Mis-wired system?? outdoor a/c unit not coming on


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Old 04-25-11, 08:23 PM
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Mis-wired system?? outdoor a/c unit not coming on

When switched to Cool and the temp. is lowered, the fan comes on but the outdoor a/c unit does not. I've checked that the outdoor unit is getting 240 volts with a multimeter. The low voltage wiring is as follows:

At the thermostat: R to Rh with jumper to Rc; B to B; W to W; Y to Y; G to G

At the GMC a/c unit: R to Yellow wire on unit; W to Blue wire on unit

At the Ducane furnace: Two wires from a/c unit - R to Yellow terminal; W to C terminal

Five wires to thermostat - R to R terminal; B to C terminal; W to W terminal; Y to Y terminal; and G to G terminal

Any suggestions? I suppose replacing the tstat is the best way to start troubleshooting. I can use the multimeter to test low voltage but which wires should I test?

Thanks!
 
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Old 04-26-11, 04:49 AM
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So, at this point I've determined that the blue wire at the tstat and at the furnace don't need to be connected for 1 stage cooling with 1 stage gas heat. Still not working but I'm trying to simplify and eliminate potential errors.
 
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Old 04-26-11, 07:06 AM
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If you get 240V at the outside unit, your low votage wire is OK (another word, the contactor is working, but to be sure, check the contactor again anyway). at this point, I suggest you check the capacitor first before replacing Tstat.
 
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Old 04-26-11, 08:55 AM
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I may be missing something - the a/c unit is direct wired to the panel through a disconnect box so it has 240v at the terminals regardless of low voltage factors.
You mentioned a contactor - I should test that the 240v is making it through the terminals to the contactor, which requires signal from the low voltage wiring, correct?

This unit was installed in January and has never been run. Unfortunately, the contractor is in the wind. So I wouldn't expect anything to need replacement. I'll do more research on the contactor and capacitor but I'll also check the low voltage wiring to make sure that when the tstat is in "cool" mode, 24v are present.
 
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Old 04-26-11, 04:23 PM
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check for 24v at the coil side of the contactor. you do not need to disconnect anything for this. if you have 24v: your contactor should "pull in" and send power to your unit. if you cant tell whether it is pulled in just check for 240 on the load side of the contactor (opposite the line side which is the side going directly to the disconnect). Its sounds like your 24v wiring is incorrect. Do you have a heat pump?? usually O/B is your changeover valve for heat pump operation. rh and rc is your transformer voltage. w terminals would be for your heating circuit, y termianls are for your cooling. its hard to know exactly without seeing it, because some people decide using different colors is the right way to do things. check for 24 at the contactor first, if you have 24 then you cut you possible problems in half. where are you located?

contactor is inside the outdoor unit just so you know.
 
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Old 04-26-11, 04:50 PM
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It is a gas furnace. As I understand, rh and rc are for the transformer, g is for the fan, y is for cool, and w is for heat.

I found the contactor and capacitor and I believe I understand how they operate. I did not detect 24v on the wires themselves when I removed the nuts (before I read your post) but did not test the contactor yet. I've managed to get an acquaintance who works for an HVAC contractor to take a look Saturday morning. I'm going to keep my hands out of there until then. I will let you know what happens. By the way, I'm in Baltimore, MD.

Thanks again - I'll post Saturday, hopefully with good news.
 
 

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