Does You AC Need a New Capacitor? Signs It Does

When your AC is not cooling, it is could be because of so many reasons. One of those reasons is a bad capacitor. So, how do you know that your AC capacitor is bad and that you need a new one?

air conditioner capacitor

You can tell if your AC capacitor is bad by visually inspecting it or by testing it with a multimeter. If the capacitor is leaking oil or is swollen, it is clearly bad. Also, if you test it and its capacitance is less than the microfarads indicated then it is bad and will need to be replaced.

But before you even go outside to check your AC condenser unit, what are some of the symptoms of a bad AC capacitor?

The following are the signs of a bad AC capacitor:

  • The AC won’t start at all
  • An increase in energy bills
  • AC takes too long to come on
  • The AC turns off randomly
  • Humming noises from the outside unit
  • AC running but condenser fan isn’t turning
  • Burning smell from the outside unit
  • AC running but not cooling

The problem with the above signs is that they can be caused by so many other AC-related problems and not just a bad capacitor. The best way to clearly tell if you are dealing with a bad capacitor is by testing it.

Warning! Not everyone should test or even attempt to replace a capacitor. Capacitors store a high charge and can shock you when you touch the terminals even when the power to the AC has been turned off.

Ideally, AC capacitors should be tested and replaced by licensed HVAC technicians. And that is why replacing AC capacitors is expensive ($100 to $300) depending on where you live.

And if you think or someone told you that you can use your AC with a bad capacitor, it is a really bad idea. Running an AC with a bad capacitor will not only increase you energy bills but it will also strain other components like motors causing them to overheat and burn out.

Why ACs Need Capacitors

Let us start by defining a capacitor. A capacitor is a device that stores charge in a circuit and then releases when it is needed. Usually motors will need more voltage than the one supplied by the power line to start.

For that reason, you will notice that AC capacitors have a voltage of at least 370 volts while the supply lines have 240 volts of current. As such, the capacitor is connected to the motor windings to supply it with initial electricity jolt needed to start it.

There are start and run capacitors in AC units. As their name suggests, the start capacitors helps to start the motor while the run capacitor helps to keep the motor running optimally during the cooling cycle.

The start capacitor in an AC compressor has 2 functions. It increases the starting torque which helps the motor easily move from idle to a point where its speed is as needed. Secondly, it ensures that the motor is turning in the correct direction.

Therefore, when the capacitor is bad, it will not start or will only do so after a sometime. Also, the motor may turn in the backwards direction meaning that no cooling will happen.

Run capacitors on the other hand provides more torque to the motor, allowing it to run optimally and efficiently thereby lowering power consumed by the motor. When the capacitor is bad, the motor will need to work hard to meet your cooling needs which will cause the windings to overheat and burn out.

How to Tell if Your AC Capacitor is Bad

As I mentioned, the signs of a bad capacitor may be similar to those exhibited by other AC-related problems like a dirty air filter or low levels of refrigerant. So, how can you tell that indeed your AC capacitor is bad?

1. Outside AC Unit Humming?

If your outside unit is making humming noises, you most likely have a bad capacitor and the noises are coming from the compressor motor. Every time the thermostat calls for cooling, the motor struggles/strains to come on and hence the humming noise.

2. Condenser Fan Not Turning

condenser fan

Is your AC system running but the condenser fan is not turning? That could be another symptom of a bad capacitor. As I have mentioned, capacitors supply the motor with a powerful electricity jolt to start but if the capacitor is bad that won’t just happen.

Just to be sure, grab a long screwdriver or stick and manually turn one of the fan blades through the vents. If the fan keeps turning normally on its own after that, you clearly have a bad capacitor.

3. Swollen/Bulged Capacitor

Good capacitors will always have a nice cylindrical shape, with same diameter from top to bottom. However, most bad capacitors will start to swell/bulge but that is not always the case.

To inspect the capacitor, you will first need to remove the outside unit’s side panel. Before that though you will need to turn off power to the AC unit at breaker box and also unplug the furnace so that you can work safely.

If after removing the side panel and inspecting the capacitor you notice that it has swollen or mushroomed, it is clearly bad and will need a replacement. You don’t even need to test the capacitance of such a capacitor.

4. Oil Leaking From Capacitor

Capacitors are normally filled with oil in order to regulate the inside temperatures. Oil also helps to remove air from the capacitor.

Needless to say, a good capacitor should have all its oil contained inside. If however upon inspection you notice that the capacitor is leaking out the oil, it is clearly bad and a replacement is the only solution.

5. Low Capacitance/Microfarads

You can only know this by testing the capacitor using a micrometer. Capacitors have 2 ratings; capacitance and voltage. Voltage is simply how much charge is passing through the capacitor while capacitance is the amount of charge that the capacitor can store/accumulate.

These ratings are usually clearly written on the exterior of the capacitor. Capacitance usually have a margin of error (usually 6%) which you need to factor in when testing the capacitor’s capacitance.

If after testing the capacitance you come to the conclusion that the capacitance of the capacitor is lower than recommended, you have a bad capacitor and you should replace it.

How to Test a Capacitor

The following are the steps to take when testing an AC capacitor:

  • Turn off power to the AC unit at the breaker box. Also importantly, unplug your furnace inside as well.
  • Use a screwdriver to remove the side panel from your AC outside unit.
  • Locate the capacitor. It will be a shiny cylindrical piece with wires connected to its terminals at the top.
  • Discharge the capacitor. An easy way to do that is take a screwdriver with an insulated handle and while holding the handle, short the 2 capacitor terminals using the metal part of the screwdriver. If you have a dual capacitor, short the COMMON and HERM and then COMMON and FAN.
  • Before you disconnect the capacitor, take your time to note how the wires are connected around it. You should probably grab your phone and take a picture which will come in handy when connecting it back (or replacing it).
  • Disconnect the wires from the capacitors and also undo the capacitor from the strap holding it to the unit.
  • Place the capacitor on a working table or any flat surface and get your micrometer.
  • Set your micrometer to the capacitance setting (MFD).
  • If you have a single capacitor, just connect the 2 probes to the 2 terminals and note the displayed capacitance reading.

Note: In a double run capacitor, there will be 2 capacitance readings (something like 45/5 uF). The High reading is for the compressor (HERM) while the lower reading is for the condenser (FAN).

  • If you have a dual capacitor, connect the probes to COMMON and FAN and note down the readings and then COMMON and HERM.
  • If the capacitance reading is lower than the one indicated (factoring in the allowed margin), your capacitor is bad and will need a replacement.

Tip: It is not unusual to find out that one side of the dual capacitor is good while the other is bad. It happens all the time. If such is the scenario with your capacitor, just replace it.

What to Know When Replacing AC Capacitors

When replacing an AC capacitor, make sure that you buy a replacement with the same rating as the old one. I am talking about capacitance and voltage and not necessarily brand.

For instance, if your capacitance is rated 45/5 microfarads and 440 volts, buy another one of the exact same rating. That will help your AC motors run optimally and efficiently.

Also, if you have 2 single capacitors and one of them fails, it is a good ideal to replace both of them with a dual capacitor. As you can imagine, it is easy to replace one capacitor instead of 2 in the future.

And that is how you know if your air conditioner capacitor is bad. As I mentioned, if you are not comfortable testing and/replacing it, call in a professional technician to do it for you.