Condensing Unit Contactor Voltage


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Old 10-24-09, 01:16 PM
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Condensing Unit Contactor Voltage

Hello:

My contactor does not pull in. I checked the voltage it is 20 volts.

Is this enough for the 24 volt contactor to work or do I need to check elsewhere for why the voltage is 20 and not 24 volts?

Thanks.
 
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Old 10-24-09, 01:29 PM
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If that is the static voltage...I'd say you probably need a new transformer. I'd disconnect the leads from the xfrmer and see what the voltage is.

No expert..just what I'd do.
 
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Old 10-24-09, 02:11 PM
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20 volts is with the leads disconnected from the contactor.

I think the problem is not with the contactor as I jumped the contactor with the leads from the other air con unit. This unit was putting out 25 volts and made the contactor contract.

So, 20 volts is definitely not enough for the contactor to contract.

Back at the thermostat for the problems unit, the fan circuit is reading 25 volts, but the AC circuit is reading only 20 volts.

How can this be? Two different transformers?
 
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Old 10-24-09, 02:11 PM
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If you have a 24V coil on the contactor and you are showing 20V across the coil and it isn't drawing then it's either the coil or the voltage that is the problem. The cheapest and simplest solution is to use another 24V source and connect it to the coil independently and check for draw action. From there the diagnosis is pretty simple.
 
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Old 10-24-09, 02:16 PM
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Sounds to me like some faulty wiring, poor connectivity through the the terminal connectors (corrosion possibly?) or possibly a bad thermostat?

I would start at the source of the 24V power and start working my way down the circuit until I found the problem.

If you find the problem is in the t-stat maybe try asking this question in that section and describe the t-stat you're using. Doubtful you're the first one to experience such a problem if it IS a t-stat. The question is will anyone who knows of the issue check the post
 
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Old 10-24-09, 02:18 PM
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Yep...I've read of different supplies for different modes (heat vs A/C)...again..no Pro..just helpin out on slow days.

Sounds like a trans...
 
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Old 10-24-09, 02:36 PM
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Thanks for the help so far. Please keep it coming.

Since its not the contractor, it is most likely the wiring or the transformer. Either way, I need to find where the transformer or transformer circuit might be.

I can't find a standalone transformer like on my previous house.

So, where is the mostly likely place that the transformer/transformer circuit for this system might be?

Thanks.
 
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Old 10-24-09, 03:05 PM
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ACKKKK Lost a reply...

Anyway..most that I have seen are mounted near or in the box with all the wires leading to it near the furnace/air handler.

If you post the brand/model of all the units..it may help the Pro's when they finally get away from the seervice call/race/game that they are on.

Its kind of one of those busy times of year for them.
 
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Old 10-24-09, 06:19 PM
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I would guess if you follow the 115V line voltage it will lead you to the transformer(s) as they have to receive their power from a 115V source before they can transform it to 24V and send it anywhere. If you start testing voltage "downstream" of the 115V supply until you find where it changes to 24V then you will find your transformer.

I'm assuming you don't have a manual for the unit? Most manufacturer's will have manuals available for download on their sites and most manuals will show a full wiring schematic. This may be useful as well if you know how to read it.
 
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Old 10-25-09, 08:50 AM
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It's pretty easy to jump out your stat.

Remove power to your furnace, pull the stat, write down the existing terminal = color combination, and jumper R (24vac), G (fan) and Y (condenser).

Restore power to your furnace. If you then get 24VAC and the contactor holds, it's only your stat.
 
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Old 10-25-09, 02:34 PM
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In my thermostat, you can pull off the front panel exposing the electrical connections.

The red and yellow wires for the cooling circuit read only 20 volts.

I am going to try to trace back these wires to the transformer.

Is there any possibility that the transformer is located somewhere in the condesing unit?
 
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Old 10-25-09, 06:31 PM
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Most systems have the transformer in the furnace.
24 VAC 40VA transformers are inexpensive.

You might unwire all field installed low voltage wires from your furnace and measuring R to C at that point. (Write down the wires at each terminal before removing any wires)
 
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Old 10-27-09, 01:16 PM
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Thanks.

I'm going to dig around in the furnace unit.
 
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Old 10-27-09, 01:40 PM
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Most of the time the transformer will simply die it is a rare bird that outputs low voltage most likely your new puppy or friendly hungry squirrel has chewed the tstat wire a bit. Find the bad spots and repair them or pull a new wire and you should be good to go.
 
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Old 10-31-09, 06:07 AM
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If it helps, I just replaced mine. It was located in the furnace in the garage. Symptoms started with the gas valve sometimes not opening (thought it was the gas valve, voltage read 23-24 volts every time I checked), then a few days later horrible smell-nothing. Transformer died. Replaced transformer ($17), gas valve opens every time now. I'm thinking that the output must have been fluctuating before it went bad completely, weird for a transformer.
 
 

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