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blown transformer

Have you examined the wire at the outside unit, for both dog chews and weed wacker attacks?
Turn power off to both inside and outside units, Placing a small fuse in the low voltage circuit will help preserve xformers. Was the t-stat in cooling or heating? With your Ohm meter start checking low voltage coils til you find the coil that is short to ground. I would check the outdoor contactor coil 1st. Best Wishes J.Lockard


  • pat vpat v Member Posts: 3
    blown transformer

    heres the problem. The system is 3 years old and has never had a problem, I have 249v going in and 29 going out and as soon as i hoock up the low volt and turn on the power the transformer burns out.i know there is probably a short on the low volt side but where should i look first and how.
  • Empire_2Empire_2 Member Posts: 2,343
    Jim has it right.....

    Chasing down a short is an Art and I have see guy's spend countless hours looking and looking and some find the problem fast. Call in a Pro if you have to. Since your short can be ANYWHERE, the system as a whole should be isolated to find out where.
  • bob_50bob_50 Member Posts: 306
    Beside a short

    I have also seen x-former fry when mice get into the control box and get debris in the contactor preventing it from pulling in.
  • kal_2kal_2 Member Posts: 60
    go to radio shack...

    and buy a lamp holder with a 24v bulb - disconnect one of the leads of the 24v output and put the 24v bulb in-line - it should burn brightly, and wont short out, and now you can go and disconnect section at a time until the bulb burns only half bright – indicating a normal load, if it goes out completely then you have disconnected everything and need to back-up – now you can easily go step by step and find your short – without worrying about a burnout – also look where wires go through holes as vibration usually wares them through to the bare metal - this is not rocket science – I personally use 2 12v pizo electric horns in series so I can hear it without looking - but don’t do it when the HOs are around they don’t take well to loud sounds
  • Empire_2Empire_2 Member Posts: 2,343

    You could buy tools like a clamp on ammeter and learn how to properly track down shorts.
  • BarbarossaBarbarossa Member Posts: 89
    Go with the bulb kid.

    The ampmeter will not limit the short current and the Xtmr will blow again the lightbulb load is a better bet for limiting damage and costs.
  • mtfallsmikeymtfallsmikey Member Posts: 765
    Inline fuse with alligator clips attached

    Works well too...strangest thing I ever saw in a condensing unit J-box was a baby mouse and small copperhead snake, both fried when they got across the joke!, but the only witness to this is no longer with us.
  • Empire_2Empire_2 Member Posts: 2,343
    I believe you Mikey

    I have found both myself. Only once though. The mouse was skeleton and he bit in one of the wires and grounded itself out. Strange........

    Mike T.
  • dana_3dana_3 Member Posts: 57
    blown transformer

    you can get at radio shack a 3 amp mini .lo voltage circuit breaker. disconnect the lo- voltage wiring at the furnace or air handler 24v trans.attach one leg of the mini breaker to the R terminal, and the other to the Rwire to the tstat. then as you rewire each other item back in.(with the power off ) as they get wired in. turn the power back on. you will then find your problem. without a need for a bunch of 24v fuses or transformers. OR you could call in a tech to troubleshoot for you
  • Empire_2Empire_2 Member Posts: 2,343
    In a short,.....?

    All I can say is USE your clamp on ammeter and there will be no need for horns, a trip to Radio Shack, or buying a bulb to do what was mentioned. No disrespect, but are you men kidding me? I guess if it works and you are comfortable,... go for it. I guess to each his own and I am sorry if I ruffled any feathers, but any short can be found in 3 minutes or less with the tried and true clamp on ammeter. Please don't take this the wrong way, but this is one question that I am passionate about.


    Mike T.
  • pat vpat v Member Posts: 3

    thanks for all the great suggestions. I found the prob it was in the contacter coil.
  • mtfallsmikeymtfallsmikey Member Posts: 765
    No ruffled feathers here, T

    As you say, to each his own. seems like in the old days, a 24v. transformer was..."tougher"? I've seen anticipators fry on T87's before the xmfr. would go. Your clamp-on works for you, but I was taught to do it a different way...More important than tools is the technique, and all of the above will work, but troubleshooting anything requires common sense and a logical sequential approach. Ladder diagrams, which were NOT included with equipment schematics back in "the day" make things a lot can "see" how each device is powered, and in what order, start to finish. I wish more people in our industry had your passion, and pride in their work. I guess it has something to do with how and by who we were trained by. i've been very fortunate in that regard.
  • Empire_2Empire_2 Member Posts: 2,343
    Like my Father said before me:

    What the hell are Ya doing with that clamp on meter,....You don't need that!!!:-)

    Mike T.
  • kal_2kal_2 Member Posts: 60
    lucky you...

    thats a rare one, open coils or fuzed contacts are far more common - as well as abraded through insulation
  • kal_2kal_2 Member Posts: 60

    a stupid bulb in series with a start or run capacitor will test it fortwith..

    off = open - dead

    full brightness = shorted - dead

    medium brightness - for run type and half brightness for starting type = good
  • jim lockardjim lockard Member Posts: 1,059

    Bad contactor on the outside unit as I predicted, not bad for a cyber trouble shoot. Glad you got it fixed. Best Wishes J.Lockard
  • Eugene SilbersteinEugene Silberstein Member Posts: 1,380
    Brings back memories

    That brings back memories of my dearly departed father. He was full of tricks like that.

    I, personally, was never a big fan of his tricks.

  • kal_2kal_2 Member Posts: 60
    so what did you use before...

    the "capacetance" range on your DMM ??? - dont tell me you were a "parts changer" ;)
  • Eugene SilbersteinEugene Silberstein Member Posts: 1,380
    I have been fortunate

    I have been fortunate enough to have been raised in the era of digital (at least decent quality analog) meters. I still enjoyed my old man's stories (and lectures) though.

    Now I get to lecture others...

  • Personally,

    I like to replace my burnt transformer with one that has the little breaker built in and then test each circuit separately to see which one blows the breaker. I always replace a bad transformer with one with the little breaker, it just seems like the right way to do it.
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