Air Conditioning, Electrical Motor, and Electrical Service Questions

A lot of back-story, but I’m not sure which bits are important.

I noticed today that my air-conditioning wasn’t working. The thermostat said the AC was running, and the furnace blower was blowing air, but it wasn’t cold. After futzing around a bit, I checked the outside heat-exchanger, and its fan wasn’t running. There was a humming sound coming from it, so it had power.

I unscrewed the cover, which the fan is attached to. I pulled the cutoff out from the electrical mini-box out by the heat exchanger. I don’t know the term, but it has copper blades, and cuts power to the AC fan, at least. With the cutoff removed, the humming mostly went away, but there was still a little humming, so there was at least some power.

I lifted the cover, and the fan blade rotates freely. The casing for the motor was hot. Inside the heat-exchanger unit, there were some wires, and a big capacitor (about 2 inches diameter by 4 inches tall). I wiggled the wires a bit, carefully, and the remaining humming would vary a little bit.

I set the cover back in place, and re-inserted the cutoff. The fan rotated a little, maybe 20 degrees, then stopped. The loud humming resumed. I tried removing and re-inserting the cutoff, with the same behavior again.

Any ideas? I’m guessing the motor has gone, and it’s just time for a service call, but I’m not a motor expert, so is that probably the case? The compressor would be the main load, though, and the cutoff would presumably cut power to that. If the fan power wasn’t being cut, why did it rotate when the cutoff was replaced? If the fan power was being cut, what still had power with the cutoff removed?

Is there anything I can try, even if it would only work a day or two? Otherwise we’ll live with no AC tomorrow, and call for service on Monday, to avoid the weekend rates.

I have an electrical service question also. I flipped the AC breaker to off, and noticed my stove went off. It turns out that the Stove and AC breakers are mislabeled. The AC breaker (labeled “range”) is 50 amps, and the range breaker (labeled AC) is 40 amps. Do those numbers sound right, or could the breakers be swapped and it should be the other way around? If it helps to size the AC amps, my house is about 2600 square feet, and I live in SE Michigan. House was built about 15 years ago.

I would normally assume the breakers were simply mislabeled, but I found that they had wired two separate circuits together, so that both breakers had to be set to off to kill the power to the two circuits. (And I’m the first owner. I bought from the builder.)


As for the fan motor, if it’s a capacitor-start motor then the starting cap could be bad.

It makes sense to me that the range breaker would be the 50A and the AC the 40A. They should both be double pole breakers, that is with two toggles that are mechanically connected so that if one tries to trip both will operate.

According tothis calculator, a 50A breaker with a 50 foot run requires a #8 wire while a 40A breaker could use a #10, but #8 could be used. Can you see what size wire is hooked to each breaker? It will be marked on the wire.


Does that fit with the fan rotating 20 degrees then stopping? I’d have thought that with a bad starting capacitor, the fan wouldn’t move at all, but that if you got it to spin a little, it would start up. That’s the opposite of the behavior I’m seeing.

if both breakers have to be off to kill all power to each device then things are seriously miswired. if someone was careless/stupid/whatever to do that then it might be well worth it to have an electrician inspect inside your breaker box to check on the quality of things.

Both breakers are double. I’ll check if I can see the wire gauge.

The AC wire is #6, I believe: 3 CDRS AWG 6 COMPACT AL–TRIPLE E ALLOY […] and a lot more writing, but I’m hoping the AWG 6 part is the relevant bit. Also that COMPACT AL–TRIPLE E doesn’t mean Aluminum (It’s about a 1995 house, so I assume not). It’s about a 70 foot run. For that length run, both 40 and 50 amps need #6 according to the calculator.

I can’t easily get to the oven wiring.

I have had the breaker box moved by a competent electrician, when we had part of our basement finished. I’d assume they’d check the connections after the move as SOP.

If you want to check the cap. Turn your t-stat so it calls for AC, go out to the compressor and when it’s humming, take a pencil (or a screwdriver or a stick or something) and reach in and turn the fan blades. If they start right up, it’s the cap. You can test them with a multimeter, but this is a quick and dirty way to do it. If you have a friend with a Grainger account or you can find a HVAC supply house that’s open to the public you should be able to replace it on your own fairly easily (the cap or the motor).
If the motor was bad, it likely wouldn’t spin freely as a humming motor that’s bad tends to be due to binding somewhere (usually bad bearings).

ETA, you may want to read up on discharging the cap before you remove it. I believe you can short it with a screwdriver. I’ve never done that, but all the caps I’ve replaced were done before I learned that should do that. I suppose I was lucky.

Something else, if you pulled the disconnect and still heard the motor humming that’s really strange. First off, are you sure it was still humming? Are you completely sure it wasn’t vibration from the furnace fan traveling up through the lineset (copper) or a neighbor’s compressor running?

If it were me, I’d open the disconnect and make sure it was wired properly and that neither of the wires went around the copper blades you pulled out. Like someone else said, if it was still getting half of the 220, something’s really wrong. If you have a proper disconnect (and it sounds like you do), you’d have to go out of your way to wire it so that disconnecting it keeps half the power going to it.

I’ll give this a try tomorrow.

It wasn’t the fan still humming. I’m not sure what it was. Vibration through the line is possible. There’s a little box inside the exchanger unit with some electrical connections that the sound seemed to come from. That may have just been mechanically vibrating.

I can check this tomorrow too.

With the disconnect, well, disconnected, there should be NO power to the compressor and therefore no noise. I could understand a little residual hissing as the freon does it’s thing, but that’s it.
Ya know what, that was probably the relay, it’s low voltage and get’s it’s power from the furnace. It might have been chattering a bit. The wires, if you trace them, will follow the copper pipes back to the furnace. Don’t worry about that (at least not right now). They probably shouldn’t have been all that audible, but people aren’t used to hearing their AC without the fan running. It sounds very different. It could be any number of things ranging from ‘totally normal’ to dirty contacts. But you should probably focus on one problem at a time.
BUT, if some time down the line, you have a problem with your compressor not coming on, I’d start there. For now, (if it was me), the first thing I’d do is try hand starting the fan motor to see if the cap is the problem.

Yeah, there is a small wire that goes along with the coolant pipes from the furnace to the unit.

I’ll try it, but I’m still wondering how this fits with the fan rotating a little bit, like 10 degrees*, when I first plug the disconnect back in. Wouldn’t it keep going then?

  • I said 20 above, but it might not have been even that much. (left my protractor inside the house… ;))

Be very careful here. You are discribing things as being wrong. If you have a multi meter and know how to use it you need to test out the wiring. If you have not used a multi meter you may be playing with your life. 120 VAC can kill you.

Had the same problem last year - it was the capacitor. Friend had the same problem - it was the capacitor again. I am quite sure - its your capacitor :).

Since they banned PCBs - capacitors nowadays have shorter life spans. Sure - it is more environment friendly :slight_smile: but it is more expensive

I know, I read that. I’m not totally sure what to make of that. The whole motor may very well be shot, but I would start here. Also, when you do this, you’ll be able to tell if it moves freely (as opposed to binding or feeling it ‘catching’ the magnetic fields) with power applied to it. If it doesn’t, the motor is bad.

I wonder if this is the behavior of a good cap and and bad (but not frozen) motor? Most of the time I see (capped) motors dying, they bind up in the process (just coincidence I’m sure). So maybe the cap is giving it the initial push but then the motor isn’t going anywhere with it.

I was thinking about this this morning, and by replacing the capacitor, do you mean that big capacitor? Is that really for the fan motor, or is it for the compressor motor?

I looked at it again this morning, and the “box with wires” is that relay Joey P mentioned. I didn’t see the coil last night from the angle I had. Also, I checked the big cap with a voltmeter, and it’s discharged. It’ll be another hour or so before I get to testing the fan.

I’ve also got the parts list, spec sheet, and wiring diagram.

OK, looks like the cap. I gave the fan a spin while it was humming, and it ran. Right after I spun it, it initially seemed to slow, then gained speed and seems to run correctly. Lots of airflow out the top of the heat exchanger.

But the AC still doesn’t work, in that it doesn’t cool. The furnace blower is running, but the air coming out the registers isn’t cold. The air blowing out the top of the heat exchanger is not hot, even after running for five minutes.

Does the capacitor start both motors? It’s a dual capacitor with three terminals. The parts list shows it as CAPACITOR, DUAL, 45/7 MFD 440 V. (That’s one of several listed, I need to track down which exact model I have. The bracket holding the cap in place covers most of its labeling.) Is the 7 MFD for starting the fan, and the 45 MFD for starting the compressor?

A couple of things you need to be aware of.

If the compressor hasn’t been off for more than 10 minutes, it may not be operating correctly. That is, if you try to start it, then turn it off, then try to start it again after a minute or two, it may run, but it may not be compressing, if that makes sense. The valves have to have equal pressure on each side on start-up (or something like that). I’d leave it off for 20 minutes or so, then try to start it using the spin the fan method.

Also, if the cap for the motor is out, you can start it by spinning the fan, but it may run hot. Eventually, the thermal cut-off may shut the fan motor down.

It is possible that the system dumped the freon when the fan wasn’t working. There may be a relief valve to release the pressure if the high side gets too high. I have not seen it, and I have had the fan go out on me three times (one at a house I was renting, and twice at the house I am currently living). In Texas, in the summer. I never had the freon dump on me, so I don’t know if it happens on residential AC units (although I have seen it on automotive AC units).

You should be able to replace the capacitor yourself. The replacement should look enough like the original that you should be able to connect the same wires to the same terminals. Take some pictures before you disconnect anything so you can be sure you have it wired correctly. Note that some capacitors have a metal case that must be connected to a ground (the frame of the housing) to work properly. Look at the wire terminals for signs of overheating (burnt insulation, soft wires, discolored connections, etc…). If everything is OK, and the capacitor is what is bad, it should work. If the capacitor does not fix it, I think it would be time for a service call.

excavating (for a mind)

Some blowers have a speed regulator (at least commercial units do). If you started the AC and the motor wasn’t running (at first) the compressor will cut out due to the high pressure (which is normal in this scenario). With the compressor not running and the pressure coming down, the fan motor may not run at full speed right away, but once things start cooling off and the compressor comes back online it should go back up to full speed. I don’t know if residential units have a speed/pressure regulator, but if they do, this may be happening.

I would just turn everything on, kick start the motor and let it all run for a while and see if the compressor kicks on after a little while.

Looking online it seems a dual cap could be starting both motors (does one wire run down to the compressor?), but I don’t know much more then that. I don’t know if they typically both go bad at the same time or if that would be an odd coincidence. I suppose since they at least share a hot wire, it’s possible. And there’s nothing you can do to start the compressor on your own. Given that, if the compressor doesn’t kick on on it’s own after 10 or 15 minutes of the fan running, I’d shut it all down and leave it off until you can replace the cap. You wouldn’t want to burn out the compressor.