# 24vac transformer burns out



## paponertelo (Sep 2, 2010)

Hi. I have a Trane gas heat/ AC unit thats about 18 years old. Untill 2 weeks ago it was working perfectly. Sudenly, it stopped working and nothing would come on. I checked the control unit and there was no power.  My trane has a white-Rodgers 50A50-403 control unit.  I bought a new 50A55-843 which is the replacement and still nothing worked. After checking everything, I noticed that the transformer was dead and ordered and replaced said transformer.  I installed the old control unit back and when I switched on the power, the unit's light started flashing normally.  I also heard a humm from the transformer. 

I went upstairs and turned the AC T-stat on. The blower started running but the compresor/condenser unit did not kick in.  I went to the control unit and noticed that the red light was flashing very rapidly and then it went out. I verified all conections and even checked the breaker and all was okay. When I touched the new transformer and it was hot. The diagnostic light does not come on and not even the blower will work.  The old control unit does not have a fuse to protect the transformer. I'm at a loss, what can I test next? 

One last thing, last year I replaced the contactor for the compresor/condenser unit because it would not stop running.

Thanks for your help.


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## kok328 (Sep 3, 2010)

I suppose you could try the new control unit with a new transformer and there's nothing stopping you from installing an inline fuse to protect your components.


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## paponertelo (Sep 8, 2010)

I was about to try that. Got a new transformer and the new control unit has a fuse for the transformer circuit.  Will report after try.


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## RichCPZ (Sep 25, 2010)

paponertelo,

I was wondering what your out come is?  I have the same problem and was considering a new system. But if it is the board then I may spring for the part. I have already changed and tested the t-stat, replaced a broken switch and went through 2 transformers. 
The only thing is seems to slowly burning out the transformer. When I test I got the 24 volts needed but it slowly get cut down to about 10-13 volts and doesn't blow the fuse either. I guess the fact that the transformer slowly heating up, building up resistances, and burning itself out. I had a HVAC guy out and he suggest I change the circuit board his cost to do it about 500.00...but, I can do it myself for about 100.00. 
Please give me an update, and any suggestion on what else I may try..


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## silentdub (Oct 1, 2010)

A transformer would NOT have a voltage drop from 24 to 13 from heating up.

Something is bring it down. I am betting that there is some kind of short on the outside unit.

I would disconnect the outside board and see if you get the 24 volts back and also make sure the inside unit works properly.

Remove the board, check for any burn marks or places where it can be touching metal. Check all connections too, especially out to the unit.


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## RichCPZ (Oct 2, 2010)

silentdub said:


> A transformer would NOT have a voltage drop from 24 to 13 from heating up.
> 
> Something is bring it down. I am betting that there is some kind of short on the outside unit.
> 
> ...



The tech and I checked the unit outside and anywhere a wire may be shorting out against the unit inside and out. He still thinks it's the board and I just bought one for 69 bucks..So, I'm in the process of installing it. If it not the problem I'm just gonna spring for a new unit.:banana::2cents::banana:
I'll update this later ....


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## RichCPZ (Oct 3, 2010)

OK, Here is my update,: Installed new board and the fan started blowing..GREAT..then switched to the A/C setting, blew fuse, replaced fuse - blew again..Called Tech then replaced contactor in the A/C unit... same result...check all wires for grounding without any success and then started system up again, Transformer blew? So, ...Gonna install a full new system this week and call it a day! Lucky it was only about 200 bucks to make this decision...


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## silentdub (Oct 3, 2010)

RichCPZ said:


> OK, Here is my update,: Installed new board and the fan started blowing..GREAT..then switched to the A/C setting, blew fuse, replaced fuse - blew again..Called Tech then replaced contactor in the A/C unit... same result...check all wires for grounding without any success and then started system up again, Transformer blew? So, ...Gonna install a full new system this week and call it a day! Lucky it was only about 200 bucks to make this decision...



That sucks, sorry to hear that.

If you are looking for an excellent system, check out my thread on the goodman, I love it.


I had an issue where I ran new wires from the inside unit to the outside, brand new from the box and something was wrong with them. They were shorted somewhere, I replaced them again and all was ok.

Pick yourself up an ohm meter, a cheap 4 dollar one will do from harbor freight tools. It will help you ohm out the lines.

something that the board is connected to is cauing the problem. What you could do, just for the hell of it, disconnect the board from all of the components, plug everything in, then plug each one in one at a time, when the fuse blows, you found the problem.


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## RichCPZ (Oct 4, 2010)

Yeah, we kind of did the process of elimination, and found the wire. Cut off part of the wire that was "worn" and then blew out the transformer. The Tech and I are replacing the whole HVAC unit...I will assist him with the tear-out and install for $3500.00.
For me it's a cool deal... I get a new system ( Goodman ) and an education in HVAC. I have an ohm meter but, never use it beyond checking batteries.
Thanks for your help...!


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## paponertelo (Jan 4, 2011)

Sorry for the delay and thank you to kok328 for his help in logically determining what was wrong with my system.  As we discussed, I replaced the transformer and the control unit (the new control unit is fused) and the system is now working. 

Like to note that the fan speed is now lower but the heat is operating beautifully.  I believe the fan speed is now lower because I did not change any of the switches on the new control unit which serves as the replacement for a large number of Trane systems.  And because this speed is lower and thus, much quieter, I do not intend to change the setting as its easier to listen to the TV now.

Thanks and I wish all of you a very prosperous new year.


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## kok328 (Jan 4, 2011)

I just love it when this help forum pays off.  In this case it paid off to the tune of $3500.


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## paponertelo (May 27, 2011)

Guess what?  My same Trane unit would not start in AC mode last night.  It worked perfectly all through the winter in furnace mode.  I had no response when I switched to AC so I went and checked the control unit and the LED did not light up.  The transformer is working because I can hear it hum.  Earlier today, I remembered that the new control unit (White Rodgers 50A55 843) has a fuse and when I checked, it was burnt.  I got a new fuse and when I turned on the power, the control unit LED started working and made a sound like a relay.  Went to the T-stat and turned on the fan/blower and it kicked in and worked.  Then, I switched to auto and changed the mode from off to cool, nothing happened.  Checked and found the fuse burnt once again.  I replaced fuse with a new one and control unit lit up again. Did the on for the blower and it runs.  I have not tried to call for cool and visually inspected and felt the wires going to the T-stat and to the condenser/compressor unit; all seems to be okay.

I'm trying to figure out what to test and how.  It seems the problem is when the condenser/compressor unit has to kick in.  I'm thinking of going to the unit and checking the connections at the contactor and the wire outside.  Then, if it checks out, test the wires to the T-stat. After that, ???

Any recommendations, hints, testing sequence, past experiences that any one can offer/relate that may help?  Thank you and have a great memorial day.

PS: We experienced severe T-storms about a week ago, but he unit was not running at the time although it was still in heat mode.


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## hvactechfw (May 27, 2011)

paponertelo said:


> Guess what?  My same Trane unit would not start in AC mode last night.  It worked perfectly all through the winter in furnace mode.  I had no response when I switched to AC so I went and checked the control unit and the LED did not light up.  The transformer is working because I can hear it hum.  Earlier today, I remembered that the new control unit (White Rodgers 50A55 843) has a fuse and when I checked, it was burnt.  I got a new fuse and when I turned on the power, the control unit LED started working and made a sound like a relay.  Went to the T-stat and turned on the fan/blower and it kicked in and worked.  Then, I switched to auto and changed the mode from off to cool, nothing happened.  Checked and found the fuse burnt once again.  I replaced fuse with a new one and control unit lit up again. Did the on for the blower and it runs.  I have not tried to call for cool and visually inspected and felt the wires going to the T-stat and to the condenser/compressor unit; all seems to be okay.
> 
> I'm trying to figure out what to test and how.  It seems the problem is when the condenser/compressor unit has to kick in.  I'm thinking of going to the unit and checking the connections at the contactor and the wire outside.  Then, if it checks out, test the wires to the T-stat. After that, ???
> 
> ...



Most likely orange is shorted to ground if heating mode works on your HP.  Or the reversing valve coil is bad.  Switch with extra wire if you have any in the loom


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## paponertelo (May 28, 2011)

Hi hvactechfw and thanks for your insight.  I checked the orange wire with my ohm meter and the wire is fine with no grounding to cabinet.  However, the Valve switch it connects to reads ground no matter how I test it. It's a round vacuum activated/reading valve (photo attached) with 3 electrical connections, 1 orange and 2 yellow.  Any way you check it, between wire connections, chassis, or ground, it has continuity.  The terminal the orange wire is attached to looks burnt/discolored (see attached photo).  Is this the reverse valve you referred to earlier? If it is, I'll replace it with a new one.  Thanks again.

PS: is there any way I can test that this valve/switch is bad and causing the fuse burning problem without swapping the valve with a new one. If this is too dangerous or can hurt other components, I won't try it.


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## hvactechfw (May 28, 2011)

paponertelo said:


> Hi hvactechfw and thanks for your insight.  I checked the orange wire with my ohm meter and the wire is fine with no grounding to cabinet.  However, the Valve switch it connects to reads ground no matter how I test it. It's a round vacuum activated/reading valve (photo attached) with 3 electrical connections, 1 orange and 2 yellow.  Any way you check it, between wire connections, chassis, or ground, it has continuity.  The terminal the orange wire is attached to looks burnt/discolored (see attached photo).  Is this the reverse valve you referred to earlier? If it is, I'll replace it with a new one.  Thanks again.
> 
> PS: is there any way I can test that this valve/switch is bad and causing the fuse burning problem without swapping the valve with a new one. If this is too dangerous or can hurt other components, I won't try it.



DO you have a Heat pump?  Orange would be at the thermostat and not in the furnace.  That is a picture of a Pressure switch in the furnace.


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## paponertelo (May 28, 2011)

I have a Trane Gas Furnace and AC unit. The wires to the T-stat are Green, white, Yellow and Red. There is a fifth Blue wire that is not used. At the controler board there are an extra couple of wires that I believe go out to the condenser/compressor unit. Here's a photo of both.


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## paponertelo (May 28, 2011)

I have attached part of the documentation that came with the unit. The furnace unit is a TUD080C936A.  I could not attach the whole .pdf file because of attachment size restrictions on this site.  I could email if you want/need all documentation. Maybe this will help.


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## hvactechfw (May 28, 2011)

ok, You need to check the wiring from the control board inside to the outdoor unit.  red is hooked to Y on control board and white is hooked to C.  Follow those wires ( should run with the a/c copper lines to the outdoor unit) and check for missing insulation or grounded out wires.  IF nothing is found their or inside the outdoor unit then it is most likely the coil of the contactor in the outdoor unit is bad, check the resistance of the contactor coil.


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## paponertelo (May 28, 2011)

Checked the wire that runs to the outdoor unit and the insulation worn out close to the unit. Any considerations in terms of what wire to use to replace it; outdoor rated, solid versus stranded, any voltage or resistance requirements, where can I get it (Home Depot, Radio shack)?  Thanks.


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## hvactechfw (May 28, 2011)

18/2 solid wire...should be at the HD


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## paponertelo (May 28, 2011)

Thanks. Will replace and report.


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## paponertelo (May 29, 2011)

Bad news for me. Replaced the Thermostat wire to the outside unit and the fuse blew when I tried to start the AC.  I know it&#8217;s not the wire because I first tried the AC without power to the outside unit and all was okay.  I then raised the Temp on the thermostat, went out and powered the outside unit and when I lowered the temp on the thermostat so the AC would kick in, nothing happened.  Went to check the fuse and it was blown.  

Last year, I replaced the contactor on the outside unit because it got stuck and the compressor would not shut down.  But since the fuse blew when the contactor had 110 volt and not when the breaker was off, I'm assuming the contactor is bad.  Is there any test that can be performed on the contactor with a meter?  Thanks.


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## paul52446m (May 29, 2011)

paponertelo said:


> Bad news for me. Replaced the Thermostat wire to the outside unit and the fuse blew when I tried to start the AC.  I know its not the wire because I first tried the AC without power to the outside unit and all was okay.  I then raised the Temp on the thermostat, went out and powered the outside unit and when I lowered the temp on the thermostat so the AC would kick in, nothing happened.  Went to check the fuse and it was blown.
> 
> Last year, I replaced the contactor on the outside unit because it got stuck and the compressor would not shut down.  But since the fuse blew when the contactor had 110 volt and not when the breaker was off, I'm assuming the contactor is bad.  Is there any test that can be performed on the contactor with a meter?  Thanks.



The out door relay will snap closed Werther you have  the power on to the outdoor unit or not. Are you sure you don't have a line volt wire touching low volt in the out door unit. With the stat not calling for cooling, turn power to out door unit. then check low volt wires to cabinet ground and see if you have any voltage.  If you have a bad contact in the relay, the relay can chatter real fast and that would increase the amps fast and blow the fuse.
Check all wire connections on the relay to make sure they are tight, and check the contacts of burned points.   Paul


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## paponertelo (May 30, 2011)

paul52446m said:


> The out door relay will snap closed Werther you have  the power on to the outdoor unit or not. Are you sure you don't have a line volt wire touching low volt in the out door unit. With the stat not calling for cooling, turn power to out door unit. then check low volt wires to cabinet ground and see if you have any voltage.  If you have a bad contact in the relay, the relay can chatter real fast and that would increase the amps fast and blow the fuse.
> Check all wire connections on the relay to make sure they are tight, and check the contacts of burned points.   Paul



I tested the contactor as you suggested and there was no voltage on the T-stat wires. All four connectors on the contactor had 110 volts; is this normal? I cut power to the outside unit and tried to hear the relay click when the call for cooling was engaged but it just burnt the fuse; it never clicked. Then I tried to check the 24 volts to the contactor by taking the blue and yellow wires (the T-stat white and red wires are connected to these with wire nuts) of the contactor and noticed that the lower part of the contactor is loose and it moves (I don't remember these terminals having any play when I replaced this contactor over a year ago). With a new fuse and the T-stat lines disconnected from the contactor, I called for cooling and the blower fan came on. Went outside and checked the terminals from the T-stat and the blue wire had 24 volts and the yellow had no voltage; the fuse did not blow. (Contactor photos attached) By the way, I'm using 3 amp fuses like the one the control came with. 

Based on these test, I'm convinced the contactor is bad/malfunctioning and will replace it. I'll keep you informed and thanks to everyone for their help.
.


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## paponertelo (May 31, 2011)

I just replaced the contactor and the AC is running perfect (and very cool, Yay).  The coil on the old contactor is burnt and almost melted.  I believe now I know what happened from the beginning that caused all the problems.

When the unit stopped working last Sept, the control board must have had a short that allowed line voltage to get to the 24v circuit and burnt the transformer and also fried the coil on the contactor.  Once I replaced the control board and transformer there was no need for AC at that time and when heat was called for, the furnace worked perfectly.  Once the season changed and I had to use the AC, the system would not work.  Since I replaced the control board with a new model that is fused on the 24v circuit, every call for cooling would send the voltage to the fried coil and it blew the fuse due to high amps.  So, replacing the contactor eliminated the failed coil and now the 24v circuit is working properly.

My thanks to all who have contributed to identifying and fixing these problems and helping me understand the chain of events.  My total cost of repairs since Sept is a bit under $200, including the new control unit ($100), 2 transformers ($17 ea), 50 ft of T-stat wire ($9) and a new contactor ($38). 

Have a wonderful (and very cool) summer.


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