A/C problem, thermostat has no power


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Old 08-05-09, 05:35 AM
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A/C problem, thermostat has no power

I have this split body A/C, that I haven't been able to turn on this season, when I noticed the thermostat does not draw any voltage, (thermostat is manual one without battery) I traced it up to the air handler unit on the attic.

The setup for air handler on the attic is rather simple, only thing b/w thermostat and the motor is a switch relay and a cylinder (capacitor ?) mounted on the motor. When I measured the relay incoming power of 110 volts, but no voltage on the coil side, so I thought it's the relay failure, but after replace it with a new relay with same rating- Honeywell R8222D , I still cann't turn it on.

I couldn't find 24 vac transformer anywhere in the circuit, nor could I find any fuse in the unit, and I followed the exact hookup as before. please help me with this, my wife is running out of patience. I am not very handy when it come to hvac, but I know basic electrical and mechnical concept and can do basic troubleshooting Thanks a lot !
 
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Old 08-05-09, 05:53 AM
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There has to be a transformer there somewhere. Find the t-stat wires in the airhandler and they should land on or near the transformer.
 
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Old 08-05-09, 06:46 AM
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As stated there has to be a 24v power source somwhere, be careful of what your doing, if unsure, could cause more harm then good.
 
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Old 08-06-09, 05:44 AM
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I traced the control circuit down to basement, and found a transformer there, and there's voltage on the secondary wires that goes to outdoor.

And it looks like the previous owner disconnected this wire from condensor, (they used window a/c to save money). now I have to connect this wire back to control circuit in the condensor. This 24 vac line has two wires: yellow and red, and control wire come out of condensor has blue,pink,and white, how shall shall I connect them ?

Thanks for your reply
 
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Old 08-06-09, 10:03 AM
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Can you post a picture of it?
 
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Old 08-06-09, 07:13 PM
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here's the condenser's picture, the power line come up in the middle, control line goes down to the righ bottom leads to air handler. That disconnected 24 vac line is not shown.

Thanks for your help
 
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Old 08-07-09, 05:17 AM
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yellow and red to blue and white
 
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Old 08-07-09, 07:19 PM
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Thanks for the reply, I've connected yellow to blue, red to white, the A/C turned on now !!! Both condensor and and air handler seems to run fine.

However, after I turn it off from thermostat, the air handler turned off, but the condensor kept running... Is it wiring problem or shall I troubleshoot the contactor ? (the pink control wire lead to the air handler/thermostat is not connected to anything here)
 
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Old 08-10-09, 05:59 AM
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Yes, check the contactor, it may be welded closed. That may be why it was disconnected. With stat off is there still 24v to coil of contactor?
 
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Old 08-10-09, 08:24 PM
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If the contactors' terminators are labeled as follow:
+----+
| A B |
| C D |
| E F |
+----+
E and F are from 110 V power, C and D are 24V across even with thermostat off. Because I've connected yellow to blue, red to white, C & D will always be 24 V across right ?

Thanks
 
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Old 08-10-09, 08:38 PM
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Try blue wire to y and the white wire to c at the furnace.
 
 

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