AC not working after replacing old thermostat with Nest

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Old 10-07-19, 02:46 PM
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AC not working after replacing old thermostat with Nest

Hi. I've been having a pretty bad day today and it got even worse. I wanted to replace my old thermostat for a Nest smart thermostat. I switched the breakers off, removed the old thermostat and installed the new one. I received an error "Error E195: No power to R wire detected". Than I removed the new thermostat and reinstalled the old one, unfortunately it was not working as well. I tried to twist red and white wires together to see if the AC unit turns on but it did not. I have no idea what to do and I dont want to call a technician just yet, maybe its an easy fix.

My AC system is Trane. The outdoor unit model is XE1100 and the indoor air handler model number is TWE036C140F1.

The wires that I have are Yellow, Orange, Red, White, Green, Blue, Brown, Black.Brown and black wires were taped and not connected to anything.

I attached pictures of the wire that connects to thermostat. It is showing no power with a non contact voltage tester on either side. Also attached pictures of how the old thermostat was connected and how I connected the new one.

Below is a video showing the circuit and voltage tester.
https://youtu.be/H1rSvcTaSVs

I would really appreciate if someone could help me. Thank you

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Last edited by PJmax; 10-07-19 at 03:37 PM. Reason: cropped/resized/enhanced pictures
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Old 10-07-19, 03:29 PM
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Welcome to the forums.

You probably switched power off to the condenser...... however the circuit originates in the air handler. You either blew a fuse or the 24v transformer.

A non contact pen is almost useless for 24v.
It's designed for high voltage..... which is usually 50v and over.

You will need to locate the 24v transformer. There may be an inline fuse attached to one of the wires.
 
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Old 10-07-19, 03:40 PM
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I just got the multimeter. Could you please help me locate the transformer?
 

Last edited by PJmax; 10-07-19 at 03:54 PM. Reason: removed un-needed pictures
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Old 10-07-19, 03:50 PM
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Post a larger picture of the wiring area in the air handler.

These parts should be in your air handler.
Transformer
Fuseholder
Fuse
 
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Old 10-07-19, 03:52 PM
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Thank you. I will look for it.

Could you please look at my previous post? I edited it with pictures. This might be the transformer replacement.
 
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Old 10-07-19, 03:55 PM
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I removed the pictures as un-needed.
That is a power supply that runs the germicidal light. It's printed right on it.
 
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Old 10-07-19, 04:08 PM
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I uploaded pictures of the air handler wiring.

Just googled germidical light, I guess its just not working. I thought that it was used as a transformer.

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Last edited by PJmax; 10-07-19 at 04:20 PM. Reason: resized pictures
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Old 10-07-19, 04:12 PM
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The germicidal light is ultraviolet so DON'T look at it if it's powered.
The tubes only have a limited life and it may be bad.

I don't see the transformer. It may be inside that metal box where the wiring connects.

That Scientific-Atlanta box allows the power company to shut down your A/C in cases of high energy demand in exchange for a reduced electric rate.
 
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Old 10-07-19, 04:25 PM
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I just removed the germicidal light and all the wiring, the wires were hanging loose and it looked messy. The glass was broken anyway. Is it something necessary that I should install?

There is also some switch that was taped so that the button is always on. And the second picture is the wire that comes from that "switch".

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Last edited by PJmax; 10-07-19 at 05:42 PM. Reason: resized picture
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Old 10-07-19, 04:28 PM
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That switch was originally set up to kill the light when the blower door was open to keep from exposure to the UV light. I'm not really sure how effective they are.
 
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Old 10-07-19, 04:47 PM
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Thank you for all info. I was wondering what all of those are for a long time.

I think I found the fuse as well as the transformer.

The fuse (similar to those in extension cords) popped out as soon as I turned the cap, now its really hard to screw it back. Is that normal?

The transformer was located at deep in the air unit.

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Last edited by PJmax; 10-07-19 at 05:18 PM. Reason: resized pictures
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Old 10-07-19, 05:10 PM
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That fuse holder can be tough to get together. Check the fuse with your ohmmeter. It's probably open.
I rarely change them anymore. I replace the fuseholder and fuse to the new plastic type fuse.
Widely available in automotive parts stores. You can use an ATC 4A fuse for protection.

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That item hanging deep inside is not the transformer. It's some type of added in relay.
I left a link to the transformer in a previous post.

The fuseholder you found should attach to the transformer.
 
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Old 10-07-19, 05:25 PM
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I really appreciate your help.

That tube fuse is ceramic and I am not able to tell if its bad by looking at it, I will test it with a multimeter in a moment. I am heading to home depot before they close because its too hot in my house now (Florida...).

I dont know if I understand. I should buy the fuse and the connector that you attached, instead of the original fuse? Should I remove the the old fuse and the old fuse holder? I would appreciate if you could help me out a little more with the wiring of that fuse.

Do you think this is what is causing the issue?
 
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Old 10-07-19, 05:37 PM
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You shorted the C wire to the R wire when replacing the stat.

As far as I know..... the fuse is part of the cap. HD will not stock that replacement.

Yes..... go to an auto parts store. Get a fuseholder like I pictured. Then buy a box of ATC 4A fuses.
The fuseholders are in the wiring aisles along with light bulbs.

I'll help you when you get back.
 
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Old 10-07-19, 06:16 PM
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I've been at Autozone and Home Depot, they only had 3 and 5 amps. I bought both, as well as the fuse holder. Will this be fine? The old fuse was a 3A.

The metal box plate is screwed on both sides so it might be hard to take it off without removing the air handlers fan, but I'll try.

edit.

i tested the old fuse. It showed 0.FL on the ohmmeter. I broke the fuse apart and it didnt look damaged, however it is showing zeros instead of 0.FL when I touched the little wire (in the middle) inside the breaker but now when I touched both ends. The tube fuse was removable from the cap.

Do you think its better to buy a glass fuse instead? Maybe it would work without replacing the fuse holder, at least temporarily?
 

Last edited by m4gnum; 10-07-19 at 07:11 PM.
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Old 10-07-19, 07:12 PM
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Yes... you will need to get to the wiring on the back if that is a mounted fuseholder.
The factory fuse is 3.2A so a 3A should be fine. 5A is too big.
 
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Old 10-07-19, 08:33 PM
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I drove to Walmart to buy those glass fuses but it was closed due to some power outage...

I was able to cut that fuse holder out and pull the black and red wire out. I connected them to the new fuse holder and put a 3A fuse. Unfortunately it still didnt power on

I tried to switch the new thermostat for the old one, also tried few fuses but nothing.

I think that that I can hear the AC powering on just for a millisecond right after I switch the safety switch on, but I am not sure about that.

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Last edited by PJmax; 10-07-19 at 10:21 PM. Reason: resized picture
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Old 10-07-19, 08:37 PM
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Is the blower door in place ? Usually there is a safety switch that will kill the unit.

If you have a meter..... check for 24vAC from R to C at the thermostat.

If I'm not mistaken..... the fuse does not come out of the black cap.
You need to buy the factory fuse which comes as a fuse with a cap and runs about $10.
 
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Old 10-07-19, 08:58 PM
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Yes, the blower door is in place.

The fuse did come out of the cap.

I damaged the old fuse holder already while replacing it for the new fuse holder that you recommended. I attached the picture of the new one above.

I tested the R and C and it is showing 0.007 to 0.009 V, as in picture attached.
The other way Red to C and Black to R - showing to 0.010 to 00015

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Last edited by PJmax; 10-07-19 at 10:24 PM. Reason: resized picture
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Old 10-07-19, 09:35 PM
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On your meter...... use the next step down..... with ~ over the V.
Now select AC by pushing the AC/DC button.
You are measuring for 24v AC..... not DC.
 
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Old 10-07-19, 09:48 PM
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Thank you. I havent noticed that its at DC. It was however at V with ~. The picture did not catch it clearly.

So, now its showing 0.103 with the safety switch on and 0.0010 with switch off.

The air handler is definitely powering on for a millisecond when I turn the switch on. The "scientific atlanta" box light comes on for a few seconds too but it turn off, however after 30-40 seconds the light comes back on and stays on.
 
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Old 10-07-19, 09:55 PM
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The two heavy wires at the top are 120 or 240 which is why the pen activates. Many of those pens don't work under 50vAC. There may be 120v in that Carrier box too.

Using your meter...... your probe should be able to fit in the end of the plastic fuse to the metal tab. Measure from both tabs to ground. See if there is 24v there.
 
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Old 10-07-19, 10:02 PM
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Do you mean the fuse that I installed today?

In my previous post I pasted a wrong video link. I wanted to show a video of the AC and the light turning on after I switch the safety switch on. Here is the correct link if you could take a quick look: https://youtu.be/VpbrCfaVSxs
 
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Old 10-07-19, 10:08 PM
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I see the light. I don't know if that's 120v or comes off the 24v supply.

Yes..... there are two metal tabs in the top of the fuse.
 
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Old 10-07-19, 10:08 PM
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I tested the fuse. Touched red probe to both metal pieces at the top of the fuse and the black one to the metal box. The meter is jumping between 25 and 27V on both sides of the fuse.
 
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Old 10-07-19, 10:18 PM
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That's very good. The transformer is good. That fuse is in the R wire. So that means 24v should appear at an R screw terminal on the terminal block. See if you can find 24v on R. If yes..... you need to trace where the wire goes from the R terminal. It needs to go to the thermostat cable.

It may go down to the S/A box and come back up.
 
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Old 10-07-19, 10:30 PM
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You checked from R to C at stat and didn't get 24v. Try measuring from R to Y or G or W.
Those should also show 24v.

I can't see anything major going bad since I'm sure the fuse blew.
 
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Old 10-07-19, 10:44 PM
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Yay! I traced the red wire and one cable (wire red wire in it) was coming from the attic, than it was attached the the cable that is going into the thermostat and there was one more (third) cable connected to them - the plastic switch with a button that was taped. I taped that button again, installed the thermostat and it worked!

So the fuse holder and fuse replacement solved the issue. I just got into another issue by removing the tape from that switch.

By the way, do you happen to know why that thing is connected there? Should I leave it as is?
It was hanging out outside the ac unit since I remember.

Should I replace that 3A fuse with a 4A in the near future?

Many thanks. You saved me lots of money and additional stress. I am so grateful. Is there anything I can do to return the favor?
 
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Old 10-07-19, 10:50 PM
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3A should be fine. It it blows out by itself.... change it to 4A otherwise leave it.
I'm not really sure exactly what that switch does. Leave it like it was.

All good here. Glad you got it going. Name:  thumb.jpg
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Old 10-08-19, 07:51 AM
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Again, thank you very much for your time and your detailed help.
 
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