I have a couple of questions about testing and replacing the capacitor on my home heat pump external condenser unit.
1. How can I reliably test the capacitor (what tools do I need?)
2. What safety precautions do I need to take when handling the capacitor (aside from the obvious task of shutting off power to the interior blower and external units). This is a capacitor and it stores electrical current. I'm assuming I need to wait a period of time after I've killed power to the unit before handling it.

  • The techs have a capacitance mode on their multimeters. They remove all the connections and test the capacitor. They know what the values are supposed to be. If you remove wire connections, be sure you label them or take a picture so you can reconnect them properly. Commented Jul 19, 2019 at 14:49
  • Always short caps prior to messing with them never trust the discharge resistors if they have them those resistors fail and getting a hard jolt from a cap can hurt.
    – Ed Beal
    Commented Jul 19, 2019 at 16:08
  • short them out, wait 30 seconds, short out again; a lot of caps can actually (partially) self-recharge without adding charge.
    – dandavis
    Commented Jul 20, 2019 at 19:57

1 Answer 1


Changing a cap is easy you have the basic idea. First pull the fuses or disconnect the power to the unit the disconnect should be close to the outside unit. Next if your cap has 3 connections use a screwdriver to short the common or c post to the fan then common to herm , if 2 post just short them together it only takes a second to fully discharge a capacitor. Make sure to note the wire colors connecting to the posts. To check the cap if your meter doesn’t have a capacitance meter built in use the ohm meter (no wires connected to the cap put in ohms and put the meter on the cap you are looking for a slowly raising value if it shows totally shorted the cap is bad if there is no charging or slowly increasing resistance try swapping the leads and now it should be discharging or the valve dropping, this is a crude test but it will point out a bad cap in most cases. Other things to look at, of the cap is bulging at the top or bottom or both even if it reads good it’s time to change it I just had a 5/55 fail to start a unit this week measured the cap it showed good but then I noticed the top was really bulged, I replaced the cap 15$ if you can wait and get it on line or 2x from a local parts house. Make sure the voltage is the same or higher on the new cap, you want the same capacitor values but if you can not find the correct one one with a value within 10% will usually work fine. The 5/55 on the cap I just changed is 5 micro fared and 55 micro fared and the voltage was listed as 400v that is a common size so the parts house had it. I installed the new cap making sure the strap was tight and not over the size info (if the name and size can get worn off I write them on the case or inside the unit so the next time it is available). Depending in the age of the unit I might change out the contactor a bad starting cap can cause damage to the contactor, so if it’s an old unit or a really cheap contactor it’s only another 20$ to do this now and will prevent call backs. Once the covers are back in place it’s ready to see if this did the trick.

I am mentioning this next part because many times I think a low charge has caused harder starts wiping the cap out. I have found more than 50% of the time when changing a cap the super heat and sub cooling values are off (loss of some of the charge) I will add as needed to bring the system back up to the proper temperature/pressures and double check for leaks but almost never find them. The majority of the time it takes 2-6 lbs in residential systems from my experience if the system takes more than this I can usually find a leak or the system is very old and has not been serviced since install.

So make sure with the power off to short the terminals of the cap’s, if any bulging they need to be replaced, it is much easier to toss a bulged cap than clean up several hundred feet of foil/plastic & oil if the cap blows apart.

Hope that helps and good luck

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