Blank display on thermostat


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Old 03-27-17, 08:59 AM
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Blank display on thermostat

Hi!
Thanks to some excellent advice I got on this forum I installed a Honeywell RTH9580 thermostat a few months back. It controls a Carrier FB4ANF024 unit and has a common wire powering it. It was working fine until today. When I woke up it was a bit chilly so I took a look at the thermostat. The display on it was blank. I tried turning power on and off but there was no change. I looked at the wires but they seemed to look OK. I also tried disconnecting the thermostat from the base and reconnecting it. Still no luck. Then I tried reconnecting the old thermostat, a Totaline P474-0140. When I turned the power on the fan started blowing and the red light between the up and down buttons turned on but the temperature display was off, like in the picture. Also the switches were set like in the picture. I noticed a soft clicking noise coming from the thermostat. I disconnected it and put in batteries. I turned on the power and the fan came back on. The clicking was gone and it was displaying the temperature now. It was showing 66 degrees. I adjusted the temperature on it to 72 degrees and moved the switch to "Heat". Gave it a few minutes but the compressor never turned on. The small fuse in the air handler looks OK. Is there some other troubleshooting I can do?
 
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Old 03-27-17, 10:27 AM
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The next step would be to check for 24vac at either the thermostat (R and C) or at the air handler red and brown wires where that the thermostat connects.

Turn off air handler when making wiring changes as any brief short will blow out the protection fuse.
 
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Old 03-27-17, 11:35 AM
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Of course about turning off the air handler.

There is no voltage at the thermostat terminals.
 
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Old 03-27-17, 11:38 AM
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Then either that protection fuse is bad....and looks good or the transformer is out.

The transformer is inside the air handler.
It should be black and yellow on the primary side connected to 240vac.
The output should be on the red and brown wires for 24vac.
 
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Old 03-27-17, 04:43 PM
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The transfomer output reads 28.11 VAC so it looks like it's good. Since there was no voltage between the C and R terminals I thought I'd measure the voltage between the C terminal and other ones and for two of them: (as labeled on the Totaline P474-0140) W2 and G it reads about 27 VAC.

The fuse is good. I checked it for continuity.
 
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Old 03-27-17, 04:50 PM
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It looks like the 24vac goes to the printed circuit board. If you have it TO the board but not FROM the board.... the board is faulty.

Did you check from C to both side of the fuse with it in place ?

Your manual..... hvac partners/docs/1009/Public/0D/SM03-5.pdf
 
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Old 03-27-17, 09:05 PM
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Thank you for your help. The problem is solved. I was able to find the problem but it was not what you might think. Two things happened that helped me: the document you provided got me thinking of how it was all connected (wired) in more detail and then I got a piece of information that I was missing: unbeknownst to me my wife decided to do some dusting around the house over the weekend. Thanks to your document I took a closer look at the wiring (which seemed OK to me before) and as I was doing that my wife mentioned she may have moved the cables out of the way when she was cleaning there. Ding, ding, ding! We have a winner! After tracing one of the connections from the transformer I noticed it didn't go anywhere! I found the break in the circuit. When she moved the cable out of the way it must've strained that one wire and broke it clean off. I'm not sure why I didn't notice it before. I guess now I knew what I was possibly looking for. There was another unused wire in that cable so I swapped it and lo and behold! It's working now.

Again thank you for your assistance. Now I have another problem. One of the screws holding the thermostat in place is not doing its job anymore. The plastic drywall anchor became loose from swapping the thermostats and connecting/disconnecting it for troubleshooting. But that's a topic for another thread. Oh, the joys of home ownership! LOL

Thanks again!
 
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Old 03-27-17, 09:17 PM
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Wait a minute..... your wife was dusting INSIDE the air handler.

These are pretty neat.... Easy Anchors. They screw into sheetrock in place of your mollys.

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Old 03-27-17, 10:35 PM
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I don't think he had molly's, just those typical cheap expanding anchors. I like the ones you showed or winged plastic anchors like this...https://www.walmart.com/ip/Westward-...&wl13=&veh=sem
 
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Old 03-28-17, 07:45 AM
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Exactly! Cheap expanding anchors. Now I know what their official name is!

Now she may have to clean inside the air handler
The label that's on the fan case is covered in black stuff. Maybe mold? But it is dry inside... Maybe that's a topic for another post.

In any case thanks guys!
 
 

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