# Its that time of year, AC not working



## falconbrother (Apr 27, 2009)

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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## kok328 (Apr 27, 2009)

What are the voltage readings on the contactor coil for the compressor at the time of call for cool (24VAC)?


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## falconbrother (Apr 27, 2009)

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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## kok328 (Apr 27, 2009)

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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## falconbrother (Apr 28, 2009)

You rock dude.  Seriously.  Thanks!


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## falconbrother (Apr 28, 2009)

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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## kok328 (Apr 28, 2009)

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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## falconbrother (Apr 29, 2009)

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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## falconbrother (Apr 29, 2009)

Just so I'm clear, the control board controls heat and AC, with central air, correct?


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## kok328 (Apr 29, 2009)

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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## falconbrother (Apr 30, 2009)

My crappy old multi meter was working intermittently.  I bought a better one at Harbor Freight and will check out the transformer.  Thankfully it is still reasonably cool here in NC.  But, soon it's gonna get hot outside.  

Again, I really appreciate all the help.  I am one of those people who don't mind calling the service guy when I need to but, I really like to be, independent and able to fix my own stuff.  I have found , over the years, that with some mechanical ability and a knowledge of tools one can fix a lot of stuff.  Having the nerve to jump in their and try and knowing when your over your head and need to stop and call the man is the biggest battle.  I usually will push it a little bit and try things that are a bit scary at first.  In those cases I try to never do anything I cant reverse if I see that I am really messing things up.


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## falconbrother (May 4, 2009)

You were right, the Transformer was burned out.  I inspected the 24 volt wiring as best I could and replaced the transformer.  The unit worked with fan and heat.  But, when I switched to AC the outside unit did nothing and the new transformer burned up.  Nice!

I have a theory and I think I have to call the pro on this one.  Our house was built in 98 and it was equipped with a control so the city could cut out our outside unit during times of peak power usage.  They stopped using that system about 18 months ago, or more.  But, I think it has shorted out and screwed me.  

As far as I can see there is a red and a white 24 volt wire coming from the control board to the outside unit.  One of those wires goes to the contactor and the other is routed through the city's contraption.  Any, no call is making it to the outside unit.  If I close the contactor the outside unit spins up nice.  The issue is in the 24 volt system and I feel pretty sure that it is that junk the city insisted be placed on my house.  

Can I bypass it by just bypassing it?  There is basically two wires that act like a switch I suppose for the contactor control.  Do you think I could just close that circuit and let the calls for AC come straight to my outside unit from the Control board.


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## kok328 (May 4, 2009)

Yes, bypass the city's cut out.  There should be a brown casing wire that follows the copper refrigerant lines to the compressor outside.  The city spliced into one of the two wire inside the brown casing.  Just find where the wire enters the city contraption and exits the city contraption, disconnect those two ends and wire nut them together.  Now the 24VAC signal will reach the contactor coil and engage the compressor and fan.  You might consider either fusing the transformer or purchasing one with a built in breaker, just in case the city comes back and shorts you out again.


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## falconbrother (May 6, 2009)

Hey, the problem seems to have been a shorted contactor.  I have bypassed the cities contraption, they no longer use it anyway.  Thanks so much.


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## falconbrother (May 6, 2009)

I spoke with an HVAC friend of mine and he laid out the typical scenario for a bad transformer.  His opinion is that during the spring within the first few hot days lots of people turn on their AC.  The power company switches phases and sends a jolt of electricity down the line and capacitors and contactors get damaged.  The local HVAC supply says they have sold 1000 capacitors this spring.  Anyway, I have a contactor on order and it should be here today or tomorrow.  I replaced the transformer but, haven't put any power to the system till the contactor is installed.  That will be the moment of truth.  

I really appreciate your help.  You suggested early on that one of the first things I needed to check was the transformer.  I will admit that it wasn't one of the first things I checked.  Heck, I didn't even know where it was till I had to really start running down wires.  Getting to it looks hard but is very easy.  

Have a great day!


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## kok328 (May 6, 2009)

Good job!  Have a cool summer.


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## RichCPZ (Sep 24, 2010)

Hello, I was pleased to find this post as, I am having the same problem and the same air handler unit. I blew two transformers. Lucky they were only 9.88 each. So, there is a short going on some where.
I don't know if it may be the circuit board or not, I even had a HVAC guy out and he installed a new thermostat, power switch,  a fuse on the line and his transformer blew also, he said it maybe about 500.00 bucks to replace the board , so I was glad to see you found it for 90 bucks on ebay. Here in Atlanta I have a HVAC store I can visit first to see if they have the board. But, was curious about the outcome of your problem. Did it turn out to be the board or was it the "contactor" 
I would love to fix this myself since I'm already about 200 bucks in the hole with the HVAC guy..Whom, I haven't been billed by yet, I just asked him for a price on a new unit and was told 4700.00 bucks "ouch" but, still not back with the 1500.00 tax break. So, I'm in a toss up! 

Thanks, for any help


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