A/C stopped working after a burning smell

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  #1  
Old 07-07-15, 05:54 PM
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A/C stopped working after a burning smell

Hi,

Last night I went and manually lower my AC until at turned on but after 5 seconds or so it stopped working and I noticed a smell of smoke.

This morning I tested the motor by supplying current directly and it worked, I also checked the door switch by connecting the cables together (by passing the switch) and it didn't make a difference.

Any idea what could be wrong at this point? What should I check next?

Any suggestion?

Here are some pictures just to show what I'm working on.

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Last edited by PJmax; 07-07-15 at 06:36 PM. Reason: reoriented/enhanced pictures
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  #2  
Old 07-07-15, 06:39 PM
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At this point is the blower motor working or not ?
You mentioned it was but then you said nothing was working.

What was the outside unit doing ?
 
  #3  
Old 07-07-15, 06:52 PM
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Blower motor works fine, switch also seems to be working fine. Outside unit does nothing, it doesn't turn on at all.

Thanks
 
  #4  
Old 07-07-15, 07:07 PM
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Did it trip the breaker? You may have a dead short somewhere. You definitely smoked something. That Magic Chef unit is 30+ years old. There's not much life left in it.
 
  #5  
Old 07-07-15, 07:15 PM
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Does the blower motor turn on with the thermostat ?
Does the heat come on with the thermostat ?

If nothing seems to work then more than likely the transformer is what you smelled burning.
This could be just a natural failure or you have have a shorted wire to your outside condenser or your contactor may be shorted.

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In the picture.... the yellow arrow is pointing to the transformer. Smell it to see if it smells burned.
 
  #6  
Old 07-07-15, 07:17 PM
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No the breaker didn't trip. Yes it is old, the house is 25 years old.
 
  #7  
Old 07-07-15, 07:28 PM
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@PJmax

No, blower motor doesn't turn on on any position, I only know it works because I tested it.

We smelled the transformer and we all agree that it is the same smell we experience last night.

How hard would it be to change the transformer?

What is the contactor?
 

Last edited by fs_tigre; 07-07-15 at 08:02 PM.
  #8  
Old 07-07-15, 08:05 PM
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The contactor is outside in the A/C condenser. It turns the compressor and outside fan on.


What you'll need to do is to replace that transformer. That looks like the transformer is mounted on a fan center. I would disconnect all four wires to the transformer and wire a new one up next to it. You need a 24vac 40va/watt transformer.

I would install a 5A fuse in one of the transformer lines. An automotive fuse and holder works good. This will protect the transformer while you are testing for a short.

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  #9  
Old 07-08-15, 04:40 AM
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@PJmax

I will get the new transformer and give it a try. Thanks a lot for your help.

Where can I get the transformer? Any place you would recommend?

I looked around but I'm having a hard time finding one for that model.
 

Last edited by fs_tigre; 07-08-15 at 05:39 AM.
  #10  
Old 07-08-15, 07:38 AM
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You can get it many places on line. The link below is one available from the depot.

Honeywell 24-Volt Transformer-AT72D - The Home Depot

amazon/Honeywell-AT140A1000-40Va-120V-Transformer


You need a 24 volt AC 40va transformer. You may have a local HVAC company that will sell it to you.
 
  #11  
Old 07-08-15, 09:57 AM
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First of all, thanks a lot for your help.

I'm more worry about the size, will all of them fit? Or can I remotely mounted (externally)?
 
  #12  
Old 07-08-15, 05:01 PM
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Might want to look into a complete furnace fan center if your worried about it.
 
  #13  
Old 07-08-15, 05:08 PM
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Hi,

A couple of questions.

1- What is the black box, is this part of the transformer?
2- Do I need to buy a new board?

The transformer seems to be reverted to the plate. Do I need to remove it from the plate?



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  #14  
Old 07-08-15, 05:16 PM
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@guyold

AAhh, that may answer my previous question, that is the furnace fan center, right?

The hard part will be to find the right one for my model (G6A75DC8-13).
 
  #15  
Old 07-08-15, 05:43 PM
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Yes... that is the fan center. If your handy you can mount the new transformer to the side of the junction box that the center is mounted on. That black box is the relay.

You can also replace it with a universal fan center (90-113) like in the links below....

White Rodgers 90-113, L37-811, 120VAC-24VAC, SPDT Fan Control Center. - Furnace Parts Outlet

90-113 - White Rodgers 90-113 - Fan Control Center, 120 VAC Primary 24 VAC Secondary, SPDT Relay

I haven't been able to locate much tech info on your unit. The original fan center will no longer be available. The sub I listed should work.

There should be a wiring diagram/schematic on the back of the blower door. If you could take a picture and post it I could confirm the part.

http://www.doityourself.com/forum/el...rt-images.html
 
  #16  
Old 07-08-15, 05:48 PM
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They are pretty interchangeable , just for fan speed and color code for motor.Take pictures and if you get stuck there are many fine people here to help, but not me. Good luck.
 
  #17  
Old 07-08-15, 06:00 PM
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Thank you all for your time.

Here is the schematic.

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  #18  
Old 07-08-15, 06:15 PM
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Thank you for the excellent picture. I have listed the incorrect part. You need a DDT fan center.
That is a part number... 90-112. You can search around using that number.

90-112 - White Rodgers 90-112 - Fan Control Center, 120 VAC Primary 24 VAC Secondary, DPDT Relay
 
  #19  
Old 07-08-15, 06:38 PM
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Thanks a lot for your help, you are awesome!

I will try to find that part in locally otherwise I will order online.

Again thanks a lot!
 
  #20  
Old 07-08-15, 06:39 PM
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Any HVAC supply will have it.... unfortunately some of them only sell to the trade.
 
  #21  
Old 07-08-15, 07:02 PM
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Hmm, I hope I can find some dealer that can sell it to me. Thanks a lot.
 
  #22  
Old 07-09-15, 07:52 AM
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@PJmax

Hi,

Quick question. I called the appliance parts store and they said they don't have 90-112 only the 90-113 which they said should be the same.

90-113

The main difference I see is the Relay SPDT vs DPDT.
 
  #23  
Old 07-09-15, 08:13 AM
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You can replace a SPST relay with a DPDT relay, however you may not be able to go the other way. It all depends whether both poles are being used. If they are, you need to stick with a DPDT relay. On the other hand, if only one pole is being used, you can use a SPST relay, however there's a chance that you may need to make a wiring change depending upon which pole is being used. I think you'll find it's easier to just go with an exact replacement.
 
  #24  
Old 07-09-15, 10:25 AM
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Sounds good, thanks a lot for your help.
 
  #25  
Old 07-09-15, 09:32 PM
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The exact replacement is no longer made. That is a fairly old furnace.

Your furnace requires two switching poles which is why I spec'ed the DPDT model. Most newer furnaces can use the SPDT model.
 
  #26  
Old 07-17-15, 07:43 AM
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Hi,

First of all let me tell you that you guys have been REALLY helpful, @PJmax immediately identified the problem, I’m impressed, thanks a lot (this has been confirmed, transformer is bad).

Ok, I finally got the Fan Control Center (90-112), now I would like some to verify my connections. I'm only interested on the wiring going to the furnace not to the Thermostat(y, w, g etc.).

Furnace Schematics.
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Fan Control Center (90-112) Schematics.
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Here is how I’m planning to do it.
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Does that look right?

How about the Violet(common) and the Black (common) wires, just cap them?

You guys are awesome!

I hope @PJmax can see this and help.

Thanks a lot.
 

Last edited by fs_tigre; 07-17-15 at 08:04 AM.
  #27  
Old 07-17-15, 08:12 AM
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I here and see all. Actually you're lucky as I'm on the road today..... just checked in.

You need some corrections.

The two red wires require a normally closed connection.
That would be the common/violet and the NC wire which is yellow. Cap off red/yellow.

The two black wires require a normally open connection.
That would be the common/black and the NO wire which is red. Cap off brown.

I'm glad you stopped back as this is only half the battle. We don't know why the transformer burned up. If you were in A/C mode it may have been a contactor issue. In post 8 I discussed a fuse and fuseholder. I would recommend that between the R terminal on the fan center and the wire(s) that was connected to R.
 
  #28  
Old 07-17-15, 08:35 AM
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@PJmax Thanks a lot.

Here is the revised schematics, I hope I got it right.

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I will also add the fuse as suggested. Do I leave the fuse permanently or it only needs to be there while testing?. Thanks a lot.
 
  #29  
Old 07-17-15, 07:29 PM
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Your diagram is correct. The fuse should remain forever. It's good to have as a safety to protect the transformer from accidental shorts. Most newer furnaces come with a fuse right on the control board.

As an aside..... I had the same problem you had. The first time I fired the air up I got that burning smell. My transformer was smoked. I had a similar fan center that I replaced.
The problem was a shorted contactor in the outside condenser.
 
  #30  
Old 07-18-15, 07:11 AM
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@PJmax, my AC is working back again since yesterday, thanks to you.

A couple of questions.

I noticed that the metal piece located on the relay gets a little hot after a while (15-20min), is this normal?
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The other thing I noticed is that the transformer vibrates (Harmonic Distortion) when the unit is off, this may be normal on all transformers I just have never touched one when not in use, is this normal?

The one thing I haven't done yet is the fuse because I couldn't find a fuse holder on my local stores. Do I put this in the main line of the transformer or in the R low-voltage cable?

Thanks
 

Last edited by fs_tigre; 07-18-15 at 07:47 AM.
  #31  
Old 07-18-15, 08:58 AM
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As long as the A/C and the blower are on.... that relay is active. It's normal to run fairly warm.

You don't have to use the exact fuseholder I showed you. Any 5A fuse and holder will work.
The protection goes between the R terminal and the wire that is currently connected to it.

Protection isn't mandatory but it will save the price of the fan center if something in the control circuit shorts.
 
  #32  
Old 07-18-15, 09:16 AM
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Got it. Again thanks a LOT for your help! I really appreciate it.
 
  #33  
Old 07-18-15, 09:21 AM
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You're welcome. Enjoy the cool.
 
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