How to Test an AC Capacitor

Think of your air conditioner as a finely tuned machine, where every part is essential for keeping your home cool. One of the key players in this system is the capacitor. Though small, it has a big impact – if it’s not working properly, your whole AC will suffer.

A faulty capacitor can cause issues ranging from your AC not starting to running inefficiently. While some homeowners may feel confident testing an AC capacitor themselves, it’s often best to leave it to the professionals. HVAC.com explains what an AC capacitor does, signs it might be malfunctioning, and how to test it.

What Does an AC Capacitor Do?

An AC capacitor stores and releases energy to start the motor and keep it running efficiently. It acts like a temporary battery, giving an initial power boost and then maintaining a steady flow of electricity to keep it operating.

There are two main types of capacitors in an AC unit: the start capacitor and the run capacitor. The start capacitor provides the extra voltage needed to start the motor, while the run capacitor provides the continuous voltage necessary to keep the motor running. Without a functioning capacitor, your AC unit won’t start or run efficiently.

Some air conditioners have separate start and run capacitors. In other models, a dual capacitor is used to provide power for both starting and running the system.

Signs of a Failing AC Capacitor

A failing capacitor can lead to several noticeable issues with your air conditioning system. Here are some common signs to watch out for:

• AC Won't Turn On: If your AC unit is not starting at all, a faulty capacitor may be the reason.
• Humming Noise: A loud humming noise coming from your air conditioning unit might indicate that the motor is struggling to start due to a bad capacitor.
• Intermittent Operation: If your air conditioner starts and stops frequently or doesn’t run continuously, the capacitor might be failing.
• No Cold Air: When the cooling system is running but not blowing cold air, it could be a sign that the capacitor is not functioning correctly.
• AC Turns Off on Its Own: If the AC unit shuts down randomly, the capacitor might be overheating and shutting off to prevent damage.
• Burnt Smell: A burnt smell near your condenser unit can indicate a capacitor failure, as it might be leaking or have a burnt-out component.

If you notice any of these symptoms, you may want to test the capacitor to determine if it’s responsible for your air conditioner’s poor performance, or contact a reputable HVAC company.

How to Test an AC Capacitor

Testing an AC capacitor involves several steps and requires some specific tools, but doing so can give you a clear answer as to whether this component is responsible for your cooling system troubles. Here’s a step-by-step guide on how to test an AC capacitor:

1. Turn Off the Power

Before testing the capacitor, ensure the power to the air conditioning unit is completely turned off. Disconnect the power at the circuit breaker to avoid any electrical shock. Most air conditioning units have a service disconnect box located near the outdoor condenser unit. This box contains a lever, switch, or pull-out handle that disconnects the electrical power to the unit. 

Open the box, and move the lever or switch to the OFF position, or pull out the handle – depending on your unit, you may need to flip or rotate the handle after pulling it out to ensure the power is fully disconnected.

2. Locate the Capacitor

Use a screwdriver to remove the access panel on the outdoor condensing unit. The capacitor is typically located near the compressor and fan motor inside the AC condenser. It is a cylindrical or oval-shaped component (similar to the size of a soda can) with two or three terminals on top, often attached to the unit’s housing or a bracket.

3. Check the Capacitor’s Condition

If the capacitor is damaged – such as having bulging, leaking, or a burnt appearance – do not attempt to discharge or handle it yourself. These signs of physical damage indicate that the capacitor is likely faulty and could pose a risk of further harm or injury. Instead, contact an HVAC technician to safely inspect and replace the damaged capacitor.

4. Discharge the Capacitor

Like a battery, capacitors store electrical energy, so you need to discharge it before testing. Using a well-insulated screwdriver, firmly hold the handle and carefully touch the metal shaft across the terminals of the capacitor. This action creates a short circuit, allowing any residual electrical charge to safely dissipate. For added precaution, you can repeat this process on all terminals, especially if the capacitor has more than two. Safety should always come first – consider contacting a professional if you’re unsure.

5. Remove the Capacitor

Use a marker or tape to label the wires connected to the terminals of the capacitor to ensure you can reconnect them correctly later. Carefully detach the wires from the terminals using needle-nose pliers, making sure not to touch the terminals directly. Once the wires are disconnected, use a screwdriver to remove any screws or fasteners securing the capacitor in place. Then gently lift the capacitor out of its mounting bracket or housing. 

6. Test the Capacitor

Set your multimeter to the capacitance setting (often indicated by a symbol like “MFD" or “μF”). Place the multimeter probes on the capacitor terminals. The multimeter will display a reading. Compare this reading to the rated capacitance value printed on the capacitor. If the reading is significantly lower than the rated value, the capacitor is faulty and needs to be replaced.

Potential Dangers of DIY Testing

Testing an AC capacitor might seem simple, but it comes with risks. Capacitors can hold a charge even when the power is off, which can lead to electric shocks if not handled properly. Plus, if you discharge or handle the capacitor incorrectly, you could injure yourself or damage your AC unit.

There’s also the risk of misdiagnosis. Capacitors are just one part of an AC system, and other issues might cause similar symptoms. Testing or replacing the capacitor incorrectly might not solve the real problem and could create more issues down the line.

Because of these risks and the complexity involved, it’s usually best to call a professional HVAC technician. They have the right knowledge and tools to safely handle and test capacitors, and they can accurately diagnose whether the capacitor is the problem or if something else needs attention.