DIY Home Improvement, Remodeling & Repair Forum > Appliances > HVAC > 24vac transformer burns out




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Old 05-28-2011, 02:50 PM  
paponertelo
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Thanks. Will replace and report.



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Old 05-29-2011, 12:32 PM  
paponertelo
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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.



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Old 05-29-2011, 05:28 PM  
paul52446m
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Originally Posted by paponertelo View Post
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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Old 05-30-2011, 08:23 AM  
paponertelo
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Default Did the test

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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.
.
img_8419.jpg   img_8421.jpg  
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Old 05-31-2011, 08:14 AM  
paponertelo
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Default That did it!

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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