No compressor or condenser fan when A/C is turned on


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Old 03-13-19, 05:34 AM
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No compressor or condenser fan when A/C is turned on

Looking for some help here as I am lost. When my thermostat senses a need to turn the AC my air handler inside comes on, but the compressor and fan on the outside unit does not. I can see that the contactor isn't pulling in, and the compressor and fan comes on when the contactor is pushed in manually which led me to measure for the 24v on the sides of the contactor. I measured and got 0v and 11 ohms across the contactor. I twisted the r/g/y wires together at the thermostat to rule that out and got the same result. Where does that 24v come from? I have a schematic on my furnace door, but it's too degraded to read.
 

Last edited by pbedens; 03-13-19 at 07:34 AM.
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Old 03-13-19, 06:27 AM
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The 24VAC for the contactor comes from the furnace. I would suggest that you check for 24VAC where it comes into the outside unit. Usually there are wire nuts where the cable bringing the 24VAC from the house connects with the internal wiring in the outside unit. If you have 24VAC at the wire nuts but not at the contactor, you may be low on refrigerant, assuming you have a low pressure switch. If you don't have 24VAC at the wire nuts, you may have a break in the cable (an animal may have chewed through the cable).
 
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Old 03-13-19, 07:27 AM
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Thanks. It's a pretty old unit so I'm not sure about the low pressure switch, but I'll check today. Any other things I should check?
Another question, I measured the voltage at the contactor on the second outside unit (I have 2 separate ones) and it only measured 15v, but that unit is working fine. Does this seem off to anyone?
 
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Old 03-13-19, 09:01 AM
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How did you measure it? Across the coil or from the coil to ground?
 
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Old 03-13-19, 09:58 AM
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I measured it across the coil.
 
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Old 03-13-19, 10:08 AM
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Be certain your meter is working correctly. And getting a good solid connection with the conductor.
15V would burn out the contactor quickly.
 
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Old 03-13-19, 10:25 AM
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Hi, do you have a float switch in the condensate pan in the AH? find out where the wires going out to the condenser are connected inside the AH and test for 24 VAC there.
It not advisable to manually push in that contactor for the compressor in, a shorted compressor can give a pretty good ARK.
Geo
 
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Old 03-14-19, 04:04 AM
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I measured incoming power on both outside units and it's reading around 145v on both so I'm sure my multimeter is trashed. I did measure the low voltage wires coming to the outside unit and I have the same 15v as I do on the second unit (even though the voltage is wrong it shows that they're the same) so it has voltage coming out. I don't have a float switch on the pan for this unit. I have a pressure switch on the outside unit, but not sure if it's a low or high pressure as there is only one.
 
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Old 03-14-19, 04:16 AM
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You’ll need a new meter to continue troubleshooting.
Does polarity work? Could be a lead issue.
 
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Old 03-18-19, 01:41 PM
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Well, after buying a new meter and troubleshooting everything I could think of I broke down and called a tech. turns out the system is completely void of refrigerant. Here's to hoping the hole in the line is at least accessible without having to tear anything out. Thanks for all the help.
 
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Old 03-18-19, 02:00 PM
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I wish you luck. The typical place for a leak to develop is at the evaporator coil inside the furnace.
 
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Old 03-18-19, 03:05 PM
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They checked the evaporator coil and got nothing. i'm having a roof put on and had the sheathing re-nailed and I'm hoping a nail didn't go through into the refrigerant line since it runs up the outside wall through the second floor joists and up into the attic.
 
 

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