No voltage to Thermostat


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Old 07-08-14, 08:44 AM
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Unhappy No voltage to Thermostat

I have a 19 year old Trane XE 80. Yesterday I turned the air on and I did hear it kick on. About an hour later I heard nothing running and my Honeywell programmable thermostat said "system on". However the furnace blower and compressor were not running. I checked for 24 volts in the wires going into the thermostat and got nothing. About 6 years ago I had to have the furnace circuit board replaced. Does this sound like the same issue? I take it is not a repair for an amatuer?

Thanks,

Brian in MI
 
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Old 07-09-14, 08:04 AM
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195 views and no one can help me?
 
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Old 07-09-14, 09:47 AM
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Without more info I can only give you a few pointers.

The only way you can check for 24vac at the thermostat is if it's using a C connection. Then you would check from C to R for 24vac.

If there's no C in use at the thermostat you can go to the furnace control board and measure there for voltage between C and R.

No voltage there...... check for a blown fuse on the control board. It will be one of those little plastic car type fuses.

No fuse.... post the make and model of the furnace/air handler.
 
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Old 07-09-14, 11:26 AM
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Wink It was the fuse!

Sure enough the fuse was bad. Put in a new one and everything works fine now.

Thanks!

Brian in MI
 
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Old 07-12-14, 11:12 AM
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Thanks for the reply. I had a setback. I tried turning the air on today via the honywell programable thermostat I have and nothing no blower no compressor. However when I move the switch for the fan to on the blower motor comes on. Which way should I go now?

Thanks,

Brian in MI
 
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Old 07-12-14, 12:13 PM
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You didn't blow a fuse this time. I wondered what blew it before.

Putting your thermostat into A/C mode..... go to your air handler/furnace where the control board is. You should measure 24vac from Y to C and G to C. If that is correct.... go outside to compressor unit... find where the thermostat wiring from the house connects. It'll probably be in wire nutted connections. Check for 24vac there.

If ok... turn off power to the compressor unit check the status of the contactor.
Check for 24vac on the small wires on the contactor.
 
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Old 07-13-14, 12:08 AM
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Is your temperature above setpoint? Did you wait over 5 minutes?

Most digital thermostats will initiate a 5 minute timer when the compressor cycles off. If the compressor cycled off less than 5 minutes prior to you adjusting the thermostat the compressor and blower wouldn't start until the balance of the 5 minutes has passed.
 
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Old 07-13-14, 06:55 AM
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No voltage from Y to C or G to C. Safety switch taped down. Tried the heat and furnace fired up and blower motor came on fine.
 
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Old 07-13-14, 07:45 AM
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Update The blower motor does kick on when set to AC but not the compressor. Check the small wires at compressor that come from control board with a volt meter and getting 3.5 to zero volts. I am concerned because I smell a burnt wire smell around the compressor. Time for me to go no farther and call in an expert! Also did check 30 amp fuses and they are good. Thanks for all the help!
 
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Old 07-13-14, 09:26 AM
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The contactor looks like this.



Is the contactor the source of your burned smell?
 
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Old 07-13-14, 09:47 AM
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contactor

Hard to tell if it is from there but I can tell you the compressor and fan started when I manually push it in!
 

Last edited by sportsdadmi39; 07-13-14 at 09:48 AM. Reason: spelling
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Old 07-13-14, 09:53 AM
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No voltage from Y to C
If you don't have 24vac between Y and C at the air handler.....then you won't have it outside either. It sounds like a broken wire at thermostat or defective thermostat.
 
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Old 07-13-14, 09:55 AM
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I'd kill power to the condenser and measure for 24 volts to the contactor leads when they are not connected to the contactor.

Are you familiar with your meter? If the blower starts, you should have 24 volts G to C.
 
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Old 07-13-14, 10:31 AM
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Exclamation Another setback!

Now I think the transformer is done. I wanted to try to check for 24 at the contactor again so set thermostat to air and went downstairs to wait for blower motor to kick on before I went out side. Waited 10 min nothing. We to back of furnace to make sure safety switch was still taped down and smelled something hot. No power coming out of transformer wires and man was that thing hot! Could the transformer been the issue all along? When I checked Y C earlier and circuit board only got 14.5. G C was only 11.5. Wires outside at compressor were not 24 but can't remember exactly the reading. Time for a professional?

Thanks,

Brian
 
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Old 07-13-14, 11:22 AM
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You've got a short. It's between your air handler and the compressor. It's a wiring problem or it's a shorted, burned, melted contactor coil on the contactor. I'd disconnect the contactor first and then recheck voltages.
 
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Old 07-13-14, 11:33 AM
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Unhappy Try another transformer?

Is it worth it for me to get another one a try again or do I risk burning out another one?
 
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Old 07-13-14, 11:41 AM
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Is the current transformer bad ? Have you tried measuring it for voltage ?

You'll need to replace it if it's bad but you still need to find the problem. The transformer is NOT the problem.
 
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Old 07-13-14, 11:47 AM
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Yes it is bad no voltage coming out and when it went it was hot enough to burn my finger!
 
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Old 07-13-14, 04:21 PM
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Lightbulb Photos

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When I measured Y C earlier today with A
C set to on the voltage was 14.5 G C was 11.5 I thought maybe photos may help.
Thanks again!
 
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Old 07-14-14, 07:33 PM
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I'd like to see a picture of your meter setting when you are measuring for 24 volts.
Seems like 0 and 27 volts are the 2 readings that I would expect.
 
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Old 07-14-14, 07:40 PM
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Need a transformer first

I have to pick up a new transformer before I can measure for any more voltage. I am also going to get a 3 amp inline fuse from Radio Shack for the 24 volt line coming out of the new one just in case there is a short somewhere. One thing I did test today was continuity for the 24 volt wires outside at the compressor. I got a good reading on them.

Thanks Again!
 
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Old 07-14-14, 07:50 PM
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You can ohm the primary and secondary side of your bad transformer to see if it was a low voltage problem.

If it was a high voltage problem I'd check for a tight connection from the panel to the grounding steak outside (with the main shut off).

You were measuring for 24 volts AC right?
 
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Old 07-15-14, 06:18 AM
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yes 24 volts

Yes I was measuring for AC. Thanks again for the help!
 
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Old 07-15-14, 09:31 AM
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Great idea with the 3A inline fuse. New transformers today WILL NOT get as hot as your old one without blowing out. They will also not take any short circuit issues.

In the pic below, make sure power to compressor is off, check for 24vac across those two wires. Either untwist them before checking the voltage or unplug them off of the contactor.

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Old 07-17-14, 06:58 AM
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Thumbs down no voltage to transformer

Well I was all excited to get back to work on this today. I mounted the new transformer and hooked up the same wires that went into the 120 side of the last transformer turned on the breaker then nothing. I checked the wires coming out of the circuit board and no power (yes safety switch off) in the wires. Time to call in an expert? I still think I have issues on the wires going outside anyway because the resistance in jumping all over the place when I tested (disconnected from contactor) them. The two wires going into the contactor test good and had continuity. Thanks!


Brian
 
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Old 07-18-14, 04:09 PM
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Thumbs up Back in business!

I had a friend of the family come by today and we are back up and running! He was able to get 120 to the transformer right away so I must have done something wrong there! He found that the wire from the air handler to the compressor was bad so we replaced that. Then that the contactor was bad as well. I guess that is why I smelled something burnt out there. Anyway thanks for all the help here I learned a lot!

Brian
 
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Old 07-18-14, 06:00 PM
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Awesome...... good job sticking with it. Thanks for letting us know what the problem was.
 
 

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