Central ac not working


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Old 05-10-15, 11:43 PM
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Central ac not working

went to turn my ac on for the first time this year and nothing just a slight buzzing from the outside unit. so I pulled the electrical panel of and contactor is closing have 27 v from signal wires. But when I test power at power wires I have 120v at both wires the ground but when I test between the wires it reads 0v and if I understand right it should be 240v.

any help is greatly appreciated
 
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Old 05-11-15, 05:21 AM
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If you have 27VAC at the contactor signal wires, 240VAC across one side of the contactor (input), but 0VAC on the output side, it would appear that you have a bad contactor. The contactor is a relay. The 24VAC (nominal) is the control signal. When the thermostat calls for cooling, it sends 24VAC to the signal wires. This energizes the contactor (relay) and closes the circuit, thus sending 240VAC to the compressor and condenser fan.
 
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Old 05-11-15, 05:33 AM
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Bob, it appears he has lost one leg of his 240 volts. He has 120 volts to ground, but 0 between the two legs.


drebinpk, turn off the the disconnect on the wall near the A/C unit and measure the line wires coming into the disconnect to see if you have 240 volts there. Again, make sure you have the disconnect off so that there is no power going to the A/C unit. While you are in the disconnect, see if there is fuses. If there is, pull them out and check them with your meter set to Ohms.
 
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Old 05-11-15, 05:44 AM
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Tolyn, if the contactor is a single pole switch (and open), it's possible to still read 120VAC from either side of the contactor (output side) to ground, yet not have 240VAC between the two wires. If the contactor is a double pole switch, then you're correct, it's likely not the contactor based on the description.
 
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Old 05-11-15, 05:51 AM
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Didn't think of the single pole switch aspect. I only recall seeing double pole contractors in A/C units so I now see where you are coming from.
 
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Old 05-11-15, 06:55 AM
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But my only thing is if i lost 1 leg wouldnt i get the 120v on only one if thw wires straight to ground nd noth both. They both have 120v ac straight to ground from both sides of contactor with contactor on
 
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Old 05-11-15, 07:01 AM
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You shouldn't be measuring the voltage referenced to ground. You should be measuring the voltage across the input and output pairs of wires. Look at message #8 in the post linked below for a picture of a typical contactor and the voltages you should be reading.

http://www.doityourself.com/forum/ai...y-running.html
 
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Old 05-11-15, 07:42 AM
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If you only have one leg, the voltage of the good leg is going through the compressor if the relay is pulled in and that is what you are reading. You could also have a bad contactor if one side of the contactor is not making contact. This is why I said to check voltage at the disconnect LINE side. This will tell you if you have a voltage issue or there is something wrong with the unit.
 
 

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