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Is the continuity between these terminals on control board normal? Keep blowing fuses

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  • Is the continuity between these terminals on control board normal? Keep blowing fuses

    I have a carrier furnace and central A/C unit that keeps blowing the 3 amp fuses on the control board. After replacing it once, it ran again for 4 days, then I replaced it again and it blew immediately.

    Using a multimeter, I can tell I have continuity between the terminals as shown with the thick red line in the following image. Power if off to the furnace/blower, but the thermostat is on and I switch between "off" and "cool" Are any of these states abnormal?

    Image attached
    Last edited by welliamwallace; 07-03-2014, 07:45 AM.

  • #2
    i cannot see any image.

    with the power off, the thermostat set to OFF, and the wires disconnected at both the thermostat and the furnace control board-

    - there should be no continuity between any 2 thermostat wires
    - there should be no continuity between any wires and ground

    - there should be continuity between the contactor wires.
    - there should be no continuity between any thermostat terminal and the Common wire
    - in the Cool mode there should be continuity from R to G to Y terminals on the thermostat

    Intermittent shorts are usually caused by worn insulation, pinched wires on housings or staples or a failing contactor coil.
    Last edited by Plumber Punky; 07-02-2014, 09:46 PM.
    ~~

    ... it was plumbed by Ray Charles and his helper Stevie Wonder

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    • #3
      Not sure why my link isn't working, and it won't let me edit.
      See continuity image here: http://i.imgur.com/aZrMNjF.png

      This is of course with all wires still connected to the t-stat and control board, 3 Amp fuse broken.. It looks like there is no Common wire between the thermostat and the control board. (The wire connected to the Common terminal on the control board goes to the outside condensing unit)

      Question:
      -When you say continuity between the "contactor wires" Is that the two wires coming from the outside condensing unit?

      In the cool mode, there is NO continuity between the R and the Y contacts (either on the control board or the thermostat), but there is between R and G. Again, see image.

      Does any of this give a hint where the short may be?

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      • #4


        Yes, the contactor wires go to the outdoor unit. They connect to the 24 vac coil and should have continuity. Resistance will vary depending upon the manufacturer of the coil.

        With the fan and ac off you should have nothing between Y and G on your thermostat terminals.
        ~~

        ... it was plumbed by Ray Charles and his helper Stevie Wonder

        Comment

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