How Do You Test A Capacitor On An Air Conditioner?

You test a capacitor on an air conditioner with a digital multimeter. This tool checks if the capacitor stores power correctly. A bad capacitor can stop your AC from cooling. It might even keep the fan or compressor from starting. This guide will show you safe steps to check your AC capacitor. You will learn about AC capacitor testing methods and how to use your meter for a multimeter capacitor check AC.

Why Your AC Needs a Capacitor

Your air conditioner needs electricity to run. Big parts like the compressor and fan motor need a strong jolt of power to start. They also need steady power to keep running. This is where a capacitor comes in.

A capacitor is like a small battery. It stores an electric charge. Then, it quickly releases that charge. This burst of power helps the motors start. It also helps them run smoothly.

Think of it like a push-start for a car. The capacitor gives the motor that first strong push. Without it, the motor might just hum. Or it might not move at all.

Start Capacitors and Run Capacitors

Air conditioners usually have two types of capacitors. Each type does a different job.

Start Capacitors: These capacitors give a large burst of power. This power helps the compressor motor start. The motor needs a lot of energy to get going. Start capacitors only work for a few seconds. Then, they turn off. Not all AC units have a separate start capacitor. Some only use a run capacitor. The air conditioner start capacitor test checks this part.
Run Capacitors: These capacitors keep the motor running smoothly. They store and release small amounts of power. This helps the motor work well. It makes the motor use less energy. Most AC units have a run capacitor. This can be a single capacitor. Or it can be a dual capacitor. A dual capacitor helps both the fan motor and the compressor. Run capacitor troubleshooting often deals with these.

Both types are key for your AC. If one goes bad, your AC may not work right. This is why testing them is important.

Signs of a Failing Capacitor

Before you test, look for warning signs. Your AC might tell you its capacitor is bad. Knowing these bad AC capacitor symptoms helps.

Common Issues You Might See

AC Not Turning On: This is the most common sign. Your AC unit might not turn on at all. Or it might try to start and then stop.
Unit Hums But Does Not Start: You might hear a buzzing sound from the outdoor unit. This means the motor is getting power. But it is not enough power to start spinning. The capacitor is not giving the motor the push it needs.
Outdoor Fan Not Spinning: The large fan on top of the outdoor unit might not spin. The compressor might still try to work. This makes the unit overheat.
AC Blowing Warm Air: The fan might run, but the air is warm. This means the compressor is not working. The capacitor helps the compressor start. If it fails, the compressor may not run.
Tripping Circuit Breaker: An old or bad capacitor can pull too much power. This can trip your home’s circuit breaker.
Higher Electric Bills: If the capacitor is weak, the motor works harder. This uses more electricity. Your power bill might go up.

Visual Checks for a Bad Capacitor

Sometimes, you can see a problem with the capacitor. Always turn off the power first for safety.

Swelling: A bad capacitor often looks swollen. The top might bulge out. It can look like a can ready to burst. This is a clear sign of failure.
Leaks: You might see oil or fluid on or around the capacitor. This means it has leaked. Leaking fluid is a sign the capacitor is bad.
Burn Marks: Look for any burn marks. These might be on the capacitor itself. Or they might be on the wires. Burn marks mean it got too hot.
Rust or Corrosion: Excessive rust or corrosion can also point to issues.

These visual signs make diagnosis easy. But even if it looks fine, it could still be bad. That is when you need to test it.

Safety Before You Test

Working with electricity can be dangerous. Capacitors can store a lot of power. This power can hurt you even when the AC is off. Always follow safety steps. Discharging AC capacitor safety is key.

Important Safety Steps

Turn Off Power: This is the most important step. Go to your home’s main electrical panel. Find the breaker for your outdoor AC unit. Turn it to the “OFF” position. It is best to turn off the indoor furnace breaker too. This makes sure no power reaches the unit.
Double-Check Power: Use a voltage tester. Touch the probes to the main power wires inside the AC unit. Make sure there is no power. Do not skip this step.
Locate the Capacitor: The capacitor is usually a metal cylinder. It is often inside the outdoor unit’s electrical panel.
Discharge the Capacitor: This step is vital. Even with power off, a capacitor can hold a charge. This charge can shock you. Use a tool with an insulated handle.
- Method 1: Insulated Screwdriver: Get a screwdriver with a strong, insulated handle. Make sure the metal part of the screwdriver touches both terminals at once. This will short the capacitor. You might see a small spark. This is normal. It means the capacitor is discharging. Keep the screwdriver on the terminals for a few seconds. Do this for all sets of terminals (common, fan, herm for dual capacitors).
- Method 2: Resistor Discharge Tool: Pros often use a special tool. It has a resistor built in. This tool safely drains the charge.
- NEVER use your bare hands.

Tools You Will Need

Having the right tools makes the job easier and safer.

Digital Multimeter: This is a must-have for a multimeter capacitor check AC. Make sure it can read capacitance (MFD or microfarads). Most modern digital multimeters have this setting.
Insulated Screwdriver: For discharging the capacitor. A 4-in-1 screwdriver often works.
Needle-Nose Pliers: To help with wires and terminals.
Gloves: Insulated gloves add an extra layer of safety.
Safety Glasses: Protect your eyes from sparks if they happen.

Always put safety first. If you are not sure, call a pro.

The Testing Process: Step by Step

Now, let us get to the actual AC capacitor testing methods. This process is simple. But you must follow each step carefully.

Step 1: Prepare the Unit

Turn Off Power: Go to your breaker box. Turn off the breaker for the outdoor AC unit. Turn off the indoor furnace breaker too.
Remove Panel: Use your screwdriver. Take off the access panel on the outdoor AC unit. This panel covers the electrical parts.
Locate Capacitor: Find the capacitor. It will look like a metal can. It usually has 2, 3, or 4 terminals on top.
Take a Picture: Use your phone to snap a photo of the wiring. This helps you remember where each wire goes. You will need this when you put things back.
Discharge Capacitor: IMPORTANT! Use your insulated screwdriver. Touch it across the terminals of the capacitor. Hold it for several seconds. Do this for all sets of terminals. You might see a spark. This means the stored power is gone.

Step 2: Disconnect Wires

Once discharged, remove the wires. Gently pull the wires off the terminals. Note which wire goes to which terminal. Write it down if needed, or rely on your picture. The terminals are often labeled:
* C or COM: Common
* FAN: For the fan motor
* HERM: For the compressor motor (hermetically sealed compressor)

If you have a single capacitor, it will likely only have two terminals.

Step 3: Set Your Multimeter

This is where the multimeter capacitor check AC comes in.

Turn On Multimeter: Turn your digital multimeter on.
Select Capacitance Setting: Look for a symbol on your meter. It looks like ||- or Hx. This is the capacitance setting. It might be labeled “F” for Farads. Capacitors are measured in microfarads (µF or MFD). Set your meter to this function.
- If your meter does not have a capacitance setting, it cannot test capacitors. You will need a different meter.
- Some meters have an auto-ranging function. This means you do not need to pick a range. Others might need you to pick a range (e.g., 200 µF, 2000 µF). Pick a range higher than the capacitor’s stated value.

Step 4: Test the Capacitor

Now, perform the AC compressor capacitor testing and fan testing.

Connect Leads: Touch the red probe of your multimeter to one terminal. Touch the black probe to the other terminal.
- For a single capacitor: Touch one probe to each terminal.
- For a dual capacitor (3 terminals: C, FAN, HERM):
  - Test Common (C) to Fan (FAN). This checks the fan part of the capacitor.
  - Test Common (C) to Herm (HERM). This checks the compressor part.
Read the Display: Your multimeter will show a number. This number is the capacitance reading MFD AC. It should appear quickly.
Compare the Reading: Look at the label on the capacitor. It will show its rated MFD value. For example, “45/5 MFD.” This means 45 MFD for the compressor and 5 MFD for the fan.
- A good capacitor will read within +/- 5% of its marked value.
- So, a 45 MFD capacitor should read between 42.75 MFD and 47.25 MFD.
- A 5 MFD capacitor should read between 4.75 MFD and 5.25 MFD.

Example Readings:

Marked MFD	Acceptable Range (±5%)
30 MFD	28.5 MFD to 31.5 MFD
35 MFD	33.25 MFD to 36.75 MFD
40 MFD	38.0 MFD to 42.0 MFD
45 MFD	42.75 MFD to 47.25 MFD
50 MFD	47.5 MFD to 52.5 MFD
5/30 MFD	FAN: 4.75-5.25 MFD
	HERM: 28.5-31.5 MFD

What if Your Meter Does Not Have a Capacitance Setting?

Some basic multimeters only test resistance (ohms). A faulty capacitor ohms test is less reliable. It can tell you if a capacitor is totally dead (open circuit) or shorted.

For an Ohms Test: Set your multimeter to the highest ohms (Ω) range. Touch the probes to the capacitor terminals. A good capacitor will show numbers that quickly rise and then fall. Or it might show a low reading that climbs. This means it is taking a charge. A bad one will show nothing (open circuit) or stay at zero (shorted). This test only gives a rough idea. It cannot tell you the exact MFD value. It is always better to use a meter with a capacitance setting.

Step 5: Interpret the Results

Good Reading: If the reading is within 5% of the marked MFD value, the capacitor is likely good.
Low Reading: If the reading is lower than 5% of the marked MFD value, the capacitor is weak. It needs to be replaced.
Zero or “OL” (Over Limit): A reading of zero or “OL” means the capacitor is dead. It is not holding any charge. Replace it.
Very High or Unstable Reading: This can mean the capacitor is shorted internally. Replace it.

If your capacitor tested bad, you will need a new one. Make sure the new one has the exact same MFD rating. Also, check the voltage rating. The new capacitor’s voltage rating must be equal to or higher than the old one.

HVAC Capacitor Diagnostic Steps for Specific Issues

Testing a capacitor is part of bigger HVAC capacitor diagnostic steps. Here is how the test fits into common problems.

AC Unit Not Starting At All

Check Breakers: First, check your home’s electrical panel. Make sure the AC breaker is on.
Check Thermostat: Is the thermostat set to cool? Is the temperature set low enough?
Listen to Outdoor Unit: Do you hear anything? A hum? No sound at all?
Visual Check: Look for swollen or leaking capacitors.
Test Capacitor: Follow the steps above. A dead capacitor is a common cause for no start.

Outdoor Fan Not Spinning, Compressor Hums

Listen Closely: You hear a hum from the outdoor unit. But the large fan blade is not turning.
Power Off Safely: Make sure all power is off.
Try to Spin Fan: Use a stick to gently push the fan blade. If it spins up and stays running, the fan part of the capacitor is bad.
Test Capacitor: Pay close attention to the “FAN” section of a dual capacitor. If it reads low or dead, replace it. The run capacitor troubleshooting for the fan side is very important here.

AC Blowing Warm Air

Is Fan Running? Check if the outdoor fan is spinning.
Is Compressor Running? Listen for the compressor. It makes a low hum or rumble. If the fan is running but the compressor is quiet or just humming, the compressor capacitor part might be bad.
Test Capacitor: Focus on the “HERM” (compressor) section of a dual capacitor. A low or dead reading here points to a failed capacitor for the compressor. This is a key part of AC compressor capacitor testing.

Run Capacitor Troubleshooting: More Details

Run capacitors are always working when your AC is on. They are built to last many years. But they can fail.

Single Run Capacitors

These only have two terminals. They power either the fan or the compressor. You test them by connecting your meter to these two terminals. Compare the reading to the MFD value on the capacitor.

Dual Run Capacitors

These are very common. They power both the fan and the compressor. They have three terminals:
* C (Common): Power input.
* FAN: Output for the fan motor.
* HERM (Hermetic): Output for the compressor motor.

When run capacitor troubleshooting a dual capacitor, you must test two separate circuits:
1. C to FAN: This checks the fan capacitor.
2. C to HERM: This checks the compressor capacitor.

It is possible for one part to fail while the other still works. For example, the fan might run, but the compressor might not. This would mean the C-to-FAN reading is good, but the C-to-HERM reading is bad. Even if only one part is bad, you must replace the entire dual capacitor.

Air Conditioner Start Capacitor Test: What to Look For

Start capacitors are different. They are only used for a moment. They usually come in separate casings. They might also have a “potential relay” nearby. This relay turns the start capacitor off after the motor starts.

Testing a Standalone Start Capacitor

Safety First: Power off AC, find the capacitor, discharge it.
Remove Wires: Take pictures and disconnect wires.
Test with Multimeter: Set your multimeter to capacitance (MFD). Connect the probes to the capacitor terminals.
Check Reading: Compare the reading to the MFD value on the capacitor. Start capacitors have a wider tolerance range, often +/- 20%. But still, if it reads zero or way off, it is bad.

Start capacitors fail differently than run capacitors. They can go bad if the motor struggles to start often. Or if the potential relay does not turn them off.

The Role of the Potential Relay

If your AC has a start capacitor, it will likely have a potential relay. This relay is a small electrical switch. It senses when the motor has started. Then, it opens the circuit to the start capacitor. This takes the start capacitor out of the loop. If this relay fails, it can either:
* Keep the start capacitor in the circuit too long. This overheats and damages the capacitor.
* Never connect the start capacitor. This means the compressor gets no start boost.

Troubleshooting a start capacitor often means checking the relay too. But testing the capacitor itself is the first step. If the capacitor tests good, then the relay might be the problem. This is a job for a professional.

Replacing a Capacitor

If your test shows a bad capacitor, you need to replace it.

Get the Right Replacement:
- MFD Rating: This must be the same as the original. If your old capacitor is 45/5 MFD, get a 45/5 MFD replacement. Do not use a different MFD.
- Voltage Rating: The new capacitor’s voltage must be equal to or higher than the old one. For example, if your old one is 370V, a new 370V or 440V is fine. Never use a lower voltage.
- Physical Size: Make sure the new capacitor fits in the old spot. Sizes can vary.
Install New Capacitor:
- Make sure power is OFF.
- Use your photo to connect wires to the correct terminals (C, FAN, HERM).
- Secure the new capacitor in its bracket.
- Put the access panel back on.
Restore Power: Turn the breakers back on.
Test AC: Turn on your AC unit. Listen for the fan and compressor to start. Check if it blows cold air.

When to Call a Pro

While testing a capacitor can be a DIY task, sometimes it is best to call a professional.

If You Are Unsure: If you feel unsafe or unsure at any step, stop. Electricity is dangerous.
Complex Problems: If the capacitor tests good, but your AC is still not working. The issue might be with the compressor, fan motor, or other electrical parts. These need expert diagnosis.
Warranty Concerns: Trying to fix your AC yourself might void its warranty. Check your warranty details first.
Specialized Equipment: Some issues need tools that only HVAC pros have. For example, testing refrigerant levels.

An HVAC technician can perform full HVAC capacitor diagnostic steps and fix other problems.

Conclusion

Testing your AC capacitor is a valuable skill. It can save you money and keep your home cool. By following these AC capacitor testing methods, you can find out if a bad capacitor is your problem. Remember to always put safety first. Use the right tools. And know when to call a professional. A little know-how can go a long way in keeping your air conditioner running smoothly.

Frequently Asked Questions (FAQ)

Q1: How long do AC capacitors usually last?

AC capacitors typically last about 5 to 10 years. Their lifespan can vary. Factors like heat, power surges, and how hard your AC works can shorten their life. Regular use and age make them weaker over time.

Q2: Can a bad capacitor damage the AC compressor?

Yes, a bad capacitor can harm your AC compressor. If the capacitor is weak, the compressor motor tries to start but cannot get enough power. This can make the motor overheat. Overheating can burn out the motor windings. This leads to very costly repairs or a full compressor replacement. Replacing a bad capacitor quickly saves the compressor.

Q3: What MFD reading is considered good for an AC capacitor?

A good MFD (microfarad) reading for an AC capacitor should be within 5% of the value printed on its label. For example, if your capacitor is labeled “40 MFD,” a good reading would be between 38 MFD and 42 MFD. If it is a dual capacitor, like “45/5 MFD,” then the 45 MFD part should be within 5% of 45, and the 5 MFD part should be within 5% of 5.

Q4: Can I replace a capacitor with a different MFD rating?

No, you should always replace a capacitor with one that has the exact same MFD rating. Using a different MFD value can cause problems. It can make your motor run too hot or too cold. This can shorten the motor’s life. The voltage rating of the new capacitor must be equal to or higher than the old one. For example, if the old one is 370V, a 370V or 440V new capacitor is fine. Never use a lower voltage.

Q5: Is it normal for a capacitor to get warm?

Yes, it is normal for a run capacitor to feel warm when the AC unit is running. They are constantly working and generating some heat. However, if a capacitor feels very hot to the touch, it might be a sign of a problem. It could be failing or working too hard. Always check it when the power is off and the capacitor is cool before testing.

Carlos Gadd

My name is Carlos Gadd, and I am the creator of AirPurityGuide.com.. With a passion for footwear, I share my experiences, insights, and expertise about shoes. Through my blog, I aim to guide readers in making informed decisions, finding the perfect pair, and enhancing their footwear knowledge. Join me on this journey to explore everything about shoes!