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  1. #1
    Join Date
    Oct 2012
    Location
    Los Angeles, Ca
    Posts
    5

    Confused blown 5 amp fuse

    Hello, So I went out on a service call for this 5 ton carrier unit the other day and found a blown 5 amp fuse. I was told that this was the 5 amp fuses protects the low voltage side. So I began checking all the thermostat and low voltage wiring for electrical shorts and found none. I replaced the fuse to see if it would blow open again but it didn't. So I amped out all the low voltage wiring to the contactor and relay but everything was below 1 amp. The unit ran for about thirty minutes and everything was running fine. If the fuse is rated for 5 amps but all of my low voltage doesn't amp over 1 amp. What caused my fuse to blow. Any opinion's would be great. Thanks

  2. #2
    Join Date
    Mar 2009
    Location
    Mid-Mo
    Posts
    3,596
    You more than likely still have a short, it's just moved and is t touching whatever caused it to short. Good places to look are where the stat wires penetrate walls, panels, etc. Put in a resettable 5 amp breaker and start wiggling wires. Assuming you have checked all other components.....

  3. #3
    Join Date
    Oct 2001
    Posts
    6,966
    even with low voltage it is the initial draw on the TR that blows the fuse,do you have an economizer package running 24Vs off it,and check all contactors with power OFF for free movemant...cocked contactor will take a fuse right out...what type of stat if electronic it might be pulling on it long stat wire run also pulls that being out side the unit
    "when in doubt...jump it out" http://www.youtube.com/watch?v=U1qEZHhJubY

  4. #4
    Join Date
    Aug 2012
    Posts
    2,720
    hvac 101.

  5. #5
    Join Date
    Apr 2009
    Location
    massachusetts
    Posts
    79
    Bet its the economizer.If the factory molex style plug is still there you could bypass it for now.

  6. #6
    Join Date
    Aug 2012
    Location
    Palmyra, Missouri, United States
    Posts
    224
    i bet it is the low or high pressure switch wiring starting to short on the refrigerant piping inside the condensor.

  7. #7
    Join Date
    Jan 2012
    Location
    Fresno, ca
    Posts
    42
    I've also had the CLO board short out and kill fuses. Definitely get a resettable 5amp breaker. Take your time and check the wiring diagram. if you don't have the economizer "bypass jumper harness" then you will need to cut and wire nut the number 5 and the number 2 wires together. * You need to study the wiring diagram first... Don't just start cutting wires.

  8. #8
    Join Date
    Jan 2012
    Location
    Fresno, ca
    Posts
    42
    oh yeah, dont leave the econmizer bypassed. Figure out what's wrong and explain all the benefits of the economizer to the customer. Fix it.

  9. #9
    Join Date
    Oct 2012
    Location
    Los Angeles, Ca
    Posts
    5
    Quote Originally Posted by ryan1088 View Post
    You more than likely still have a short, it's just moved and is t touching whatever caused it to short. Good places to look are where the stat wires penetrate walls, panels, etc. Put in a resettable 5 amp breaker and start wiggling wires. Assuming you have checked all other components.....
    I checked out all wiring in the panel and also tried wiggling the t stat wires but my fuse didn't blow.

  10. #10
    Join Date
    Oct 2012
    Location
    Los Angeles, Ca
    Posts
    5
    This unit did not have an economizer it had one of those constant outside air dampers. I forgot to mention that I found a loose connection on the compressor contactor which was causing the wire connector to turn cherry hot red and was melting the wire insulation but this was on the high voltage part of the contactor. So I'm not sure if this could have caused the fuse to open.

  11. #11
    Join Date
    Oct 2012
    Location
    Los Angeles, Ca
    Posts
    5
    Quote Originally Posted by maxster View Post
    even with low voltage it is the initial draw on the TR that blows the fuse,do you have an economizer package running 24Vs off it,and check all
    contactors with power OFF for free movemant...cocked contactor will take a fuse right out...what type of stat if electronic it might be pulling on it long stat wire run also pulls that being out side the unit
    The unit had no economizer with a venstar t2800 Stat. Cocked contactor? Is that the same thing as having a pitted contactor ?

  12. #12
    Join Date
    Feb 2008
    Location
    petaluma,ca
    Posts
    48
    Is this unit direct or belt drive, if belt drive, follow low voltage wires over the top of the blower and check for chaffing. I have often seen the wires shorting on the 5/16 screw heads. Is the blower vibrating at all?

  13. #13
    Join Date
    May 2002
    Location
    evansville indiana
    Posts
    943
    I would bet on a wire rub ....
    mikeacman

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