A/C or Heat doesn't work in "on" or "auto". Fan will not come on at the "on" position either. I changed the batteries in the thermostat. I also read somewhere to disconnect the "Y" and hold it to the "R" on the thermostat for two minutes. That will tell me if the thermostat is the problem. Still nothing worked. I checked to make sure the outlet the furnace was plugged into had power... it does. I then removed the cover from the circuit board on the central air unit. It is a Ducane, it has a Honeywell circuit board. I looked at that wiring for a little while and decided to remove the in coming hot and the "cool" wire leading to the fan. I wired those together and the fan came on. I then (by mistake) put the cool lead back in place but accidentally put it on the "continuous on" lead. The fan turned on and wouldn't turn off, can you imagine?:) After that I cut power and put it back the way it's supposed to be... nothing. Does this mean the circuit board is bad? Or a relay on the board? What is the next step? Replace the board? Or am I completely off base?
Thank you for any help you can give.

Jumper R to G at the unit. (do not forget to make sure door swith is closed, use tape) If the fan runs go to the thermostat jumper R to G. If the fan runs from the board and not the t-stat you have a bad wire connection from t-stat to unit. If the fan does not run from jumping R to G at the board it may be a bad board or relay let us know.

So not R to Y?

Y makes the outside unit come on. G makes the blower run.

Ok, here is the deal, I have done R to G, and R to W. I did not do R to Y. I have also switched the "hot" black wire on the circuit board marked "cool" to the "cont" and the fan comes on to run continuous. Also, I have checked that the safety switch for the door access panel is functioning properly so that isn't the problem. If I have taken these steps is it safe to assume that the circuit board is the problem and needs to be replaced? What does the transformer do? Could it have any part of this? Please give any advice on what to check next to make sure I don't order a $200 part and it not be the problem. I sure appreciate it.

Also, if it is the circuit board, it is an honeywell st9120c. I found online this circuit board and it appears to be the same. Does it make sense to you?
Furnace Control Circuit Board. (6 Pin Molex Plug) (Armstrong, Honeywell, Lennox): American HVAC Parts (http://americanhvacparts.com/Merchant2/merchant.mvc?Screen=PROD&Product_Code=CIB9120C2028&Category_Code=)

It is not smart to buy a circuit board until you have exhausted all other possibilities for problems. You say you did R to G and R to W but you didn't say what happened when you did that. So what happened when you connecter R to G on the circuit board.? The main blower motor should have started, Did it?? When you touched R to W on the circuit the hot surface igniter should have begun to glow if all things were right. So if these things didn't happen then it is possible that it is the circuit board if it is not the transformer.
You ask what the transformer does. It changes 120 volts into 24 volts. Your furnace has two different wiring systems. The high voltage operates the two fan motors and the 24 volts operate all of the controls. Also something you have not mentioned is a fuse that is on the circuit board. If you have a short in your low voltage wiring this fuse will blow.

I really appreciate the help, nothing happened with the R to G and R to W combos. The 5 amp fuse is in tact on the circuit board as well. What else can I do? How do you check the transformer?

With the power off pull the 5 amp fuse and look at it close. Is it burnt in the middle? If not put it back in place and turn power on check voltage from R to C if you do not have 24 volts check sec 1 and sec 2 on the control board if you do not have 24 volts check L1 and L2 do you have power(115 volts)? If you have it at L1 and L2 and not at sec 1 and sec 2 you have a bad transformer. (power needs to be on and the bottom door switch closed to do this test) If you have it at sec 1 and 2 and not at R and C you have a open limit switch. Good luck and let us know.

Awesome, I'll get on that and let you know. For now, I'm off to shmooze at some banquet. Thanks a bunch.

Here is what I've got and don't got.
Fuse is good.
No Voltage at R and C. If I understand what SEC 1 and 2 is, There is no voltage there. I checked Sec X (the 24v output from the transformer, no voltage there. I have 115 at what is calls "s1" and "s2" on my circuit board (s1 being the main power supply to the board, s2 being the output to the transformer. Basically, 115 going into the transformer, nothing coming out.

So... I need a transformer? Or I need to double check something?

Thank you so much.

If you have 115 volts in on the transformer and nothing out -bad transformer.

Force air furnace won't turn on to heat home. It has been working good but srarted tiday then stopped now won't come on. What can be the problem?? I have an electric heating system

I found this other question and answer I copied and posted below. Do I need a fuse before my transformer for any reason, or does that 5 amp fuse handle that? Also, does the replacement need to be identical? Where is the best place to get one? Radio shack?

Thanks again for saving me so much money.



BobkatAug 7, 2005, 09:32 PM
My old lennox hvac transformer blew, it was a Basler be11349rcd29, 120volt to 24 volt 50/60 cycle, it has a white and a black wire comeing from it, I want to add a fuse to the correct line, I don't know much about electric stuff, how or where do I find a correct and affordable replacement part, and to where and how do I attach the fuse, or would there be a replacement unit with a fuse box on it, a good useful part number replacement would also help,
Thanks

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labmanAug 8, 2005, 01:20 AM
Do you have a Radio Shack nearby? If so, go buy a 273-1512 heavy duty transformer, a 270-1281 inline fuse holder, a pack of 270-1005 one amp fuses, and maybe 64-3057 assorted wire connectors. These are out of an old catalog, so I can't promise they haven't changed the part numbers. Also, make sure if the old transformer has a amperage marked on it, it isn't over 2 amps output.

If you don't have a Radio Shack, you may be able to find the stuff at a hardware store or home center. Auto stores may have the fuse holder you need. The part numbers I gave you are for the traditional 1/4'' fuses. You may have to buy 5 mm ones now. Just make sure the fuses and holder match.

Shut the power off to the furnace. The transformer should have 2 black wire on each side, plus a yellow wire one. On the side with only the 2, connect one to the white wire from the furnace. Connect the black wire from the furnace to the fuse holder, and the fuse holder to the other wire on that side. Put in one fuse. Ignore the yellow wire, maybe capping it with a wire nut so it never connects anything. Connect the 2 black wires on that side to the red and blue wires to the furnace controls. You have a new, fuse protected transformer likely all for less than $20.

If you have power at the transformer.(115 volts) and nothing on the low voltage side- two wires leaving the transformer to the control board. You have a bad transformer.

T-top, what I'm asking is, when I replace the transformer, with a new transformer, should I add an inline fuse for any reason... like the guy was talking about above?

AND does the replacement transformer need to be to the same specs? Where do you get something like that?http://i175.photobucket.com/albums/w129/hartrobinson/DSCN3907.jpg

I've gotten a transformer and installed it. Now, the fan will run, but as soon as I turn the thermostat to Cool. The 5 amp fuse blows. Is something with the outside unit causing that? What is the next step?

All right. You have a short in the pair of wires that runs out to the relay for the compressor. A small chance the relay coil is shorted. All the relay needs is a pair of wires running from the Y and com terminals at the furnace out to it. Sometime they run a multi conductor cable and use the yellow and blue wires. Sometimes they run a pair of any colored wires. Trace the cable and repair/replace as needed. You may be able to switch to other good conductors in the cable. I learned the inline fuse trick after chipmunks chewed the insulation off my thermostat wiring in an inaccessible place.

Can't resist this, I have had worse chewing problems since.

http://www.photolocker.net/images/Labman/KatetheAC.jpg

force air furnace wont turn on to heat home. it has been working good but srarted tiday then stopped now wont come on. what can be the problem??? i have an electric heating system

Go to the sticky at the top of the forum. Work you way through it, and if you don't find your answer, post back with details. Do you have power, a flashing red light, relays that click, are the breakers all on, etc.

Labman, t-top-letmetellu,

You were very helpful. Labman, you hit the nail on the head, it was the coil. Turns out, according to the guy I had to get to help me, the coil failed, and had an internal short. The short fried the transformer. All of this happened without blowing the 5 amp fuse initially. Then, with the new transformer, the 5 amp blows. Don't ask me. I'd love to hear anyone's explanation for that. At any rate, thank you all very much. It cost me about $80 including paying the moonlighter $55. I love this site now!

I would say you have a bad transformer- if you have 120 volts to the transformer and nothing coming out then it's bad... be sure when purchasing a new one that the voltage and the VA are the same

Jumper R to G at the unit. (do not forget to make sure door swith is closed, use tape) If the fan runs go to the thermostat jumper R to G. If the fan runs from the board and not the t-stat you have a bad wire connection from t-stat to unit. If the fan does not run from jumping R to G at the board it may be a bad board or relay let us know.


I jumpered R to G at the unit with the switch taped closed and nothing happened. When I jumper R to G at the T-stat the fan outside comes on, but not the blower motor inside.

So, it might be the board or relay?

This was helpful for me, and we are having the same problems with our unit. Three 3yrs old bank put it in when we bought the house. The fuse blew once replaced due to unit not turning on as well as fans not turning on, but t stat lights up. If we have a short in the low voltage wiring what do we need to do to?
Thank you for your help, you guys are very helpful

The heat on my unit is not working. When I turn the thermostate to the heat position the motor runs but there is not heat coming out.