[KenLoh]

Hello All, I'm new here and have a question that I hope someone can help me with.

My A/C thermostat started acting weird a few days back. I have a simple digital stat, with a greenish-glow backlit screen, with settings for cool, heat, and fan. All of a sudden, the screen went blank, no light, and the A/C of course would not kick in. Initially I thought the stat must have gone bad, and I was going to get a new replacement. Decided to take it off the wall mount and check it out. Removed the green and red wires and put them together, the fan didn't kick on... no power? Tested it with a multi-meter, no power at all from any of the wires. So... it's not going to be a simple stat replacement after all.

The strange thing is, after messing with the wires on the stat, and turning the main A/C breaker on and off, suddenly the stat came back to life. Lighted up and settings displayed, A/C kicked in, and this lasted for a couple hours, before it went dead again. That was late last night. Early this morning, my wife said she saw the stat working. Late morning when I woke up, it was dead again.

From my recent research on this forum, as well as eleswhere on the internet, I suspect it could possibly be the transformer. But I have no idea where that is! Just to make one thing clear, I am not savvy with electrical stuff, I'm pretty handy mechanically, and can follow instructions well though. So, I opened up the air handler in the garage, looked inside at the bunch of wires, there were some similar thin wires as the wall mounted stat's. I tested those with the multi-meter and got positive readings around 18 V.

Now I'm thinking it must be somewhere else... so I opened up the panel in the condenser(?) at the back of the house (the noisy one with the fins and fan). Tested the terminals where the power enters (A & B) and is connected to some device (attached pic below) and got around 110-120V. Tested the other terminals (C & D) which look like where the power gets distributed to those familiar thin wires that I saw on the wall stat. No Power there... so I assume I've found the culprit! Well, being an amateur, I am not sure. And that's why I'm here.

Here are some pics of the panel in the condenser/compressor (sorry, I don't know the correct name for this noisy thing! LOL!). Can someone confirm if this is the transformer, if not, what is it called? There looks like a little contact point, I guess for the timer to kick the thing on/off. Judging from my description, is this the faulty part, and should I attempt to replace it myself since I have come this far?

I am one of the many unfortunate unemployed ones, and would rather do this myself, if I can. If I have to, I don't mind getting someone to do the job, if there are big risks involved.

Thank you all.

[KenLoh]

Sorry, I left this note out... don't see an "Edit" button anywhere, so have to make another post to add:
There's no power at the terminals of those thin wires (bottom right corner of the first pic above) that look similar to (or leading to/from) the thermostat's inside the house.

[letmetellu]

First before we get into what the wires are and such I want to ask you, does the thermostat have batteries in it and does the LED light show you anything on the thermostat that says "Low Battery"

I see you are on line now so I will post this and wait for an answer.

But one warning, The things on the left side of your pictures are all high voltage and very dangerous. The small wires on the right side down low are the low voltage. The thing that you pointed out and ask what it is is called the contactor, and the two small wires on either side is what powers the contactor to make it contact to make the AC run.

I will wait for you answer.

[mygirlsdad77]

Great answer from tellu. Yes the part in question is the contactor, not the transformer. My first thought would be a low or dead battery in the thrmostat also.

[KenLoh]

Sorry for the delay... just got this.

There is NO battery in the thermostat. I checked that first. There is a wire that goes in C - which I assume is for the power?

The thermostat just came back to life, and AC is running now. However, I scanned the vents and the air coming out is only 68 deg... ooops, I hope there isn't something else wrong.

[dac122]

Ken, look in your air handler for a small cube shaped object, the transformer. With power on it should be a little warm to the touch and even hum a little. Typical specs on those are 24VAC output, not 18VAC, but check the label on its side to be sure, if readable, and test its input voltage 110-120VAC and its output with your multimeter by backprobing etc.

A transformer on the edge of acceptable voltage could cause flakiness in your tstat. Here's a link that may help explain the internals of your tstat and standard wiring color codes.

Let us know what you find and we'll take you from there.

[KenLoh]

Ken, look in your air handler for a small cube shaped object, the transformer. With power on it should be a little warm to the touch and even hum a little. Typical specs on those are 24VAC output, not 18VAC, but check the label on its side to be sure, if readable, and test its input voltage 110-120VAC and its output with your multimeter by backprobing etc.

A transformer on the edge of acceptable voltage could cause flakiness in your tstat. Here's a link that may help explain the internals of your tstat and standard wiring color codes.

Let us know what you find and we'll take you from there.

Thanks dac, I will go look in the air handler again. I'll test with the meter and take some pics as well. I noticed that with the main A/C breaker on or off, the Air Handler makes the humming noise either way. Is this normal?

[dac122]

Thanks dac, I will go look in the air handler again. I'll test with the meter and take some pics as well. I noticed that with the main A/C breaker on or off, the Air Handler makes the humming noise either way. Is this normal?

It is likely you have two breakers, one for the outdoor unit and the other for your air handler. If there is humming from the transformer after you've presumably shut off the power you probably shut off the wrong breaker.

[KISS]

This reads well. Kind of interesting reading.

I'd focus your immediate attention on the following two terminals. R & C.

The next time it fails check R & C at the furnace first and R & C at the thermostat. There should be 25 VAC between these terminals at all times.

Those two terminals should go to the transformer in the air handler. There should be 4 terminals or wires. Two go to 120 VAC and two go to R &C.

Look for any loose connections in any of these wires.

There also may be a 3 A automotive type fuse on the control board. Re-seat that.

You need 24 VAC between R & C for the tstat to have power.

[KenLoh]

Dac, You are correct! I checked again in the main breaker panel, and there IS a separate breaker for AC and Heater. I assume Heater would control the Air Handler, and AC, the Condenser.

Here are some pics of the Air Handler. 1st pic shows the whole layout, just in case you need to point out any particular parts. 2nd pic shows the terminals I tested with the multi-meter. 1-2, 3-4, 5-6, 7-8 all showed 110V. The small panel on the far right, with those wires similar to the thermostat's, show NO power. I tested both inside and outside rows - Zero V. Is this the wire that leads to the thermostat?

Which one is the transformer? The beige color part directly on the left of the row of thin 24V wires... there is a small "switch" or so it looks like, I tried it but it doesn't seem like it switches anything. Perhaps it's just a release to remove the beige plastic casing? There's a closeup pic of it in the 3rd pic.

Thanks for the help.

[KenLoh]

This reads well. Kinda interesting reading.

I'd focus your immediate attention on the following two terminals. R & C.

The next time it fails check R & C at the furnace first and R & C at the thermostat. There should be 25 VAC between these terminals at all times.

Those two terminals should go to the transformer in the air handler. There should be 4 terminals or wires. Two go to 120 VAC and two go to R &C.

Look for any loose connections in any of these wires.

There also may be a 3 A automotive type fuse on the control board. Re-seat that.

You need 24 VAC between R & C for the tstat to have power.

Thanks for your input, I didn't see your post until after I posted the pics. Well, I guess I answered your questions. If it helps at all... the tstat and hence, the AC has been coming on and off randomly since last night, probably like 3-4 times. It won't stay for long when it comes on though... maybe for an hour or so.

[dac122]

Which one is the transformer? The beige color part directly on the left of the row of thin 24V wires...there is a small "switch" or so it looks like, I tried it but it doesn't seem like it switches anything. Perhaps it's just a release to remove the beige plastic casing? There's a closeup pic of it in the 3rd pic.

Looks to me like your transformer is just behind the terminal block on the right. It appear to have that switch mounted to it for some reason. I see 24VAC on its label peeking between wires, and has that telltale ridged solid block on the bottom. Hard to see from here but it should be feeding your R and C terminals for hot and common respectively. Now check the 120VAC input to the transformer.

Let us know what you find.

[KenLoh]

Looks to me like your transformer is just behind the terminal block on the right. It appear to have that switch mounted to it for some reason. I see 24VAC on its label peeking between wires, and has that telltale ridged solid block on the bottom. Hard to see from here but it should be feeding your R and C terminals for hot and common respectively. Now check the 120VAC input to the transformer.

Let us know what you find.

I wanted to do that, check the input to the transformer, but I didn't know which is it, and which terminals for sure. And with the power on and that bunch of live wires, I wanted to make sure first. Is the transformer the block with the label, right behind the beige plastic casing? Or is the beige casing part of the transformer itself?

It's kind of tight to get behind there to the terminals on the transformer without touching any of the wires in front... I'll try.

[dac122]

Is the transformer the block with the label, right behind the beige plastic casing? Or is the beige casing part of the transformer itself?

It's kinda tight to get behind there to the terminals on the transformer without touching any of the wires in front....I'll try.

Minimally the transformer is the block with the label. The beige casing might be part of it but it is hard to tell from here. On that block part look for two 120VAC wires going into that block part and two wires emerging from it. There might even be another label on the other side showing input and output, or if you read the label it might tell.

Regardless that's our puppy and you need to verify its working. If worried about all those wires then carefully place your probes on the connectors, pulling back any connectors to expose, or backproble them, or whatever you have to make a connection. Then power on and see. I agree maybe probing around that area while hot might be dangerous since I see 220VAC wires nearby.

Alternatively you could pull the whole transformer out and could bench test it with an extension cord. You seem to be handy enough. Just label everything or take good pics to remember how to rewire.

[KenLoh]

Ok, here are some closeup pics. I can't seem to find terminals to test. As described in the pics, the wires go into the box itself. Can I test on that part on the top of the transformer? I've also attached a closeup of the label.
I'll wait for advice...

[KenLoh]

Here's some New Discoveries! I traced the wires and A1 and A2 looks like they came from the main input (120V), and they showed 120V on the meter. B1 and B2 seems to lead down to the compartment below (check the earlier pics of larger view angle). That leaves C1 and C2 which seem to lead into the transformer (beige box). Both B's and C's pairs showed 0V on the meter.

I opened up the beige box, and the wires are soldered on. I didn't test those terminals as there's no output anyway on C1 and C2.

So... have I found it yet?? What is that part called? It's a transparent casing, if you need to have a closeup shot, I can go get one.

[KenLoh]

..... B1 and B2 seems to lead down to the compartment below (check the earlier pics of larger view angle).....

Sorry guys, I forgot the overall view pic. Here it is:

[KISS]

On the right picture, you will see this symbol: http://images4.wikia.nocookie.net/ho...symbol.svg.png

That's the symbol for a transfomer. Two of the terminals should be labeled R and C.

The thing that you last pointed too is an enclosed relay.
It does the same function as the contator, except you can't see the contacts.

The thing that's shaped like a "S" is the fuse. It's likely about 3 A and on the low voltage side.

You can post directly to this forum using "Go Advanced/Manage attachements". If you need to resize the images, you can use the free program IrfanView - Official Homepage - one of the most popular viewers worldwide

[KISS]

The transformer may be the obscured object with the white label on the far right of your picture. The Red wire-nutted wire may be going to a fuseholder. I just can't tell with the resolution of the pictures.

[KenLoh]

On the right picture, you will see this symbol: http://images4.wikia.nocookie.net/ho...symbol.svg.png

That's the symbol for a transfomer. Two of the terminals should be labeled R and C.

Thanks, that's good info. I'll look for that symbol to confirm.

The thing that you last pointed too is an enclosed relay.
It does the same function as the contator, except you can't see the contacts.

So which means it's normal that there is no power at those B1-B2 and C1-C2 terminals? If the contacts are not on, then there's no power. Is this correct?

The thing that's shaped like a "S" is the fuse. It's likely about 3 A and on the low voltage side.

I am lost here, which "S" shaped thing are you referring to?