DIY Home Improvement, Remodeling & Repair Forum > Appliances > HVAC > Its that time of year, AC not working




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Old 04-27-2009, 08:17 AM  
falconbrother
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Default Its that time of year, AC not working

Hi all, I have an Air Conditioner problem and I think it's the control board. I have a gas pack and AC. The heat worked fine all winter. The Compressor unit outside is a Goodman that was replaced a year and a half ago. The blower is a Trane and it was installed in 1998. About two weeks ago I turned on the air to test it. The blower came on but the compressor did not. I waited a day. Turned the system on and everything worked great. I tried it on Saturday and nothing at all. I believe that the thermostat is not the problem. I had a good thermostat on it and tried a new one. Still nothing. My next try will be to see if I can diagnose the control (circuit) board. Apparently these things self diagnose with a green or red light. I check all circuit breakers. They are fine. I checked all switches, fine. My guess is now that it's a possible short or the control board. I believe that the outside unit is fine but, is not getting a command from the control board to turn on.

Any thoughts??



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Old 04-27-2009, 10:38 AM  
kok328
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What are the voltage readings on the contactor coil for the compressor at the time of call for cool (24VAC)?



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Old 04-27-2009, 12:32 PM  
falconbrother
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I haven't checked it yet. I planned to do that today. HVAC is pretty new to me. I was an aircraft mechanic for 5 years and do all my own car repairs, but HVAC is a bit of a stranger. I followed the repair guy around the last time it was worked on (year and a half ago) and it seemed, overall, pretty logical. I will check it out. Suppose it reads 24 volts, what does that tell me?

I dont think the compressor is getting a call at all. That's my gut feeling. The compressor has about one and a third summers on it. I cant imagine that it is the problem.

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Old 04-27-2009, 01:44 PM  
kok328
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Working backwards from the compressor:
The compressor is 240VAC. It is held out from running via a contactor. The contactor has a 24VAC coil on it that, when is energized by the t-stat (call for cool or heat on a heat pump unit), the coil closes the contactor to engerizes the compressor. If your not getting the 24VAC signal to the contactor coil then the contactor won't close to complete the circuit to engage the compressor. The T-stat in turn gets it's 24VAC power from a 120VAC-24VAC step down transformer located in the furnace cabinet. The T-stat directs this voltage to the appropriate terminal in the terminal strip inside the furnace cabinet depending on the T-stat setting. If you have 24VAC on the red wire from the tstat, then the transformer is working. If you suspect the t-stat, you can temporarily jump R,G,Y under one wire nut and the system should be making cold. If not, let me know which component is not working (condensor, condensor fan, blower fan). Lastly, if applicable, you may have an open pressure switch, indicating a leak and/or a need for a freon charge. Let me know what you find.

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Old 04-28-2009, 02:04 PM  
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You rock dude. Seriously. Thanks!

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Old 04-28-2009, 02:17 PM  
falconbrother
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If this matters, I did climb up in the attic and open up the furnace/blower. I turned off the power. Opened her up and turned the power back on and held the safety switch down to see if the green or red light was on. There is no light at all. The whole thing is dead.

I have, in the past, had an old Singer outside unit that bugs would get in and clog up what I assume is the contactor. I turned off the power and freed it up. Everything worked good after that. I did notice that the blower would still work when the outside unit wouldn't come on. Of course, that was a heat pump and I have a gas heat and AC now so it might be different. I did take the cover off of the outside unit and looked around. I didn't put a multi meter to it though.

The control board for my system is like 89 bucks new on ebay. It looks slightly different but, I cross referenced on two web sites and it looks like Trane (White-Rogers) has a new and better board that supersedes my old one.

My system was installed in 1998. Wouldnt a board from that time period have the green/red light indicator? The board is a CNT 2181 which has been replaced by CNT 3797.

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Old 04-28-2009, 04:05 PM  
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Hey, no problem, glad to help. However, from reading your last reply, I'm not sure if your up and running or not.
If your saying that the whole control board is dead, then check:
- Transformer is working?
Check for 120VAC on the primary side of the stepdown transformer and 24VAC on the secondary side of the transformer (120VAC in, 24VAC out).
-Also locate the main power leads to the board and verify power is going into the board (may be 120VAC or 24VAC).
- Fuses
Look for a board mounted fuse. It might be the blade style or the glass tube style.
- overloads/breakers/limit switches (some transformers have a manual reset on overload)
- Loose or corroded connections to the board.

Let me know what you find. Hopefully, it's something simple and not a circuit board.

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Old 04-29-2009, 11:36 AM  
falconbrother
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I checked power to the board this am before I left for work. It is getting 120VAC just fine. Nothing is coming out of the other side. I wasn't really looking but, I don't remember seeing a fuse on the board at all. I also checked the safety switch and it checks out fine. At this point I feel pretty certain that the board is bad. I will check for a fuse when I get home. Thanks again.

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Old 04-29-2009, 11:46 AM  
falconbrother
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Just so I'm clear, the control board controls heat and AC, with central air, correct?

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Old 04-29-2009, 01:36 PM  
kok328
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unfortunately, yes, the control board is needed for both heat and a/c.
what about the transformer? the controls that tell the unit to engage heat or a/c are all 24VAC which comes from the secondary windings on the transformer. obviously if the transformer is dead, nothing will work. just hate to see you buy a board when you may not need one, it kinda defeats the advantage of this website.



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