Transformer Bad? - http://www.hvacmechanic.com/ Forums
Original message
| hvacwannabe
 |  "Transformer Bad?"
, posted Mon 2 Jun 18:56  
Condenser not running so replaced contactor. Ran again but contactor chattered. Next day no power at thermo and no power at contactor of course. Checked 5A auto Fuse on red line at transformer and found it blown. Replaced. Power up system. Thermo on. Arc across contactor in condensor and fuse blows again? Any ideas?
Know just enough to be dangerous!
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Replies:
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| theduke03
| "Re(1):Transformer Bad?" , posted Mon 2 Jun 19:03
You sure are dangerous. You have a short in the wiring or in a component. Check outdoor low voltage wires for obvious rodent or weed whacker damage. Critters like to chew the insulation off the wires inside the outdoor unit.
If they look ok then you must disconnect the low voltage wires at both ends. Then check ohms from each wire to each wire. All should read open/infinate. A short will read 0 or low ohms.
"My dad was the most feared furnace fighter in Northern Indiana."
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| | hvacwannabe
| "Re(2):Transformer Bad?" , posted Mon 2 Jun 19:34
Your Good cause I found some mouse turds in the attic next to the blower. I will check for an insulation problem. Confirming - A transformer could not cause these symptoms?
Know just enough to be dangerous!
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| | hvacwannabe
| "Re(3):Transformer Bad?" , posted Mon 2 Jun 20:03
No you are probably right, its outside because:
1) Replaced fuse and checked power at Thermo. Power Good 24V
2) Turn Thermo to Cool and arc outside and fuse blows at blower
Still sound like a short outside? Could the contactor be shorted? Its a 2 pole- replaced a single pole but worked for 24 hours.
Know just enough to be dangerous!
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| | theduke03
| "Re(4):Transformer Bad?" , posted Mon 2 Jun 20:17
I would never question a transformer that is supplying 24v. Having said that Your short is most likely outside.
Do this: Shut off indoor unit. Disconnect the 24v wires from contactor coil. Check ohms across the coil. usually about 10 ohms+/-. 0 ohms would be a shorted contactor coil. I am doubting that it's the contactor because it's new and you had this problem before you replaced it.
Visually inspect those wires and then (if you don't find an obvious short visually) use your ohmmeter on the wires (between air handler and outdoor unit) with the procedure I described before.
"My dad was the most feared furnace fighter in Northern Indiana."
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