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Jim & Susan · 03-18-2012, 08:58 PM

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

AIR-Quarius · 03-18-2012, 09:16 PM

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.

Jim & Susan · 03-18-2012, 09:23 PM

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?

Jim

CanoeStream · 03-18-2012, 09:42 PM

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.

AIR-Quarius · 03-18-2012, 09:42 PM

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.

TG Twinkie · 03-18-2012, 11:15 PM

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.

Jim & Susan · 03-19-2012, 04:40 AM

Thanks guys. That gives me a couple good ideas. I'll check back in tonight.

Jim

TomW · 03-21-2012, 01:59 PM

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

Crusty · 03-21-2012, 03:35 PM

Quote:

Originally Posted by CanoeStream

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.

Jim & Susan · 03-21-2012, 05:28 PM

Quote:

Originally Posted by TomW

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.

Jim

Aage · 03-21-2012, 05:33 PM

Quote:

Originally Posted by TG Twinkie

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.

Jim & Susan · 03-21-2012, 06:29 PM

So, using my "stethoscope", i was able to determine that the hum is definitely coming from the 24v transformer here:

Jim & Susan · 03-21-2012, 06:35 PM

The thing that I was referring to as a "switch" is mounted right next to the transformer:

Jim & Susan · 03-21-2012, 06:38 PM

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).

Jim & Susan · 03-21-2012, 06:44 PM

Guess I need to study these for a while to see if I can figure something out. That, and trace a few more wires.

Jim & Susan · 03-21-2012, 08:06 PM

So Tom, have any idea where I can find one of those 24v transformers? I took the control box out and started tracing wires and I suspect that's where the problem is. "Suspect" because I haven't bench tested it yet. Too late tonight, I'll deal with it tomorrow.

Jim

perryg114 · 03-22-2012, 06:07 AM

Does the AC run other than the hum?

Perry

Crusty · 03-22-2012, 06:27 AM

Transformers can be expected to hum when they're supplying a lot of current and the hum may only indicate that some component is drawing too much current. The schematics you've posted don't show the important component very well (the start relay) which makes diagnosis tough, but I'd say that a failed starting capacitor would fit your symptoms perfectly.

You can test the transformer independently by disconnecting the output wires and then measuring the unloaded output voltage. If it is 24VAC (probably higher unloaded) then that's an indication that some other component is drawing too much current and pulling the voltage down. Components which draw too much current get hot BTW and capacitors often make a popping sound when they short out.

TomW · 03-22-2012, 01:15 PM

Quote:

Originally Posted by Jim & Susan

So Tom, have any idea where I can find one of those 24v transformers? ...

Air conditioning parts houses sell 24 volt transformers. Beware, though, that most of these transformers are sold with either 240 or 208 vac primary sides instead of 120 volt like your Airstream needs. Larger parts house sell a universal transformer with multiple primaries - use the wires you need, and tape-off the rest.

You mentioned reading "4 volts DC". Keep in mind that this is all AC voltage.

Your transformer could be bad. But it could be that its output wires are being shorted. Disconnect the transformer's output wires, and reassess. If the hum stops, then the trouble is downstream of the transformer.

Tom

Jim & Susan · 03-22-2012, 04:06 PM

Well, I finally just took the entire assembly out of the Airstream and took it to the workbench in the garage. The transformer is indeed outputting 25.2v AC with no load on it. Which would be about right if I'm following Crusty's logic. I want to go back and reread his and Tom's posts right quick.

Jim