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3 amp fuse blowing

Paul_69 Member Posts: 251
i had a goodman 90 percent gas furnace that blew the 3 amp fuse on the board instantly every time i put one in. removed all tstat wires from board and heat cool digital tstat off wall. put red wire back to board only and pow everytime instant blow.so the red wire must be shorting to metal somewhere i bet. if the wires were shorted together going to stat something would be running all the time, like r to w or r to g or r to y.so i traced wire thru basement going to where wire went up thru floor. i was going to pull a wire up to tstat and i cut wire half way accross basement to see if short might be in basement area first. now just b4 i was going to pull new tstat wire i put the wires back together where i sliced them and the fuse now didnt blow!! ran for 20 minutes and no dead short. did i tug the wire away from where it was shorting or what??i did move wire a little trying to see where it went but not much. so i left it alone i thought maybe he might need a new wire run but it didnt do it again and it wasnt going to be a easy pull of wire. maybe a electrician might have had to do this anyway.well for now its running but it was very strange situation.is there anybody that had a simamlar situation? are than any sites or books etc. that explain more about troubleshooting shorts, opens etc?


  • Empire_2
    Empire_2 Member Posts: 2,343
    Ahh Shorts............

    You started great pulling all the T-stat wires off the board.  Here's what or how I attack the "Short problem".  Since you started, fuse was always blowing.  Disconnect the wires as you did and think of it this way.......  Eliminate the stat, interconnecting wire to the board.  Now jump each circuit on furnace,....R-W/ R/G/ R-Y etc.  Were looking for ANYTHING that may trip your fuse.  As you get proficient at this, you can stop replacing your fuse supply and use a clamp on ammeter.  Anything above1.2 amps is a problem.  So..... since we cycled all the individual circuits at the furnace,.....? No problems.  Switch to OHLM meter and check you stat wires to ground.  Each one is either grounded or not,........ period. 

    Now check (still on OHLM), if needed each wire on stat cable (from the furnace-up)  from say R to each wire....W-Y-G-C etc.  What you are looking for is any short (any "0" ohlm reading) from any of the wires to ground or (common) .  R to C will give some kind of reading, but that is not what were looking for.

    In essence what we are looking to do is see if the short is from the stat to the furnace, OR at the furnace and it's circuitry.  Since you pulled the cable between the furnace and the stat location, and now it works,...?  The short may reoccur but we do not know from where.  Replace the cable if you feel it necessary, but you make the call.  Just make sure you know for next time where the supposed short is so you can take appropriate action and ALSO, Identify the reason for the problem and where it's LOCATED (very important)........OK?

    Hope this helps you.  Every move we make while trouble shooting tells us where the problem is,....Why it occurred, and most importantly, How it happened, or what caused it.  In your problem it could have happened from construction, shifting or rubbing causing the short, or any number of things that can and DO HAPPEN.  When it costs a lot of money, the customer wants answers.  It's nice to say MR Jones,....You know that new addition you added to your home????  Someone drove a nail into the wiring for your furnace......:-)  How do you know this?  Ex... Mr Jones you had your addition built in summer and everything worked great, but as winter came and the heat kicked on.....The fuses were just-a-poppen......  Heating circuit shorted......??? 

    All kinds of stuff happens.  You look like, and are a star when you can pull answers not by magic, but from "OLD FASIONED TROUBLESHOOTING":=) 


    Mike T.
  • Empire_2
    Empire_2 Member Posts: 2,343

    I did not mean to sound condescending, just how I attack these things.........

    Mike T.
  • icesailor
    icesailor Member Posts: 7,265

    Here's a "short" story.

    I got a new customer. He had just bought the house from a spec builder who had a 90+ furnace installed. It wasn't converted to LP and when the new owner moved in, a day or so later, the CO detectors went off. The furnace was totally soot-ed up. It was replaced. by others. Now, it wouldn't work. I was recommended. The 3 amp fuse on the board would blow instantly. Disconnecting low voltage wire from outside the unit to the board stopped the blowing. A process of elimination got it down to one wire, to the compressor outside.  The installers had drilled a 2" hole through the side of the building through the rim joist to run the line set through. They also ran the thermostat wire through the 2" hole. There were mouse tracks on the wire outside and inside. Mice pee on their feet because they can't see well. So, they follow their nose. There was over 4" of solid wood through the rim joist there the hole was. The mice could easily gain access to the building through the hole. I pulled back in the wire and there was at least 2" of mouse stripped wire with exposed conductors touching, causing the blowing of the fuse. I ran a new wire and sealed the hole up with that electrical duct seal. End of problem.

    I've been under houses where Sparky fished long lengths of UF wire in crawl spaces and meeces had stripped long lengths of insulation from the wire without them killing themselves or the wire shorting out. If the holes through the plates are way too big for the wire, and there is rodent activity, mice will chew the insulation off the wire to get through.

    If you are in a crawl space or cellar, and you see copper tube with dark greenish black stains along the top of the tube, with funny stripes on the upper sides, use respiration protection if you need to sand on the tube. The dust can be seriously hazardous to your lungs and health.
  • don_9
    don_9 Member Posts: 395

    I have pulled my hair out many times chasing the phatom short.And many times it was not the low voltage wires..I have had loose 120 volt wire at the burner switch to burntout coils on the contactor and coils on the reversing valve to wires on equipment going across a hot discharge line.

    How about service disconnect breaker tripping due to a bad crankcase heater..

  • Empire_2
    Empire_2 Member Posts: 2,343
    Yes, I have seen the damage before.

    Besides completely grossing me out, it took a while to repair, clean up the area.  Just a note, I once had a snake zapped inside condensing unit control panel.  Also, if you see a condensing unit all deteriorated on the corner, Dog has been pissing on it.  Can't save that PUPPY.  Yea, some of the things we all see.  Great topic of discussion.

    OK; Take care boys, see ya on the back/9

    Mike T.
  • Tim McElwain
    Tim McElwain Member Posts: 4,621
    I have a method

    for finding shorts called "Burning Your Bridges Behind You" it is illustrated in my Circuitry and Troubleshooting Volume I manual. It is effective about 90% of the time. There are some unique "shorts" which really require detective work but 90% of the time is pretty good.
  • icesailor
    icesailor Member Posts: 7,265
    Finding Shorts

    I once went on a call of no water. I climbed down into a well pit to be confronted by a snake. The snake was hanging put by the pump. I caught the snake and removed him/her/it. It tried to come back. That un-nerved me. The pump hummed when I reset the breaker. I tried to spin the motor shaft with a pair of pliers. I heard a "crunching" noise inside the motor. I shined my flashlight into the pump and there were a few baby snakes inside who had crawled inside to get warm. Snakes are reptiles and are cold blooded. The pump came on and they lost their battle with and for life. The owner was grossed out when they saw the pump motor.

    Job successfully completed.
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