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Old 03-18-2012, 09:58 PM   #1
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Air Conditioner Hum, advice?

One of the kids was sleeping out in the 'Stream last night and heard a "pop" sound in the middle of the night. Now I have a "hum" coming from the a/c unit.

The camper is parked in the back yard and connected to 30 amp shore power at all times. The A/C was not being used at the time, but did have the circuit breaker on. In fact, it hasn't been used since last summer.

This is the original Armstrong unit that came with the camper. Some work was done on it in the early '90's (new compressor) and looks like some of the electronics under the cover inside were replaced at some point. This humming sound is coming from that area, under the inside shroud in the cabin.

I've shut the circuit breaker off and plan to have a look at it tomorrow (no time today). Maybe crank it up with fire extinguisher in hand?

Any advice is appreciated.

Jim
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Old 03-18-2012, 10:16 PM   #2
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I am no rocket scientist, but the hum could be coming from the 'squirrel wheel' not turning...but wanting to turn so it can deliver air. My armstrong used to do that until I oiled it from up on top. Meantime I had to take the interior cover off and reach up inside at the time I started it, and would give the wheel a quick spin while watching my fingers. But they need oiling every once and awhile and a spray can of good lubricant (not WD 40) with an extension straw will get it oiled without having to take the whole thing out and turning it on it's side. Tell me if I am wrong.
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Old 03-18-2012, 10:23 PM   #3
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It's not turned on and hasn't been since last summer. In other words, It wasn't in use at the time this "pop" was heard. Hum started shortly thereafter. Make sense?

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Old 03-18-2012, 10:42 PM   #4
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Try a sound conduction test to check location for sure. Take inside AC cover off. Hold a short pipe or any solid piece of metal in contact with the AC chassis and put the other end in contact with your skull. It should be apparent if the AC truly is the source of the sound.

And/or use a short length of hose to isolate to specific location like using a stethoscope. One end up yonder, the next to your ear.
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Old 03-18-2012, 10:42 PM   #5
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This is the motor that runs the wheel. This is what I had to oil like crazy all over to get it lubed enough to come on by itself. But when it did not turn (the wheel itself) I would have to manually jumpstart it. I found out later that the hum was from the motor going out of service.
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Old 03-19-2012, 12:15 AM   #6
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The blower motor as well as the compressor motor are capacitor start motors. If the motor hums and you can spin the squirrel cage fan and it will run. It's a good chance the capacitor has failed.
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Old 03-19-2012, 05:40 AM   #7
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Thanks guys. That gives me a couple good ideas. I'll check back in tonight.

Jim
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Old 03-21-2012, 02:59 PM   #8
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My vote was for something shorting out the 120 vac to 24 vac transformer. Shorting knocks out the words, and transformers hum real loud when they don't know the words...

Did you have a chance to look at it?

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Old 03-21-2012, 04:35 PM   #9
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Quote:
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Try a sound conduction test to check location for sure. Take inside AC cover off. Hold a short pipe or any solid piece of metal in contact with the AC chassis and put the other end in contact with your skull. It should be apparent if the AC truly is the source of the sound.

And/or use a short length of hose to isolate to specific location like using a stethoscope. One end up yonder, the next to your ear.
This is a good technique and I've found that an old golf club with the head broken off and a hole drilled in the end of the grip makes an excellent stethoscope for weird noises locating, and is the best use of a putter or driver.
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Old 03-21-2012, 06:28 PM   #10
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My vote was for something shorting out the 120 vac to 24 vac transformer. Shorting knocks out the words, and transformers hum real loud when they don't know the words...

Did you have a chance to look at it?

Tom
As luck would have it, I was just out there taking things apart and poking around at stuff. As soon as I finish my Fig Newtons and Scotch, I'm going to try Bob's make-shift stethoscope. The hum is emanating from either the transformer you mentioned or the "switch" (???) next to it. I'll check back in shortly.

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Old 03-21-2012, 06:33 PM   #11
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The blower motor as well as the compressor motor are capacitor start motors. If the motor hums and you can spin the squirrel cage fan and it will run. It's a good chance the capacitor has failed.
I'm with TG: caps often go pop when they take leave of this world. And time is their enemy: they don't last forever.
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Old 03-21-2012, 07:29 PM   #12
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So, using my "stethoscope", i was able to determine that the hum is definitely coming from the 24v transformer here:
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Old 03-21-2012, 07:35 PM   #13
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The thing that I was referring to as a "switch" is mounted right next to the transformer:
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Old 03-21-2012, 07:38 PM   #14
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The two blue spade connectors to the left of my finger supply 110v to that "switch". It looks like the transformer leads connect to the right of that. I'm only getting 4 volts DC on those posts (the two black wires to the right of my finger).
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