How Do You Test a Capacitor on an Air Conditioner?

Had your AC unit ever throw a tempter tantrum? Perhaps it’s humming loudly, operating sluggishly, or simply blowing air that isn’t very cold. If you find yourself wondering “how do you test a capacitor on an air conditioner?”. then you’re in the right spot. To test an air conditioner, the professionals use a capacitance testing meter, which comes to around $40 but is also included in multimeters. The first step here is to turn off power to your AC unit and then safely discharge the capacitor. After that match your multimeter to the measure the reading between the terminals of the capacitor to the conducting itself. It’s a fast way to troubleshoot a common problem, and to save yourself a chunky service call.

The Silent Hero In Your Air Conditioner - The Capacitor!

Now before you go poking around inside, what exactly does this cylindrical wizard do? An AC capacitor is not just any other part; it’s a component that saves an electrical charge and releases it to power up and keep your air conditioner’s compressor and fan motor running. Think of it as a jack in the box for your AC – it gives a big “kick” to get everything started and then delivers a surge of energy once the unit’s up and running.

There are typically a couple different types of capacitors found in an AC.

• Run Capacitor: This is the guy that helps the motor’s run once they’ve gotten started. These are in most house ACs.
• Start Capacitor: This one provides an extra jolt of electricity (again, just to get things started) and then it’s out of there.
• Dual Run Capacitor: Now it starts getting good. A lot of AC systems do this, containing two run capacitors in a single unit. They usually power both the compressor and the fan motor. On a run capacitor, you’ll frequently find three terminals marked ‘C’ for Common, ‘FAN’ for the fan motor, and ‘HERM’ (on smaller capacitors you may have just a single ‘COMP’ terminal) for the hermetic compressor.

Numbers On a Capacitor On a capacitor, you will notice a few numbers as you look at it. It is typically quantified in microfarads (MFD or µF), and indicates how much electrical current it has the ability to store. You’ll also find a voltage (VAC) rating on the cord to tell you how much electrical current it can manage. A heads-up: don’t get capacitors mixed up with batteries. And while they can store charge, they don’t hold onto it long enough for the capacitor to actually power anything all on its own; they operate within the context of an AC’s wiring system.

Red Flags: The Telltale Signs of a Bad AC Capacitor

Well, how can you tell if your AC capacitor is about to kick the bucket, long before you even pick up your tools? Distress signals from your air conditioner are typically Mayday calls. Keep an ear out for these common symptoms:

• Air Conditioner Won’t Turn On or Fails to Start: This is a perennial. Your machine is refusing to start, or it huffs and puffs but never really catches.
• Very loud humming or clicking sound You can hear a loud hum or clicking noise from the condenser or compressor but there is no work being done? That engine is laboring to start without a proper juice. It’s ever so much like the sound of a car that is being cranked up and refuses to take hold.
• AC Fan on but Not Blowing Cold Air: It’s summer, the AC has been running for a while, but now, the fan’s just blowing hot air.…What seems to be the problem? That typically means the compressor, the part of the air-conditioner that actually cools the air, isn’t getting the juice it needs to run.
• AC Turns On and Off Repeatedly or in Intervals: Your equipment could turn on, run, and then stop. Or it might take minutes to turn on, and then randomly cycle on and off, which is just irritating.
• Higher Energy Bills: If your AC is struggling, it’s having to work harder. And harder work, you’ll find, is electricity, and electricity, everybody knows, is bills. Nobody wants that.
• Flickering and Dimming Lights: When your AC starts, do your lights flicker or dim? This could mean the capacitor isn’t doing the heavy lifting, leaving it for your home’s electrical system to do.
• Burnt Odor: No one wants to smell anything burning, especially near their own unit, is a quick indicator. There could be a leaky/faulty capacitor or burned out parts.

However, when the capacitor finally lets go, it can do so in big, dramatic fashion. Do a brief visual check for the following:

• Bulging or Swollen Top/Sides: The capacitor’s casing may appear to be pushed up or swollen. If it appears pregnant, it’s likely bad.
• Oil Slick: Oil traces on casing or terminals.
• Corrosion, Cracks, Dents, or Burn Marks: Physical damage to the case or terminals is an obvious indication of trouble.

If you see any of those physical damages, even if it tests “okay,” to be honest you might want to just go ahead and replace it. It’s a ticking time bomb.

Safety First: The Must-Have Steps Before You Startultiply your text!

Pay attention, this is the key. Seriously. Dealing with electrical capacitors like these AC capacitors is no laughing matter; it’s downright hazardous if you’re sloppy about it. These things can carry a powerful electrical charge, even after the power’s been turned off; a jolt from one can be brutal. So, no heroics. Read these instructions carefully:

1. KILL THE POWER: This is step one, two and three. It’s a short drive to your main electrical panel where you’ll want to turn off the circuit breaker that serves your air conditioning or HVAC unit. Next, locate the outdoor disconnect switch box close to your outdoor condensing unit. Open it up and either pull the handle or flip the switch to OFF. Confirm it’s off. Don’t assume anything.

2. VERIFY POWER IS OFF: If you have it, use a circuit alert or a voltage tester on the wires entering the unit to absolutely confirm there’s no power. Better safe than sorry.

3. GEAR UP: You’re working with electricity. Wear the right safety wear – insulated gloves and safety glasses are your best friends here.

4. DISCHARGE THE CAPACITOR (VERY IMPORTANT TO YOUR SAFETY): I cannot stress the importance of this. There’s still a good chance you’ve got stored energy in the capacitor. You need to discharge it BEFORE you contact ANYTHING.

• Tool: Get a screwdriver that has a well-insulated handle. Do NOT, under any circumstances place your hands on that metal shaft.
• For a Single Capacitor (two posts) Use a screwdriver (insulated handle) and touch the metal shaft between both posts. Maintain that position for a few seconds.
• For a Dual Capacitor (three terminals: C, FAN, HERM/COMP): you’ll do these in two steps. Start by jumpering the ‘C’ terminal to the FAN terminal holding it for a few seconds. Next, connect ‘C’ to ’HERM’ (or ‘COMP’) terminal, and hold for a few seconds. You may hear a tiny spark, or pop; that’s the short circuit safely letting out the collected energy. Repeat one or two additional times just to be super safe.

After that, you’re all clear to continue. But really, if you’re not okay with any of this, do not pass go and call a pro. It’s not worth the risk.

Your Playbook: How to Use a Multimeter to Test Your AC Capacitor

Okay, now that safety is out of the way, let’s take the real test. Here is your step-by-step guide:

1. What You’ll Need: You’ll want a multimeter that has a capacitance setting (it’ll often be labeled on the dial as µF or MFD as well). You got the insulated-handle screwdriver. – Needle-nose pliers can be good for removing wires.

2. Find the Capacitor: The access panel of your AC unit should be opened (held closed by some screws). Inside, what you’re looking for is a metal cylinder — usually silver, black, or blue, and maybe a foot tall or so, depending on capacity — with two or three terminals on top where wires are connected. The compressor and fan motor will be chilling.

3. Visual Inspection (Again! ): Take one more good look before you unplug anything. Keep an eye out for those signs of trouble: bulging, leaks, corrosion, cracks, burn marks. If it looks sketchy, replace it even if it passes testing later.

4. Take the Capacitor Out: This is very important. BEFORE YOU DO ANYTHING, take a CLEAR photo of where all the wires and their terminals (‘C’, ‘FAN’, ‘HERM’) were or label them. You really DO NOT want to confuse these later. Pull the wires off the connectors one by one. If so, unscrew any brackets or fasteners that are securing it and carefully lift it out.

5. Prepare the Multimeter: Rotate the dial on your multimeter to the capacitance option. The µF mark, or MFD, can look like or sometimes a solid line with a dotted line. Check if you correct probes connections in a meter.

6. Test the Capacitor:

For Single Capacitor (two terminals):Just put one probe of multimeter on each terminal. You will receive a reading from your meter.

For a Dual Capacitor (three pronged: C, FAN, HERM/COMP): Here you’ll take two readings.

• Test the fan circuit Next, you will want to test the fan circuit.
• Test the compressor section: Place one probe to the common terminal (‘C’), and the other probe to the hermetic compressor terminal (‘HERM’ or ‘COMP’).

Stump the Multimeter: What Say You?

Oops: You have a reading (or two!). Now what? Time to play detective.

• Compare to the Rating: All capacitors have a rated capacity value (given in MFD or µF) on the label on the capacitor. This is your benchmark. If you’re testing a dual capacitor, keep in mind — it has two ratings: typically a higher one for the compressor (HERM/COMP) and a lower one for the fan.
• Know the Tolerance: Capacitors are not precision components. They have a tolerance, typically ±5% or ±6%. The reading of a proper capacitor should fall precisely within this range.
• The Verdict – Good or Bad?

Here’s a breakdown of what your multimeter reading likely means:

Old-school multimeter users: a quick note: If your multimeter isn’t equipped with a capacitance setting, you might use an ohm meter. When you make contact between that with one of the terminals, there should be movement of needle to zero ohm, then movement in the other direction to over limit (infinity). If it doesn’t — or just sits around zero — your capacitor is likely a goner.

Other Ways to Test (The Pros’ Playbook)

The capacitance test is, of course, your standby, but other practices exist, and some are commonly for the pros:

• Ohm Meter Test for Shorts: Use your ohm meter and see if the cap has shorted to ground. Set your multimeter to ohms. Connect one probe to the terminal and the other probe to the capacitor’s metal casing. If your meter does register a reading of continuity (A low reading or a 0 reading), then it means that the capacitor is shorted and there is nothing further you can do with it other than placing it in the garbage bin.
• The “Jump Test?” (No Capacitance Setting? No Problem. ): If you don’t have a capacitance setting on your multimeter, you can approximate using the ohm setting (for instance, 1000 ohms or more. With the probes touching their extremal terminals, the meter’s needle should jump some towards zero ohms, then fall back towards infinite ohms. If you switch the probes you should observe the same dance. If the needle doesn’t remain in place, or never reaches zero, the capacitor is probably bad.
• Voltage Test (Under Load – Advanced Users Only): Here’s where some of you guys might be able to flex. With the AC running and the power on (and again, this is for professionals only, based on the high-voltage danger), they can measure the voltage across the capacitor’s terminals. If there’s a big voltage drop compared with what you’re expecting, it means you have a problem. This “load test” may be more reliable on some meters than a bench test – particularly when the capacitor fails only under load.

Ratings Matter: Choosing Your Capacitor Replacement

So, you have tried it, and it is bad. Now you need a new one. This is not where you eyeball it. So it is absolutely crucial to get the ratings right.

• Capacitance (MFD/µF): This is not up for discussion. The microfarad rating of the new capacitor must be EXACTLY the same as the old one. There are reports that using the wrong MFD may result in underperforming motors, motors running hot and sometimes even burning up. Dual capacitors have two MFD ratings, one for the compressor (C) and one for the fan (Fan). Match both.
• Voltage (VAC): Indicates the maximum voltage that the capacitor can receive safely. The voltage rating of your new capacitor needs to be the same or greater than the old one. For example if yours is 370VAC, you can put back a 370VAC or a 440VAC, never lower. Paradoxically, a higher voltage rating protects the capacitor from damage. Using the incorrect ratings, either MFD or VAC, for your new capacitor can cause it to prematurely wear out and other expensive deficienies with your AC. Don’t cheap out here.

DIY Capacitor Test : Know Your Limits

All right, so you have the information. You can really do this by yourself? The test itself? Yes, many can. But replacing it is a different ballgame. Here’s an easy cheat sheet to figure out whether you’re ready to go solo or if it’s time to call in the cavalry:

You’re in the Green for DIY Testing IF ALL Of These Apply:

• ✅ You know how to work safely around your basic electrical systems. You’re not new to this wires thing.
• ✅ You own and will use the right safety gear — think insulated gloves/tools and safety glasses.
• ✅ You are 100 percent certain that you can execute all of those safety steps, particularly the power-off and discharge actions, with the utmost precision. No “oh, maybe I’ll sit down for this part”.
• ✅ The only obvious issue seems to be the capacitor itself; don’t see melted wire, burn marks, or any other damage.
• ✅ You know how to read your multimeter and what those capacitance ratings mean.

SMASH THE RED BUTTON (Call a Licensed HVAC Company) NOW IF ANY OF THESE APPLY:

• ❌You have little to no experience doing electrical work. It’s not worth being electrocuted for a few quid.
• ❌You’re not properly equipped with safety gear or insulated tools. No, your screwdriver’s rubber band isn’t going to do it.
• ❌ You observe burn marks, melted wires, or any other damage other than the capacitor. But this is indicative of larger issues.
• ❌ Your A.C. unit is still under a warranty. DIY work frequently voids that bad boy and leavers you in the lurch if something else goes sour.
• ❌ There’s any step you feel anything less than comfortable about or confident doing. Trust your intuition here.
• ❌ Those are great isolated conclusions, seems the problem in the changes circuit. Which is our way of saying the capacitor wasn’t the real villain here, and you’re going to need a proper diagnosis.

And although it seems like testing should also be a do-it-yourself project, keep in mind that replacing the capacitor is the more dangerous proposition if you don’t know what you’re doing. Professionals have the right tools, the knowledge and follow strict safety protocols. They also have the ability to test for the capacitor being the actual issue at-hand or another one of those pesky and time-consuming alternate problems that can save you time, money, a headache or two.

Rookie Mistakes… Here’s What Not to Do

So your “how to test a capacitor on an air conditioner” work doesn’t turn into a nightmare, don’t make these rookie mistakes:

• Not KILL(ing) ALL POWER: We told you above, we’ll tell you again. Shut down the breaker AND pull the disconnect. Seriously.
• Skipping the Capacitor DISCHARGE step: That’s a shocker. Discharge every single time.
• Failure to wear good protective gear: Gloves and glasses aren’t just for show.
• An incorrect or too low in a replacement Capacitor another question: You are begging for more troubles: […] which might fry the new part (or worse other more expensive parts).
• Miswiring the new capacitor: That photo you took is a nice memory prompt. That’s your map. Switch the ‘FAN’ and ‘HERM’ lines and you are in a bonafide world of trouble for your unit.

FAQs

Is it possible for an AC to work without a capacitor?

Nope, not really. Even if the indoor fan will still blow, your air conditioning system won’t work right without a capacitor to give the system a boost so that the outdoor fan and compressor to start and stay running cool. If your AC has a faulty capacitor, it will have trouble starting, (i.e. you may hear a humming sound) or the unit may not start at all.

What is the average life of an AC capacitor?

AC capacitors tend to be pretty strong. Many are meant to last as long as the air conditioning unit, which is typically about 20 years. But sometimes those flash drives kick the bucket early for any number of reasons — overheating, a power surge, a manufacturer defect, a firmware problem, or a short circuit. Keeping up with regular AC maintenance can in fact help you extend the life of them by avoiding such problems.

Where can I get a new AC capacitor if mine is damaged?

You’ve got a couple of options here. Replacement AC capacitors can often be obtained from larger home centers. HVAC contractors will also carry them, and you can often order them directly from the manufacturer. When you’re shopping, you’ll want to use your air conditioning unit’s model number so that you can be sure you’re getting the exact right part for your system. Prices can fluctuate, so it’s worth looking around.

What type of multimeter is used to check a capacitor?

You’ll also need a multimeter with a function that’s designed for capacitance. You can usually find this feature marked with something like “µF” (short for microfarad) or “MFD” on the face. This function comes standard on most multimeters in 2019, but it doesn’t hurt to verify in the product description or the meter’s user manual. The good news is that you don’t have to spend a fortune — you can pick up a durable model with this test in mind for as little as £10.

Learning how do you test a capacitor on an air conditioner will be a great help when it finally breaks down. Just keep in mind: safety isn’t something you can take for granted, and when in doubt, you’re better off contacting the pros — a professional HVAC technician is always the safer bet.