No Power to A/C Thermostat

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Old 05-14-09, 07:01 PM
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No Power to A/C Thermostat

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 gonna 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 gonna be a simple stat replacement afterall.

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. Oh, btw, there's no power at the terminals of those thin wires (bottom right corner of the first pic below)that look similar to (or leading to/from) the thermostat's inside the house.

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.











 
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Old 05-14-09, 08:26 PM
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the parts there in the CONDENSER are the compressor contactor the capcitor and the solid stat relay boardthe only low voltage is what you have coming in on the lower right corner terminal strip(does not control stat).the transformer is in the furnaces..stay with the stat tghten up the wires ther clean the pins with alcahol..your touching it and its lighting up then not.with the stat removed jump R to G FAN R to Y compressor cooling R to W heat if you get these actioons its the contacts on the stat...if you have a R and C on the base check for a constant 24Vs with the stat off the subbase.WHAT controls WHAT....the transformer in the furnace powers the stat LED and with the settingss send the 24Vs out to the condenser in cooling mode and down to the furnace in the heating mode.might get lucky with a simple Q-TIP and alk and that jumping test.
 
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Old 05-14-09, 08:53 PM
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Thank you for your response, Sminker!

Ok, when I first dismantled the thermostat, I did try connecting the R & G wires, but the fan did not come on. I even clamped the wires down and waited a few mins. Did the same with R & Y - No action.

Checked the bare wires after removing them off the thermostat itself, and they showed no power on the multi-meter. I placed my - and + leads on all the points, R-Y, R-G, R-Y, R-C, etc...no power at all.

The thermostat just came back to life a while back, and kick the AC on, it's running now. I know it won't last long. I scanned the vents, and the air coming out is only 68 deg. Is something else wrong somewhere? It should be colder than 68.

Should I still clean the thermostat contacts even though there is no power at the wires?
 
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Old 05-15-09, 06:19 PM
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that R and C should be a constant 24Vs at the subbase with nothing jumped stick with that........when the TR is powered and the unit is running is it overly hot on the metal middle section.if it is toooo hot to the touch the heat build up might be cauasing something to happen with the 24V feed what i don't know .have you read anything less then 24V solid volts say 15V 18V 17V...hope it is hot to the touch then flip a new one in availible at grainger supply Grainger Industrial Supply if you don't have a source.if you have a relay board within the furnace that 24Vs up to R and C should not be effected!hope somebody else kicks in on this one is it possible the overheating of the TR is causing it from sending 24vs up to that R/C.for your info...the board in the pix is the biggest draw on the VA rating of the TR so if there is one in the furnace also add that then the stat draw and the assorted relays...i'll take a quick guess at it is the original TR and its rating in the furnace 40-50 VA 115 primary 24Vs secondary.if it is 208 primary make sure your not tapped for 240V that will give you a dragging 19V or so and things might light up but no action.
 
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Old 05-15-09, 06:34 PM
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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.






 
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Old 05-15-09, 06:37 PM
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Here are some closeup pics. If that block on the far right with the white label (behind the row of 24V thin wires) is the transformer, I can't seem to find terminals to test on it. As described in the pics, the wires go into the box itself.



 
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Old 05-15-09, 06:38 PM
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No help for your problem...sorry...but wow..take pride in your pictures!! If only all posted pics were that clear and sharp!

Hope you get it fixed soon!
 
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Old 05-15-09, 06:47 PM
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Originally Posted by Gunguy45 View Post
No help for your problem...sorry...but wow..take pride in your pictures!! If only all posted pics were that clear and sharp!

Hope you get it fixed soon!
Thanks, Gunguy!


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 learned that thing is called a Relay? Similar to a contactor? It has a transparent casing. So does that mean it's normal that there is no power on both B's and C's?

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



 
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Old 05-15-09, 07:49 PM
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I removed that beige plastic cover from the transformer. It looks like a fuse, I can't open that black casing up, didn't want to try too hard and break the solder on those wires. I did a connectivity test on the meter on F1 and F2 and it was positive, needle went all the way to the right of the scale. (Sorry, not sure how to read the units for this test, told you I'm an amateur! ). Anyway, so does that mean the fuse is good? What should be my next step in this troubleshooting? It's getting warmer here tonight, (FL) really hope to get it fixed by this weekend, well, parts need to be bought tomorrow (Sat) I guess.






 
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Old 05-16-09, 12:39 AM
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I did some probing around the Fuse and Transformer terminals. Hope this will help.

Blue - Braided White - 26V
Blue - Yellow - 26V
Yellow - Braided White - Nothing

I also tested the row of R. W. G. C terminals...
R-C, R-G, R-W....they all showed the same 26V

Refer to pics below for a clearer description. So, are we there yet? What's the verdict? Wife wants me to call someone in tomorrow, but I've come this far...really don't wanna give up at this point. The pressure is on, as Saturday is the last work day, won't be able to get any parts or anyone in on Sunday. And it's getting uncomfortably warm in the house!






 
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Old 05-16-09, 03:37 AM
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that is the circuit breaker the white and yellow are wired to and they basically are the same wires.if you had no black CB the yellow would go directly to the TR soldered terminal...you would read blue to white or blue to yellow for the first 26V out of the secondary.what is the schematic legend name on taht glass relay?it has to be the stat with no solid state in the fan section...but that jump you did with no action even on the fan is a mystery i would suggest getting somebody in before days end..unless you have a loose wire nut for that 24Vs up to R/C on the subbase...i am spent on this one...did you try your fan jump down at the furnace terminal strip to see that reaction...R/G what do they call this an epithany jump the circuit breaker in the transformer and do your jumps at the air handler and then up on the stat.thats it
 

Last edited by sminker; 05-16-09 at 03:53 AM.
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Old 05-16-09, 09:47 AM
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I jumped R-G in the air handler....nothing happened!? But there is power at those terminals....what does that mean?

I know there's no power at the wires coming out of the wall to the thermostat.......but does this completely rule out a fauty thermostat? Is there a chance that if I put in a new thermostat, everything will work???
 
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Old 05-16-09, 04:32 PM
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no i would do a transformer change out first before anythig if you look at that schematic on the fan housing look at the wires that come off the secondary.that as said should be a constant 24V just like the line power thru the on/off switch on the side of the furnace.if the on/off is up the Tr should have 24vs at R/C at the air handler and on the R/C at the stat subbase.i tell you one thing you take great pictures can you put up one of the that (right side) schematic so i can see if the 24vs goes thru something that i am missing mostly the middle of the drawing or top and bottom if anythig...thanks
 
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Old 05-16-09, 05:50 PM
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Sminker, I finally found the problem after 3 days! Beer 4U2
I was also working on another forum at the same time. Thank you very much for your help anyway, I appreciate it.

Do you wanna guess what the problem is?

Here are some more pics of the air handler and where the wires go. I don't wanna just disappear from this thread now that I've fixed the problem. I want to wrap this thread up with the solution, so that others who may have the same issue might find this helpful.







 
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Old 05-16-09, 05:58 PM
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that was bouncing the 24Vs around it was doing the job it was designed for .........
 
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Old 05-16-09, 07:26 PM
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Originally Posted by sminker View Post
that was bouncing the 24Vs around it was doing the job it was designed for .........
Huhh? I don't get that......
 
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Old 05-17-09, 11:43 AM
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thats what was knocking the 24Vs off the stat.......right that is a limit switch to shut the AC down when it senses water back up.that is usually wired into the Y (no cooling no water) from the stat instead they are killing the 24Vs out of the TR as installed.so when the cooling runs your condensate is running if it backs up and hits the sensor you loose 24Vs off the stat.....
 
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Old 05-17-09, 01:58 PM
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Originally Posted by sminker View Post
thats what was knocking the 24Vs off the stat.......right that is a limit switch to shut the AC down when it senses water back up.that is usually wired into the Y (no cooling no water) from the stat instead they are killing the 24Vs out of the TR as installed.so when the cooling runs your condensate is running if it backs up and hits the sensor you loose 24Vs off the stat.....
YES sminker! You are absolutely right! I wished I had thought of that earlier, but it was a good learning experience.Beer 4U2
 
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Old 06-25-10, 02:31 PM
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Hi there,
I came across this thread while trying to figure out what's wrong with out AC/thermostat. The problem is sort of similar: The thermostat goes blank and comes back on after some time. This has been going on for a week. When the thermostat is on, the AC works just fine. An A/C guy came in and changed the thermostat unit thinking it might be a problem with the fuse. But the problem came back again. I'm trying to find out what exactly you did to fix your problem. I'm nowhere close to handy and the pictures seem very technical to me :-( Any help would be appreciated!!!
 
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Old 06-25-10, 05:09 PM
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The float switch was turning off power to the stat because the ac drain was backing up. Most float switches are found below the evaproator coil or air handler in a secondary pan in my area.

A shop vac to the drainline might pull out the restriction.
Some bleach in the drain after flushing it would also be a good idea. (I use nitrogen and pan tablets)

 
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Old 09-30-12, 02:54 PM
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Hello, yellow to blue (26v) and white to blue (26v) = Your low voltage/control power, which signals things like your thermostat, your heat/blower relay, water sensors, your contactor, and low/hi pressure switches.
 
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Old 09-30-12, 07:41 PM
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FoolTheViking, This post is over 2 years old, no need to "wake" up old posts.
 
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