Replacement dual capacitor ready to install.

Air Conditioning Not Blowing Cold? Replace your Capacitor or Contactor 30

Summer 2015 in Seattle has been officially declared the hottest summer here on record (check my weather station’s 2015 max temps on my Weather Underground weather history graph for geeky details). For those in the Pacific Northwest living without central A/C, it’s been miserable. And for those fortunate enough to have central air conditioning, running the A/C all day has been expensive. But things can get even more expensive if your A/C unit stops working and you call an HVAC repair man. Hopefully, this article can help you avoid that.

When your air conditioning stops working, what you’ll usually notice is that your HVAC fan runs normally, but the air coming out of your vents isn’t cold. If that’s happening to you, the #1 most likely cause is a failed capacitor in your outdoor A/C unit or heat pump. The #2 most likely cause is a failed contactor. I recently replaced both the capacitor and the contactor in one of our A/C units when it stopped blowing cold air, so I’ve documented the process to share with you here.

Disclaimer: Although these two fixes are simple enough fix for a beginning DIYer, you will be dealing with high voltage, and therefore you must take the proper precautions as explained in this article. Make sure power to the appropriate equipment is turned off, and that electricity in your capacitor is safely discharged (as explained below) before proceeding. There is a risk for serious electrical shock (enough to kill you) if you’re not careful. So be careful, and proceed at your own risk.

Recommended Tools

Before tackling any HVAC projects, I recommend having the following three tools in your arsenal:

  1. A set of rubber-handled insulated screwdrivers. They are perfect for any electrical DIY work, but especially for high-voltage stuff like HVAC. You can pick up an inexpensive 7-piece set for under $20 like these Titans, but if you can stretch your budget closer to $35, this 6-piece set from Wiha is what the pros use (they’re rated to 1,000 volts).
  2. A quality multi-meter (like the Fluke 117). Even though the HVAC-specific Fluke 116 is what most professional HVAC technicians use, the Fluke 117 will do most of the same functions as the 116 while also being slightly more useful for all your other electrical projects.
  3. A Leatherman Wave Multi-tool. I never go anywhere without my trusty Leatherman Wave, and I strongly recommend picking up the Bit Driver Extender because the 1/4″ socket in the extender is perfect for inserting and removing those 1/4″ sheet metal screws used in most HVAC applications.

What are the A/C Contactor and Capacitor?

Your air conditioner’s contactor and capacitor work together to help power your air conditioner’s fan and compressor whenever your thermostat calls for cool (the capacitor and contactor in your heat pump work the same way).

The contactor is the “switch” that receives a low voltage (usually 24V) signal from your furnace to turn on. It’s a standard magnetic relay, meaning low voltage flows through a coil inside the contactor to creates a magnetic field, which then pulls down a piece of metal that connects both sides of the high-voltage circuit usually (120V or 240V). Closing the circuit allows high-voltage to power the fan and compressor motors. A contactor with one magnetic coil that connects one circuit is called a “single-pole,” if it has two magnetic coils that connect two circuits it’s called a “dual-pole,” and so on.

The capacitor is like a “battery” that stores some of that high voltage goodness to create what’s called a “phase shifted current,” which is what causes your fan and compressor motors to spin (thanks one of my readers, Jerry, for commenting and helping me understand that better).

Your A/C capacitor will be either a dual capacitor or a single capacitor. A dual capacitor has three sets of terminals on top: one for the compressor, one for the fan, and a shared “common” terminal for both to use. A single capacitor only has two terminals, and if your unit uses single capacitors, you probably have two of them in your system (one for the compressor and one for the fan).

Accessing Your A/C Contactor and Capacitor

Your contactor and capacitor will be protected by some sort of panel on your air conditioning unit or heat pump. The Rheem outdoor air conditioning unit at our Seattle house has a panel that removes with four screws from the top and side:

Removing panel screws from a Rheem air conditioner

Removing panel screws from a Rheem air conditioner

The heat pump at our cabin has a curved panel on one of the unit’s corners. If you’re having trouble finding it, look for where the wires and plumbing enter the unit. That’s always where the panel will be.

Be careful when removing these panels, as some of the wires are “hot” and the capacitor may contain some charge… even if the unit is not running.

Here’s what things looked like under my A/C unit’s access panel:

Contactor and capacitor on a Rheem air conditioner.

Contactor and capacitor on a Rheem air conditioner.

The contactor is the rectangle switch-looking thingy on the left, and the round gooey mess on the right is the top of my capacitor. That gooey mess is a sure sign that the capacitor is leaking and needs to be replaced.

Diagnosing a Bad A/C Contactor or Capacitor

The only 100% accurate way to test for a bad capacitor is to use a decent multi-meter like the Fluke 117 (which is the one I recommend). But a quick visual inspection might be enough to indicate something’s wrong. If your capacitor is gooey (like in the above photo), it’s way to failure… if it hasn’t failed already. The other dead giveaway is a bulging top. In this image, notice the bulging top on the capacitor on the left. The one on the right looks normal:

A bulging capacitor (like the one on the left) is a sign it's gone bad.

A bulging capacitor (like the one on the left) is a sign it’s gone bad.

If your capacitor is bulged or leaking, that was probably your problem, and replace it will fix your air conditioning.

To test your contactor, a multi-meter is also helpful to confirm that you have high voltage running to the contactor (and that your problem isn’t caused by something as simple as a blown fuse or breaker) and continuity on both sides of the circuit when the contactor is “closed.”  But if you’re confident that your contactor is receiving power, you can do this quick-and-dirty test with a rubber-insulated screwdriver (the rubber is important so you don’t shock yourself). With the power to your air conditioner turned on and your thermostat not calling for cooling, hold the rubber-insulated end of the screwdriver and use the other end to hold in the “button” (circled in red in the image below) on the contactor for 5-10 seconds to manually close the circuit. The fan and compressor on your air conditioner or heat pump should run normally until you release the button.

Press this button to close the circuit on your contactor

Press this button to close the circuit on your contactor

If pressing the contactor button turns everything on, the capacitor is probably OK, but keep reading to test the contactor’s coil.

If you press the button and hear a hum, and the fan and/or the compressor doesn’t turn on, you’ve probably got a good contactor and a bad capacitor. The hum you hear is the fan and/or compressor motor getting some power… but without that “extra oomph” from the capacitor, it’s not enough to get it moving.

If absolutely nothing happens (and you’re certain you’ve got power to the contactor), you probably have a bad contactor.

If everything works manually, you still need to verify that the magnetic coil is automatically closing the circuit on the contactor when the thermostat calls for cool. Stay outside with your air conditioning unit and have a helper inside the house turn the thermostat down so the system calls for cooling. You stay outside and watch the contactor (you can do this from your phone by yourself if you have a smart thermostat like an ecobee). If the contactor’s button doesn’t get pulled down by the magnet to close the circuit when the system calls for cooling, the “coil” portion of the contactor is likely your problem and the contactor needs to be replaced.

In my case, neither the contactor nor the capacitor had failed completely, but the capacitor was leaking (as seen from the first photo) and the contactor was making a loud buzzing sound when the system was running — both of which are signs that failure is imminent. Because the parts are cheap and easy to replace, and because a healthy contactor and capacitor can extend the life of expensive compressor and fan motors, I decided to replace both the capacitor and the contactor at the same time, and it might not be a bad idea for you to do the same.

Buying the Correct Replacement A/C Contactor or Capacitor

The best way to find the right replacement for your contactor or capacitor is to go to Amazon, type in the model number of your old one, and purchase the same kind. Both items are destined to fail eventually, so if you need to replace them now, I recommend buying two so you have a spare on hand when this project is done. Or, if you’re reading this but haven’t had either of them fail on you yet (trust me, it will happen eventually… and probably when it’s least convenient), you should go through these steps to find the correct replacements and order spares now so it’s a quick and easy process to repair your A/C or heat pump when the time comes. The parts are cheap, and you’ll be the family hero when you can fix it before the house gets above 76F. 🙂

If you need to disconnect or remove anything to get a better look at the part numbers on your capacitor or contactor, do the following three things first:

  1. Make sure power to the air conditioner or heat pump is shut off. This is different than the power to your furnace. There should be a large shut-off lever or a fuse panel outside and near the unit.
  2. Don’t touch the terminals of your capacitor until it’s been discharged. Use a rubber-handled screw driver or rubber-handled need-nose pliers (the rubber insulates you from electric shock) to “short” the terminals and dissipate any residual charge in the capacitor. On a single capacitor, just use the tip of the tool to touch both sets of terminals with the metal part of the tool the same time. On a dual capacitor, dissipate both sides by using the tool touching the C and the FAN together and then the C and the HERM together. It’s worth noting that this is not exactly the “recommended” way of dissipating a capacitor (as one of my buddies mentions in the comments below). The “recommended” way to drain a capacitor is to use a 1000 Ohm resistor that’s rated for the voltage and hold it between the terminals for 5 seconds. With that said, I’ve seen lots of HVAC professionals use the screwdriver trick. As always, proceed with caution (and eye protection probably isn’t a bad idea).
  3. Take close-up photos of the connections with your mobile phone (get a few different angles) so you know how everything goes back together.

When shopping for a replacement capacitor, you’ll need to make sure:

  1. You’re buying the right type of capacitor.
  2. The capacitor is able to deliver enough voltage to help power the compressor and fan motors.
  3. The capacitor has enough capacitance to store that voltage until it’s needed.

Concerning capacitor type, there are two general categories: “start capacitors” (used only when the motor starts) and “run capacitors” (used the entire time the motor is running). Most A/C capacitors are run capacitors, but there are some units that will require a start capacitors instead, so make sure you get the right kind for your unit (again, thanks to my reader Jerry for his education in the comments about capacitors).

As for voltage, most air conditioners and heat pumps have compressor and/or fan motors that require either 370VAC or 440VAC to start. If your unit requires 440VAC, you cannot use a 370VAC capacitor. But if your unit only requires 370VAC, it’s totally fine (and not a bad idea) to use a larger 440VAC capacitor. With voltage, you can always go bigger than required, but you can never go smaller.

For the capacitance, make sure the replacement capacitor has the same micro-farad rating (shown as MFD or μF) as your old one. For dual capacitors, the capacitance is shown as “45/5” or “45+5” MFD, which means the compressor side is rated at 45 μF and the fan side is rated at 5 μF. For single capacitors, you’ll just have one value. Unlike with voltage ratings (where it’s OK to go bigger), you should not get a capacitor that’s rated higher in capacitance than the original. Keep the same capacitance in your replacement.

If your capacitor’s exact replacement is too expensive or hard to find, you can replace it with any unit that has the proper ratings. For example, my Rheem RAKA-037JAZ air conditioner came with a dual capacitor marked “Aerovox TT 10000 AFC SH1238.” I couldn’t find an exact replacement anywhere except direct from Rheem, and they wanted way too much money. But with some searching I found out that particular Aerovox is a 370VAC 45+5 MFD capacitor, so I purchased this TRCFD455 unit on Amazon for less than $11 (including shipping), which is rated up to 440VAC and 45+5 MFD. Again, it’s fine to go bigger on the voltage, but you can’t go smaller.

My existing contactor was a 24V Honeywell R8242A-1032, which isn’t available any more. The replacement is listed a Honeywell DP1025A 5006, which I found for $20 shipped on Amazon (note: the wiring for the new one was a bit different than my old one — read more on that below if you’re using this same contactor).

If you’re coming up empty, try contacting a local HVAC supply house. They might be able to tell you what you need based on your equipment’s model number. And some HVAC repair companies will sell parts direct to consumers if your house is too hot, you’re in a rush, and don’t want to wait a day or two for Amazon shipping. Remember when I said you should order spares before they break? Seriously. It’s cheap insurance!

Installing the Replacement A/C Contactor or Capacitor

Make sure you’ve followed the three steps mentioned above (power to the A/C unit or heat pump off, capacitor discharged, and photos taken) before you proceed.

My A/C unit had a large shut-off switch like this:

Shut off the power to your A/C unit or heat pump before you proceed, and don't forget to discharge the capacitor.

Shut off the power to your A/C unit or heat pump before you proceed, and don’t forget to discharge the capacitor.

Remove the access panel (if you haven’t already) and locate your contactor and capacitor. Here’s a reminder of what my old ones looked like:

Contactor and capacitor on a Rheem air conditioner.

Contactor and capacitor on a Rheem air conditioner.

Triple-check to make sure there’s no power to the unit (I did mention that 240V is nothing to scoff at, right?), then disconnect all the spade connectors from the part(s) you’re replacing. If they’ve been on there for a while, using your Leatherman’s pliers will make things easier. Remove any screws holding the part(s) in place. Check out how rusted my old contactor was on the bottom:

This rusty old contactor was on its way out.

This rusty old contactor was on its way out.

Remove the old parts (in my case, I replaced both the capacitor and the contactor). If you’re installing a capacitor, double-check it to make sure it has the correct rating. If it’s too low, it won’t start the compressor and fan motors:

Replacement dual capacitor ready to install.

Replacement dual capacitor ready to install.

Drop the new part(s) into place and secure them (you can wait to the end to secure them if you prefer). The replacement capacitor should drop into the old hole, but it’s OK if you need to figure out a different way to mount it:

Securing the new capacitor.

Securing the new capacitor.

Once the new capacitor and/or contactor are installed, re-connect all the wires using your pre-disassembly photos as a guide. If the markings aren’t clear, the C (common) terminal on your dual capacitor will have the most spade connectors (mine had four), the HERM (compressor) terminal will probably have one fewer than the C (mine had three), and the FAN will probably have only one. Here’s how my new contactor and capacitor looked when re-connected:

Reconnect your capacitor and/or contactor just like the old ones.

Reconnect your capacitor and/or contactor just like the old ones.

If you’re replacing an old Honeywell R8242A-1032 contactor with the newer Honeywell DP1025A 5006 contactor, read this next part carefully. The terminals on the older R8242A-1032 are marked T1, T3, L1, L3, C, and C. On the newer DP1025A 5006, the terminals are marked T1, T2, L1, L2, A1, A2. Logic might suggest that you simply hook up the T1 and L1 connections just like the old one, then connect the wires from the old T3 and L3 terminals to the new T2 and L2 terminals, right?


The internal wiring on the two relays is different, so when replacing a Honeywell R8242A-1032 with a Honeywell DP1025A 5006, here’s how the old contactor’s terminals map to the newer contactor:

  • 240V to fan: T1 (old) -> T2 (new)
  • 240V to compressor + capacitor: T3 (old) -> T1 (new)
  • 240V from panel: L1 (old) -> L1 (new)
  • 240V from panel: L3 (old) -> L2 (new)
  • 24V from Thermostat: C (old) -> A1 (new)
  • 24V from Thermostat: C (old) -> A2 (new)

It really doesn’t matter which old C wire goes to which new A1 or A2, and it actually doesn’t even matter if you swap the L1 and L2 incoming 240V power wires on the newer relay. But the old T1 wires must connect to the new T2, and the old T3 wires must connect to the new T1, or things won’t work.

When everything’s re-connected, turn your A/C unit’s main power back on:

Main power to the A/C unit turned on.

Main power to the A/C unit turned on.

Then tell your thermostat to call for cooling again.

If you did everything correctly (and as long as you were careful, there’s no reason you believe you didn’t), your A/C unit or heat pump compressor and fan should both be running (instead of just humming) and chilled air will once again be flowing from your vents. Congratulations on a great DIY fix!

I won’t blame you if you walk around the house demanding high-fives from the spouse and kids.

Go ahead. You’ve earned it.

As always, I welcome your questions, comments, and feedback below!

  • Jerry

    Great guide! The explanation of what the capacitor does though, is a bit incomplete. It does store energy, and it is for motor starting, but it’s not due to the electrical surge required to start the motor. It stores electricity cycle to cycle, to create a phase shifted current. This phase shifted current is fed to one of the motor windings, and it either leads or lags the main winding. This phase offset is what causes rotation inside the motor. Without a phase offset, a single phase motor would just sit there and hum and would never start. It would most likely just hum until it overheated and burned up, or tripped a motor protection fuse, thermal overload, or breaker. Three phase motors have 3 perfectly offset phases supplied by the power company. They do not need capacitors to create phase rotation. Because of this, 3 phase motors also create about 250% more torque during start than run! But few residences have 3 phase power, and even fewer have 3 phase air conditioners; most are single phase. Single phase motors are much different than three phase motors. They have the least amount of torque at start (when it’s needed the most) because they have to create two phase electricity, by shifting a phase, using a second motor winding, and for larger motors, a second motor winding combined with a capacitor. This is why a capacitor is so important. It provide the torque to rotate that motor. It’s also worth noting that there are two major different categories of capacitors. Run and start. A start capacitor is only used during start. They are only used for a fraction of a second to start the motor. As the motor starts, a switch usually opens and takes them out of the circuit. Start capacitors are only designed for very intermittent use, and are usually physically smaller and less expensive than a run capacitor of the same voltage and capacitance. A run capacitor is usually oil filled, and built to dissipate more heat because it is phase shifting current during the entire time the motor is running. It is designed for 100% duty cycle, and will not overheat in use. You can use a run capacitor to replace a failed start capacitor (as long as voltage and capacitance are the same) but you can’t use a start capacitor to replace a run capacitor. So make sure you’ve got the right type of capacitor! It’s also important to note that you can get a capacitor that’s rated larger in voltage, but you should not get one with more capacitance. From experience I can tell you that it might work initially but it also might cause some pretty big problems down the road.

    • Hi, Jerry. Thank you for the EXCELLENT education on capacitors. I’ve updated the post to include an abbreviated version of your thoughts, and am stoked to have learned so much from your comment. Thanks again!

  • Excellent point, Steve! Though I have to admit don’t keep resistors handy. Maybe I should!

  • John Pepper

    Steve, great guide and I loved the detail that you’ve went into. I had a question for you. A few days ago, my fan stopped running (compressor still kicked on though). I replaced my capacitor (the contactor seems fine and functions as advertised), but the fan still won’t kick on. I did a single check to make sure the motor worked by plugging it onto the herm side, since I knew that side was working. Sure enough, the fan started spinning. Put it back on the fan terminal, and nothing again. Any idea as to what is going on?


    • Weird. Do you have a multi-meter to see what’s happening when the fan SHOULD be kicking on?

      • tai

        Steve, I have the same problem, you mention above, my contactor burned out, I changed the new contactor, call the heat and the fan isn’t running, I pushed the contactor button and the fan is running fine. I didn’t check my capacitor yet, but you mentioned above if I push the contactor button and the fan running more likely the capacitor is ok right? I’m still not sure why the system still not working. i swapped the circuit board and the new contactor to another outdoor compressor and it works fine. do you or anyone on this forum have any idea/ Thanks

        • Hi, Tai. If pushing the contactor manually works, then the cap is fine. If you call for heat and the contactor doesn’t contact, then either the contactor is bad, or it’s wired improperly.

  • I couldn’t agree more. I always keep filters on hand, and have calendar reminders to check them often. 🙂

  • Strange. If the contactor isn’t “closing” when you call for heat, then it’s a wiring issue. Closing the low voltage on the contactor (triggered by the thermostat) is what causes it to close and pass high voltage to the compressor and fan. I’d check to see if you have continuity on the low voltage side when the T-stat calls for heat (take all proper precautions when you’re working around high voltage).

  • Hi, Cesar. It’s impossible to know the correct capacitor based on the fan motor alone, since the capacitor is responsible for helping to power both the fan motor and the compressor. Also, I would not simply use the same one I did for my unit without knowing for certain that it’s spec’d for yours. If you’re unable to find the correct ratings on your old capacitor, or via searching online, I recommend contacting RUUD directly, giving them your model and serial number, and finding the right capacitor. It’s OK to use one with higher voltage (such as a 440v rather than a 330v), but the microfarad (uf) requirements are precise. My GUESS is that you’ll probably need a 370-volt 35/3 capacitor… but again, that is a GUESS and you should call RUUD directly to find the right capacitor. Or if you still have the old one, you can get the ratings from that, or Google its part number and see what you need. Don’t guess. 🙂

  • Sounds like you might have a grounding issue somewhere.

  • Weird. Are you getting a full (or close to it) 240V on the “hot” side of the contactor?

  • Iric

    Hi Steve,

    I was having similar problems and changed both the contactor and condenser, and the compressor still runs when the contractor is depressed, but not when the thermostat calls for it. My question is, could it possibly have something to do with this transformer (pictured below)? I remember awhile back it was making a loud buzzing noise and now it makes no noise at all. I mistakenly thought it had something to do with the phone line at the time because the wires look similar.

  • My guess is that you blew something in the AC tower. If you’ve got burnt smell, I’d get a professional out there…

  • Yes, sounds like classic signs of needing a freon recharge.

  • Is their voltage “upstream” of the capacitor? It could be a lot of things. Breaker, fan motor, or as the article says, the contactor. Be very safe when testing, but I think a voltage tester is necessary to find the problem.

  • If the low voltage circuit from the thermostat is still “closed” when the contactor releases, and there’s still high voltage juice on the “hot” side of the contactor, then yeah… that’s sounding like it’s time to replace that contactor. The good news is, they’re cheap. Way cheaper than a service call… but NOT cheaper than a hospital visit. Make sure the voltage is OFF and capacitor discharged before tinkering. 🙂

  • Yep. Sounds like classic signs of a bad contactor!

    • vinny

      I just changed out my capacitor and it blows out cool air. My house was at 86 degrees at around 2 pm. Its at 830 now and my wife noticed the fan outside was off. Then after a few mins it turned back on. Its been doing this all day. Not sure what it could be since everything is running smoothly. It seems the fan is in phases. Any thoughts?

  • Mark Goodman

    Hi Steve, great article. About 2 weeks ago I had an issue where the house was cooling and then all of a sudden the fan outside would seize and stop spinning. Sometimes I could kick start it using a stick but ultimately it would seize again. An HVAC “professional” could not figure it out. After he left I swapped out the capacitor myself and everything worked fine for about two weeks. Now I am having the same problem. I realized I used the wrong specs on my new capacitor. Original one was 370v 50+3. New one was 440v 45+5. Could this be the issue? Also, I tested the Contactor using the screwdriver trick and with the thermostat not set to cool everything would run normally when pressed. When the fan was seized up, the button on the Contractor was stuck in the down position. I am hoping using the proper Capacitor will do the trick. Please let me know your thoughts. I appreciate your help.

  • Jerry

    Hello. I just replaced my capacitor…and the a/c feels good on todays NY hot and humid weather. Is it a good idea to have extra capacitors on hand? I mean, if I were to buy another new now and the one I just replaced lasts 5 years, would the extra one still be good after 5 years??

    • Hi, Jerry. Yes. I keep an extra capacitor on hand always, and they “wear out” based on usage, so an unused one on your shelf will be fine for years and years.

  • Conrad A Riel

    Replaced the contactor & capacitor a couple of weeks ago then I noticed that AC not cooling. Outside fan is running but very little cold air coming out from vents. Inside temperature is at 81. it’s set for 75. It was ok before the capacitor & contactor was replaced. What could be wrong?

  • Matt Brown

    Jerry, great guide here, pictures and descriptions.

    My contactor, when depressed at the plunger, will cause the A/C unit to come to life, but then a split second later, a small spark emits at the contactor and it shuts down. Release and depress again and same thing, starts up and immediately a tiny spark and it shuts off. The whole time, the fan is still running silently.

    Any ideas what that spark is? My capacitor is still good right? It’s not bulging.

    • Just because it’s not bulging doesn’t mean it’s good. It still could be the problem. If you have a multi-meter to test it, you can see if it’s good. But I’d probably replace it, as it’s sounding like that’s the problem. Likely the compressor side of the capacitor.

  • Amit Khatri

    Steve, I replaced both Capacitor and Contactor on my AC unit. When the unit calls for cool it still doesn’t start, Manually when I hit connect button on the capacitor it works. I initially did incorrect wiring for Contactor, tuned on unit and manually tired to check if fan rotates and it did, thereafter I fixed the connections. Wondering if that could have damaged the Contactor? Also, wondering how can i know if correct signal is passed to contactor to connect from the Thermostat?

  • Tommy Dinh

    hello steve i buy both and replace on my ac until and i call for cold but the fan not running can you help pls

  • Bennie Leggett

    Very good. Does the capacitors have a negative and positive terminals. like a electralitic .

  • Andra W.

    Hi, Steve. I know this article is from 2 years ago, but I found it while trying to figure out how to find the model of the capacitor I’ll need when my HVAC is so old that none of the model numbers/text is legible. I know it’s a RUUD and it’s original to my house built in 1986. (Yes, people are amazed it’s lasted this long!). I’ve attached a picture to show what I mean.
    Thanks to you, I do at least know that it’s a single capacitor (2 terminals). Any suggestions on how to find out what I need to buy? Thanks!

  • john

    Not getting 24 volts to contactor, what I do wrong?