If you have an air conditioning unit it will have a capacitor or two in it. Unfortunately, they don’t last forever. An ac capacitor will usually last for several years but it is unlikely that they last over ten years. If you have a problem with your air conditioning unit it’s quite likely that the capacitor is the culprit.

What is an AC capacitor?

An AC capacitor will usually look like a small metal can. It is used to start the motor that spins the fan around so if the fan isn’t spinning it could be a faulty start capacitor.

It’s used to give the motor a bit of a jolt at start-up as a motor takes far more current to start it than to run it. A capacitor is used to store up some energy and then release it to start the fan running.

The compressor that is used to cool the air also has a capacitor for exactly the same reason. It needs more power to start than it does to keep running so another capacitor is used to give the compressor a bit more power to start it going. Similar to your fridge that will also have a start capacitor.

Sometimes on an air conditioning unit, you will have two separate capacitors and sometime you will have one that has the two combined inside it or dual capacitor as it is also known.

For more information on capacitors read my article What is a capacitor and how does it work?

Before you start

Before you do anything disconnect the power from the air conditioning unit. This will vary depending on how it has been wired up maybe a fuse, fuse box or circuit breaker. Whatever it is make sure you disconnect it before you mess about with it.

How can you tell which AC capacitor you have?

Once you have located your “tin can” looking capacitor, don’t touch it you could get a shock off it even if you have disconnected the power. Have a look at the top or bottom depending on how it’s mounted, which will have wires connecting to it. If it has three connections such as spade connectors, it is the two capacitors combined type.

The 3 connections are “C” which is the common connection, “F” is for the fan connection and “H” is for the compressor. If you’re wondering the “H” part comes from the phrase hermetically sealed compressor.

If it just has two connections or spade terminals then there will be another capacitor somewhere in your air conditioning unit.

How to discharge an AC capacitor?

You can discharge an AC capacitor using a screwdriver. Make sure it has an insulated handle without any missing bits as if you touch any metal on it when you discharge a capacitor you will get a shock. If the capacitor is the two-terminal type, touch the metal blade and shaft so it touches both terminals you may see a spark and hear a pop sound so be prepared.

If it is the dual type of capacitor with three terminals you will have to discharge both capacitors. One of the terminals will be marked with a “C” meaning common. Touch the screwdriver blade and shaft so it touches the “C” terminal and one of the other two and then the “C” terminal and the remaining terminal. Both of the dual capacitors should now be discharged.

Don’t touch them though until you have tested them. You can do this if you have a multimeter across the terminals or you can use a non-contact voltage detector. If you detect enough voltage to make it glow and bleep you haven’t discharged the capacitors properly.

How to identify an AC capacitor

The AC capacitor will look like a small can, cylindrical or oval. It will have some wires from one of its ends, usually connected by spade connectors.

The capacitor will also have a voltage marked on the case. Don’t worry if it is a lot higher than the voltage of the air conditioner unit. It’s the maximum operating voltage of the capacitor, not the unit so it could be 440V which is fine.

If you’ve turned the power off to your AC unit and you’ve discharged your capacitor you can double check with a meter or non-contact so you’re sure there’s no power and no nasty shocks.

Take a picture of the capacitor with your mobile so you can see how it fits and how the connections go back on it. If you connect your new capacitor up wrong it’s not going to work.

You can now take the capacitor out it may be screwed or clipped in. Once you have it out look at the writing on it. You’re looking for the value of the capacitor or capacitors. This will be in microfarads or the symbol uF. A typical dual air conditioning capacitor will be something like 35uF / 5uF. These values obviously vary from unit to unit but they won’t be a million miles different. Once you have found what the values should be you can check the capacitor.

Visually check the capacitor

Although you can’t guarantee a capacitor will work just by looking at it there are some signs to look for that it might be bad before you test it. If there is anything leaking from it or if it’s misshapen with bulges you’ve guessed it. That’s not a good sign and even if it passes the test it’s an indication that all is not well and it probably won’t last anyway.

How to check an AC capacitor

Quite a lot of multimeters have a capacitor measuring function built into them. If you haven’t got a suitable meter it might be worth borrowing one. You’ll then be able to test the capacitor before you order a new one.

The meter I have has a capacitor range that goes up to 40uF so that would be perfect for measuring a 35uF / 5uF capacitor. Don’t worry if the value doesn’t measure exactly. Its fairly normal for a 35uf capacitor to measure anywhere between 33uF to 37uF.

If your multimeter has a continuity range you can still check the capacitor but it won’t give you a reading of the value of the capacitor but you can still get an indication if the capacitor is any good or not.

Set the multimeter to the continuity range so that it bleeps when you touch the probes together.

If you have a single capacitor when you touch the probes on the two capacitors terminal you should get a short beep and a reading on the multimeter that might be too quick to read but there will be something before the bleep stops and the multimeter reads 1 or open circuit.

You can discharge the capacitor and try it again. If you don’t get that short bleep the capacitor is probably not working. A continuous beep will indicate that the capacitor is short-circuited and also no good.

If you have a dual capacitor the smaller 5uF value will give a shorter bleep and reading than the 35uF capacitor.

This test is more difficult for a beginner as it is not a clear measurement like the first test it will also only show if a capacitor is holding a charge. Sometimes a capacitor may be faulty because it is reading something like 10uF when it should be 35uF. You’ll only be able to see this with the first capacitor measuring test.

Related article How to test a capacitor.

Where can you get a new AC capacitor?

You may be lucky enough to have a part supplier near you where you can go and buy a replacement but like most things these days you could just use the internet. There are loads of online outlets that you can try but having had my fingers burnt in the past I prefer just to stick to Amazon. I know they’ll refund me if I don’t get the item or if it’s broken on delivery or the wrong thing. Being an Amazon prime member I can get the item the next day for free.