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    mwattigney1's Avatar
    mwattigney1 Posts: 1, Reputation: 1
    New Member
    		  
    #1

    Aug 4, 2010, 09:26 PM
    A/c contactor has 24 v on the red wire to grnd and 24 volts on the black wire to grnd
    This is causing the relay to not pull in due to both wires being positive. But the other 25% of the time, the black wire becomes common ground and the relay to work correctly without touching anything. Could this be caused by a faulty circuit board?
    wmproop's Avatar
    wmproop Posts: 3,749, Reputation: 91
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    #2

    Aug 4, 2010, 11:11 PM

    What are wires tagged to on the other end, in the furnace/airhandler?
    One should tie to common,the other to yellow post or yellow wire from the y post in the tstat
    NorthernHeat's Avatar
    NorthernHeat Posts: 1,455, Reputation: 132
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    #3

    Aug 5, 2010, 03:06 PM

    Something is wired in series with this circuit and is opening.
    24V into contactor will read 24 V out of contactor until the output is allowed to go to ground. So a wire is loose or a pressure switch or other safety device is opening not allowing voltage back to neutral/ground.
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