Air Conditioning - Walk me through a diagnosis

Hi guys, I have a Trane xr12 unit that does not come on. When I lower the t/stat it says that the unit is on but the airhandler just hums like there is power going to it but the blower is not coming on. Also the condenser on the outside of the house does not come on. I checked the house circuit but it was not tripped and the fuses outside in the box do not appear to be faulty( no red light indicating they are bad). What's next? What is the order of start up? Does the a/h come on first and then the condenser or vise versa? I am just trying to figure out where the problem might be because like I said there seems to be power going to the a/h because it hums so I'm thinking the problem is on the outside and that's where I should look first. Thanks for any help.

the condenser/outside and the furnace /air handler should be two seperate circuit breakers.the control voltage that the stat controls will be off the transformer(TR) in the air handler.first go out and check the disconnect for the condenser for power(even if the CB is ok)pop the cover on the condenser control section and bump/push the contactor in condenser and compressor should bump...if they run your missing the 24Vs out from the stat and that TR in the air handler.up on the stat try to run FAN ONLY if nothing happens your TR might be shot...can you check the yellow/blue wires coming out of it for 24Vs with a meter.the TR should be warm if you grag it if it is cold or room temp it is probably open on the 115V or 24V side.easy change out...get back with the results i'll check backBeer 4U2

Sminker thanks for the reply. What does the transformer look like in the a/h?. You said it has a yellow and blue wire coming out of it? Where should I check for voltage on the wires at at the stat end or the transformer end? Also, what does the contactor look like. I've never worked on a/c before but I'm pretty handy so with a little help I'm sure I can get this thing running again. Once again thanks for the help.

http://i59.photobucket.com/albums/g314/tomd7735/Fancenter.jpg
Yours may be similar to this, but it may be built into a PC Board. Look for the low voltage terminal strip with the letters "R,Y1,Y2,G,C" on it and check for 24v between R & C or R & ground. You might be able to smell a burnt transformer, or there might be a fuse or reset on the secondary.
Tom Beer 4U2

Thanks HVAC MECH. I' going to check that now. The contactor in the outside unit is good. I pushed the button in and the fan came on so I'm thinking it is the transformer like Sminker said, but I'm going to check it. Thanks for the picture , it helps out alot and thanks for the info.

Have you tried the James' HVAC diagnostic chain (http://www.longviewweb.com/visit.htm)?
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Ok I tried to check the transformer but I really do not know what this damn thing looks like. I took off the panel and found 3 different pieces where wires are going to and from. The one thing that I think is the transformer looks like a magneto on a lawn mower, it has about forty thin flat pieces of metal stacked on top of each other and 2 yellow wires ,2 black wires , 1 red wire and 2 blue wires coming out of it. I assume that the black wires are grounds, the red wire is hot and the blue and yellow wires are for the t/stat. Is this correct or do I have the wrong piece. The other two parts do not appear to have a transformer "look" to them, they look like they are just male and female connectors that join wires together. Good news is that there is no circuit board in the a/h so I can eliminate having to look for that. Thanks again.

your new puppy has chewed the low voltage wires at the outside unit cut off the power and wire them back together then put the puppy in a cage

The wires on the outside look good. I did not check for 24 volts on the outside unit though, I just pressed in the contactor button and the fan came on so I figured everything on the outside unit was ok from reading the prev posts.Thanks for the replies, we'll get this thing running before the extreme heat hits, it's got to be the transformer or the contactor.

well shoot I was so sure of my diagnosis. If you have a voltage meter you will need to check for 24 volts on the low voltage wiring, did you say that the air handler is not running also? First lets check at the t stat what have you got from R to C and R to Y with the t-stat calling?

When you pressed in the contactor, did it stay in by itself, thereafter? Or did you have to hold it in to keep the fan going?

well shoot I was so sure of my diagnosis.

:D

Don't ya hate that when that happens?

Sorry for the lack of response guys but I've had no time to check back to the forum due to work, but anyway, in response to your question ecman51, the contactor did not stay in when I pressed it. It popped back out and the unit shut off. Here is a list of what happens when I call for cooling,1: power is getting to the a/h but it hums and the blower does not come on.2: I checked the contactor outside pushed it in and it turned the fan on but when released the fan cut out.3: I didn't check for voltage yet because I'm not sure where to start to check for voltage.Do I start at the stat then to the transformer in the a/h then to the contactor outside or vise versa . Or is there another sequence that I need to follow? Again, sorry for the delay in responding, I really do appreciate all the responses to my question. Thanks for the help. P.S. The t/stat is for a/c only there is no furnace connected to it.

There are three wires hooked up to the t/stat, red ,green, and yellow. When I touch the yellow and red the reading I get is 24 volts. But when I lower the stat and call for cooling I get nothing. Also something interesting just happened when I lowered the t/stat the unit came on and is running. SOOOOOO now I am really lost. Any suggestions? Thanks

red wire is 24 to stat--yellow is cooling--green is to inside fan. red to yellow reading 24 means stat not calling for cooling.[open circuit].Turn cooling on and reading '0' volts means circuit is closed and you should have 24 V to outside unit.If it started after you fooled with stat,it could be a loose wire or flrky stat.

My typing is BAD ...my wife thinks its very amusing to watch me..I meant flaky stat.

red wire is 24 to stat--yellow is cooling--green is to inside fan. red to yellow reading 24 means stat not calling for cooling.[open circuit].Turn cooling on and reading '0' volts means circuit is closed and you should have 24 V to outside unit.If it started after you fooled with stat,it could be a loose wire or flrky stat.


Kilowatt, is this the way the circuit should run? If I got those readings then the circuit is working properly and the problem should be on the outside right? Although I turned the a/c on all day today and it is running fine. I'm confused right now, I guess I should leave it alone if it continues to work . It was just weird how one day it worked and then it stopped and then it is working again. If it fails to work again I guess I'll post back. Thanks for the help.