No 24v to my A/C condenser


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Old 03-15-10, 06:35 PM
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No 24v to my A/C condenser

About a week ago I noticed that if i set my thermostat upstairs to "COOL" and "AUTO" that nothing would happen. If I set it to "ON" the fan would turn itself on but it wouldnt blow cold air. I initially tried changing out the thermostat but that didnt seem to help. I opened the furnace in the attic and it seems as if 110V is getting to the transformer, 24V is getting to the thermostat, and 24V is making it to the fan (green) and heater (white) but when I set to COOL and AUTO it doesnt seem to be making it to the outside A/C condenser unit. I checked the float switch and it seemed OK, i even completely bypassed it and it didnt make any difference. After setting the thermostat to COOL/AUTO i tried to read 24V at the condenser and I didnt get a reading. I checked the high pressure side of the refrigerant and i have plenty of pressure, and i was able to activate the outside fan/condenser by pressing in the switch, so there wasnt a tripped breaker and everything mechanical seems to be working. So basically what i can find is that I am not sending 24V from the thermostat to the outside A/C unit.

Any ideas or things that i could test too see what the issue is? I would think that it couldnt be a problem with the transformer since everything else seems to be working, i bypassed the float switch, i replaced the thermostat with the one that works from downstairs and downstairs keeps working and upstairs doesnt, but i didnt get a reading of 24V outside (although i am starting to question that i did that right) and the A/C unit itself works if you manually engage the switch. Maybe the switch on the A/C unit, or something physical with the 24V cable from the thermostat to the A/C unit outside (although i can't see any damage).

Thanks in advance,

-dave
 
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Old 03-15-10, 06:42 PM
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Where are you measuring for 24VAC outside? The Contactor, (which would include all safeties and timers) or the 2 wire to the condenser.
 
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Old 03-15-10, 06:46 PM
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If your wiring runs back to your Air handling unit i would reference Y to C and see if you have 24 volts. If not it should be wiring or stat. If you have a spare wire taped off connect it under Y of thermostat and check at Ahu again for 24 volts between new wire color and C. If still no voltage i would consider changing thermostat.
 
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Old 03-16-10, 05:59 AM
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I was measuring at the 2 wire to the condenser for 24V. I made sure the thermostat was on AUTO and COOL and the temperature racked down.

I have identical thermostats upstairs and downstairs. I can switch between them any time, and the downstairs always works, and the upstairs always doesnt. It could be the base plate but I have tried reconnecting all the wires on it as well.

I dont have a spare wire, I only have R, Y, W and G running to/from the thermostat. Since I can get the blower and the heater to work, i should be able to make 24V when connecting R to W or R to G right? If i use a voltage meter and connect R to G, should the fan come on as well as show 24V on the meter? If that's true, i should also be able to connect R to Y and have the condenser turn on ...
 
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Old 03-16-10, 11:47 AM
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What i would probably do is is pull off the white wire put in on Y and check to see if you have 24 volts at the furnace This way you can use reference C on your terminal block. If you still dont have it at the furnace i would change the stat. Reason i say to check it at the furnace is it eliminates issues with the common wire as a reference
Meaning you could have a damaged common wire going to the condensing unit. Much more accurate using your C on your terminal block at your furnace for a reference.
 
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Old 03-17-10, 10:16 AM
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Fixed

Bad contactor switch on the compressor. As soon as the switch was replaced, all was good. /whew.
 
 

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