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When I turn the air conditioning on in my house, air blows out of the vents but the air is not cool. The fan inside the outdoor A/C unit spins just fine as well but it seems that the compressor won't start. A few seconds after the air starts blowing out of the vents inside my house, I hear a short buzz/humming sound that lasts about half a second that I assume is the compressor trying to start. This buzzing sound happens about once every 15 seconds, so it seems like the compressor is still trying to start, but never does.

I have replaced the run capacitor in my unit (the only one I see) and it has not fixed the problem. Not sure if it matters but it seemed like before I replaced the capacitor, the compressor would only try to start once every minute or so, compared to once every 15 seconds after the new capacitor was installed.

The A/C circuit is never tripped in the electrical panel so it does not look like a short anywhere. I have also tested the resistance between all three terminals on the condenser itself and everything seems to be normal with readings between 0.5-2 Ohm depending on the pair of terminals being tested.

The unit was serviced last year and we were told we likely had a leak in the refrigerant system. We opted to have the refrigerant recharged and to try a sealant hoping to buy us a year or two longer. The unit is ~13 years old.

Make and Model: Armstrong Air Model no. SCU10B48A-7

Any ideas on how to get the compressor to run or what the problem might be?

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  • Welcome to Home Improvement! Providing the make/model number for your unit may help. Someone may have trouble shooting document for this particular unit, or at least know where to find them. Once you've edited your post to include that, take the tour and read through the help center so you can learn how to get the most out of the site. Also, it'll help kill time waiting for someone to answer. ;)
    – FreeMan
    May 26, 2020 at 17:57
  • can you spin the blades manually, like with a chopstick or something? sometimes you can start it up like that and it will work after, perhaps a bit of gunk or rust that get smoothed over by the rotation. if it's hard-stuck, then the motor is likely kaput.
    – dandavis
    May 26, 2020 at 20:01

2 Answers 2

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I installed a hard start capacitor alongside the run capacitor and that seemed to give the compressor the boost it needed to get running again. The hard start capacitor I used was a Supco SPP6.

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Did you check to make sure you don't have separate fuses that are just for the compressor? Many systems are set up that way, using what's called a "fused pull-out disconnect" mounted outside near the compressor. enter image description here

Because this fuse box is downstream of the breaker feeding the entire AC unit, the blower and controls will still work, then the sound you are hearing is the relay attempting to start the compressor, which doesn't happen because the fuses are blown. The controls then detect that the compressor didn't start, so it keeps trying.

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  • The condenser fan probably wouldn't work if a fuse was blown.+
    – JACK
    May 26, 2020 at 18:15
  • On mine, the fan is 120V, fed off of one of the fuses. If that fuse blows, the fan stops but if it's the other fuse, the fan still runs. One blown fuse means the compressor doesn't run.
    – JRaef
    May 26, 2020 at 18:18
  • Funny, on mine the fan is 240V...... still upvoted you...
    – JACK
    May 26, 2020 at 18:29
  • No fuses in the shutoff box that I can see.
    – Nick
    May 26, 2020 at 18:38
  • You cannot see the fuses unless you pull out the block, the part facing you might have a little wide bale handle.
    – JRaef
    May 26, 2020 at 19:09

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