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Old 01-14-2013, 09:12 AM   #1
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24v not present at terminal block or tstat wire leading to condenser outside?


I've been trying to troubleshoot this problem all weekend and I think I may have narrowed it down to one possibility. I'm at work now, so I can't put the tester to it to be certain. But I wanted to see if this makes sense to the pros.

I have a functioning air handler and a functioning condenser outside and a functioning tstat. The outside unit will not turn on remotely, only with the override button on the contacter. When I tested the low voltage sides of the contacter, I get no voltage. When I test the red wire from the tstat, I get nothing there either. And when I test the terminal on the tstat, I still get nothing. One caveat, the thermostat is running (lights up, engages air handler, etc.) So I'm not sure what I'm witnessing here.

Note: When I say I'm not getting voltage, I mean that I'm only getting small voltage and it bounces all around. Like .8 - 1 volt. Nothing remotely close to 24v.

What are the chances that the transformer in the air handler is bad? Would that cause the symptoms that I'm seeing? Would it be possible to still have power to the tstat with such a low voltage? Seems unlikely.

Worst case, I go another night without A/C (we're in Florida running 80's in the day and 60's at night.) But I was trying to make a diagnosis at work so that I could possibly buy a part and replace it to see if that was the problem.

Any thoughts?

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Old 01-14-2013, 10:29 AM   #2
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24v not present at terminal block or tstat wire leading to condenser outside?


When you tested power at the thermostat, did you go from R to C and is your C-common really hooked up between the thermostat and air handler? If not, that may be why the bad reading with the meter. Does your thermostat run on batteries?
Have you tested power in the air handler itself, R to C? Bet you'll find you have 24 volts there.
You say the thermostat engages the air handler and the air handler works fine, is the problem that your condensor wont turn on?
Turn the a/c on and test between Y and C in the air handler and if you get 24 volts there, then the problem may be the wire that runs to the outside unit.


Last edited by sammy37; 01-14-2013 at 10:35 AM.
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Old 01-14-2013, 11:35 AM   #3
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24v not present at terminal block or tstat wire leading to condenser outside?


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When you tested power at the thermostat, did you go from R to C and is your C-common really hooked up between the thermostat and air handler? If not, that may be why the bad reading with the meter.
I did not test between R and C, which probably shows my ignorance on that side. I tested between R and ground and R and Y, so I'm not sure if that would register a voltage or not? I thought that R and Y completed the circuit for the cooling portion, so I assumed that it should be carrying 24v? Or is that only when combined with the common?

Quote:
Does your thermostat run on batteries?
No, it's an energy stealing thermostat the runs off the existing voltage, which makes this harder to understand...unless it's just a matter of me testing the wrong wires.

Quote:
Have you tested power in the air handler itself, R to C? Bet you'll find you have 24 volts there.
No, that is my last step. I checked everything but the AH wiring yesterday. I'm thinking that's the problem area, or at least I hope.

Quote:
You say the thermostat engages the air handler and the air handler works fine, is the problem that your condensor wont turn on?
Correct. It will not turn on via the tstat. I get a clicking sound, but that's about it.

Quote:
Turn the a/c on and test between Y and C in the air handler and if you get 24 volts there, then the problem may be the wire that runs to the outside unit.
I assume you mean for me to turn on the tstat, not the button on the condensor unit, right? I just want to be clear.


And thank you Sammy for your assistance. It's very much appreciated!!
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Old 01-14-2013, 11:51 AM   #4
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24v not present at terminal block or tstat wire leading to condenser outside?


24 should be reading from R or Y to ground as that's the potential path through any conductor the electricity could take to ground if given the chance. Your meter should be able to pick that up.

What I'd do is first turn the breaker to the air handler off. Without high voltage going to the unit there shouldn't be any power to the stat which means you won't stand a chance of backfeeding and taking out any components in the air handler on this next step, mainly the step down transformer.

Remove the wire from both the R and the Y terminal in the stat, twist them together outside of the stat connections and turn back on your breakers. Is cool now on? If not than you may simply have a wire disconnected/broken. If so, change the stat.
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Old 01-14-2013, 11:51 AM   #5
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24v not present at terminal block or tstat wire leading to condenser outside?


You were testing wrong at the thermostat, Need to check between R and C, or Y and C but I have a feeling the problem is not at the stat.

In your last reply, yes turn the thermostat to a/c and check in the air handler at the Y and C terminal, you should have 24 volts.
Then go out to the condensor and PULL the disconnect and check to see if you have 24 volts between the red and white at the contactor.
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Old 01-14-2013, 11:54 AM   #6
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24v not present at terminal block or tstat wire leading to condenser outside?


you can also check at the contactor in the unit for 24 volts. follow the low voltage wires into the unit. Two will be on the contactor. Test from one to the other.

Over time contactors can go bad as well.
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Old 01-14-2013, 12:39 PM   #7
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24v not present at terminal block or tstat wire leading to condenser outside?


Both of you have been "thanked" for what that's worth, because you have provided me with a great many tests to run when I get home. One more night without A/C isn't going to kill us.

Thanks to both of you for the troubleshooting tips. I'll report back tomorrow with my results, and hopefully a firm conclusion.

Thanks again!
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Old 01-14-2013, 08:41 PM   #8
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24v not present at terminal block or tstat wire leading to condenser outside?


See below

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Old 01-14-2013, 10:38 PM   #9
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24v not present at terminal block or tstat wire leading to condenser outside?


I would find another meter first, or make sure yours is working properly and check it again, just to be sure.

Some transformers have universal conections for 110 or 220, so some wires may not be used. Just follow the instructions per wiring for your 220 volt system.
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Old 01-14-2013, 10:43 PM   #10
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24v not present at terminal block or tstat wire leading to condenser outside?


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I would find another meter first, or make sure yours is working properly and check it again, just to be sure.

Some transformers have universal conections for 110 or 220, so some wires may not be used. Just follow the instructions per wiring for your 220 volt system.

I feel like such an idiot. Your intuition was right.....and I probably need to call a pro. I've been checking the voltage using the DC setting this entire time. No wonder I can't get readings anywhere! I'm going to rewire the tstat and let someone else take care of it. I feel defeated, but it's obvious that I'm in over my head here when I can't figure out the difference between AC and DC voltage.

Thank you again for your help, I do appreciate it very much!
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Old 01-15-2013, 12:29 AM   #11
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24v not present at terminal block or tstat wire leading to condenser outside?


Now that the meter is set properly, put everything back together and go check for power at your contactor at the outside unit. Dont forget to set the stat to cool and please pull the disconect at the outside unit.
If you find your not getting power to the contactor, do like I mentioned earlier, check the power across Y and C terminals at the air handler and go from there.
These are simple checks, no reason to pay someone a fortune to do what you can yourself.
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Old 01-16-2013, 07:48 PM   #12
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24v not present at terminal block or tstat wire leading to condenser outside?


Thank you for the pep talk Sammy. I took a day to cool off (no pun intended) and rethink the problem with a clear head. I went back and tested as you told me and I found that I have no power from Y to C. I have 35+/- volts on R to C, but nothing on Y.

I'm going to replace the wire from the air handler to the outside unit on Friday. Is there anything else I should be checking at this point?

Thankfully it hasn't gotten too hot down here and tomorrow it's supposed to cool down ever more, so I might be able to get this done over the weekend with any luck and some help from you guys.
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Old 01-16-2013, 10:10 PM   #13
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24v not present at terminal block or tstat wire leading to condenser outside?


Hold up a minute. If you have no power between Y and C at the air handler with the stat set to cool, then the problem would be between the thermostat and air handler, or a problem with the thermostat itself.
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Old 01-16-2013, 11:32 PM   #14
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24v not present at terminal block or tstat wire leading to condenser outside?


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Hold up a minute. If you have no power between Y and C at the air handler with the stat set to cool, then the problem would be between the thermostat and air handler, or a problem with the thermostat itself.
Yep I wouldn't change the wire to the outside unit yet. Assuming the above is correct your next step should be go to the stat and check R to C. If you read nothing that would mean your losing power to R from the furnace, the wire from the stat to the furnace would likely be the issue. If you read 24v from R to C next check Y to C. If you get nothing at Y the problem is the stat, if you get 24v at Y you're losing power in the wire from the stat to the furnace
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Old 01-17-2013, 12:26 AM   #15
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24v not present at terminal block or tstat wire leading to condenser outside?


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Yep I wouldn't change the wire to the outside unit yet. Assuming the above is correct your next step should be go to the stat and check R to C. If you read nothing that would mean your losing power to R from the furnace, the wire from the stat to the furnace would likely be the issue. If you read 24v from R to C next check Y to C. If you get nothing at Y the problem is the stat, if you get 24v at Y you're losing power in the wire from the stat to the furnace
I thought about telling him to check Y to C at the stat also but the problem is most newer stats lose connection when you remove them from the base.
He has said that the air handler does run, so I'm thinking either bad stat, or bad wire between stat and air handler.
If he has an extra spare wire in his cable, he could easily hook that in place of the yellow at each end and then he would at least be able to narrow it down to a final problem.

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