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Old 07-17-2010, 06:48 AM   #1
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transformer failure in a Trane XE78


I have a Trane XE78 furnace that has blown the power supply transformer. The primary windings were shorted in the original transformer. I replaced the transformer with the same exact on and it went out right after attempting to turn the A.C. on. With the A.C. off the light came on the board like it should.
I checked the blower fan with a separate power source and all speeds are working fine. I opened the control board cover and there are no visible bad parts. Can anyone point me in the right right direction? Thanks for your help

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Old 07-17-2010, 07:30 AM   #2
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transformer failure in a Trane XE78


Look for a short in the 2 wire going to the AC. Burnt out primary side of a transformer means a low voltage short.

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Old 07-17-2010, 07:38 AM   #3
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transformer failure in a Trane XE78


Take a 3 or 5 amp GM blade auto fuse and 2) 1/4" stakons. Splice them into the hot wire from the transformer or between the R wire and the circuit board so next time it blows the fuse instead of the xfrmr. Cover the ends with tape. Find the short like Marty said.
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Last edited by yuri; 07-17-2010 at 07:43 AM.
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Old 07-17-2010, 09:24 AM   #4
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transformer failure in a Trane XE78


Finding a short:

For testing purposes,

replace the fuse with a power resistor so that you still get current flowing but at a level low enough to not damage the 'frmr or the components downstream.
For a 40 VA 'frmr this would be less than 1.7 A and so the resistor value is more than 14 ohms, at 40 W. Rather than buying this costly resistor use a 120v incand. bulb, 100 W or less, in a socket with pigtail leads. The bulb will probably not light with this voltage across it.

Depending on the layout, disconnect things until the short disappears as shown by a substantial drop in the voltage across the resistor. If the short is on a printed circuit board the procedure is a little more involved and you need a very sensitive DVM.

Or, replace the fuse with a small circuit breaker like that in appliances and then disconnect things. Garbage disposals use a CB that's probably close enough and you can get one at stores that sell appliance repair parts.
http://www.hosfelt.com/contents/en-us/d448.html

Last edited by Yoyizit; 07-17-2010 at 10:13 AM.
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Old 07-17-2010, 10:01 AM   #5
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transformer failure in a Trane XE78


auto fuse that R side of the TR within the furnace,go out to the condenser and remove the 2 low voltage wires where they land and tape them away...now power up the TR with that fused R side.if you blow the fuse the run out to the condenser is shorting..did anybody nail inside the house into any walls in the area of the stat drop to the furnace..might of hit the Y out to the condenser.if you DON"T blow the fuse..there is somethng in the condenser shorted...coil on the contactor for the comp/cond/fan or if you have LP/HP shorting on the safty.if you have an ohm metter set it to audible and ground one side onto the copper pipe..then touch each side of the taped wire for 24Vs should hear nothing if you hear audible the control circuit is shorted within the condenser.... i will check back after noon
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Old 07-17-2010, 02:55 PM   #6
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transformer failure in a Trane XE78


Thanks for the ideas. I have to get a second transformer and no stores are open till Monday. I will try then and post back.
Thanks again
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Old 07-19-2010, 07:27 PM   #7
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transformer failure in a Trane XE78


Well I think I am on the right track. I fused the new transformer like described. After disconnecting one thing at a time I found that the switch the starts the compressor is blowing the fuse or transformer. With the wires of the switch everything else works. I am going to get a new one tomorrow and try it.
Thanks for all the help
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Old 07-19-2010, 07:34 PM   #8
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transformer failure in a Trane XE78


Check with your meter, should have 24 volts at the 2 wires to the contactor with them in the air. Its called a magnetic contactor (contactor for short). Order a 30 amp type as it has stronger contacts. If it shorts with them in the air then the wiring is shorted and not the contactor.

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