


Run capacitors are a very common type of AC capacitor that you will find on a high volume of the units that you work on. These AC capacitors are known to store energy for different functions. The energy found within these units is commonly used to power the motor responsible for an air conditioner’s fan.


These powerful AC capacitors are known for combining the features of both start and run capacitors. They have a capacitor that can support two different motors, allowing them to manage even more from one single case. The two capacitors are combined in one area to save on space. These capacitors are more commonly found in larger HVAC systems.

On a dual capacitor, you will see three primary terminals. These terminals will be labeled with three titles–C, Fan, and Herm.

The “C” terminal refers to “common” and will connect to the contactor. This terminal is considered the source of power that is entering the system. Most terminals accommodate up to 4 lines.

The “Fan” terminal is very well-named for its purpose. It connects directly to the condenser fan motor. These terminals generally accommodate 2 lines.

The “Herm” terminal refers to the term “hermetically-sealed compressor” in this case. It connects directly to the compressors and generally has support for up to 3 lines.

What Types of Units Have AC Capacitors?

AC capacitors can be found on single-phase AC motors, particularly permanent split capacitor (PSC) motors. These capacitors store and release energy as needed to ensure that the air conditioning unit can boot up and get to work.

How to Run an AC Capacitor Test

In order to run your AC capacitor test, you will need to begin by turning off the power. It is very important that you start with this step for your own safety and that of the system.

When the system has been turned off, you will disconnect all of the wiring. The amount of lines may vary depending on the AC capacitor itself. Be sure to completely disconnect everything.

After you have disconnected the wiring, it is time to drain any of the leftover voltage. Again, this is a very important step that must be completed before you carry out any testing. In some instances, you may need to remove rust from the terminal to expose the contacts.

Dual capacitors can be discharged with a 10,000 to 20,000 ohm resistor across all three terminals. Single capacitors can be discharged by placing the resister across the two taps.

Following the discharge process, you can jump into testing. To do this, you will want to use a multimeter with the MFD setting or uF setting.

How to Tell if a Capacitor is Bad

There are many red flags that will tell you that an AC capacitor is bad, but not all of them are as obvious as you would expect. Though, in some instances, you will be able to know before testing, that isn’t always the case. Some AC capacitors will look fine until they fail testing.

When testing, you will know that an AC capacitor is bad if it falls out of the appropriate range. A good capacitor will have a uF MFD reading around 5% of the storage capacity. This will vary by device, so be sure to check the capacity. Anything significantly outside of this standard reading should indicate that the system is bad.

In some cases, you can tell that a capacitor is bad without testing. Common indicators of this include a capacitor that looks expanded or bloated, signs of leaking dialectic fluid, an oil-soaked top cap, or any other physical issues.

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