Cant figure this out! Not getting 24v at the board or thermostat

Discussion in 'HVAC' started by jayrod, Mar 6, 2013.

  1. Mar 6, 2013 #1

    jayrod

    jayrod

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    Its a Carrier split unit. Had the heater on yesterday and at some point when I wasnt around it turned off on its on. Its completely dead at the thermostat, and on the circuit board it doesnt get 24 volts either. So I tested voltage going into the transformer and it has 120 volts on all 3 wires going in but it only has .1 volts on the 24v side. Well thats not right. We have a 2 story house and both units have the exact same transformer so I measured the transformer on the working unit to be sure Im dong it right and it measures 120v on all 3 coming in, and about 27 volts going out. So I go and buy a new transformer, wire it up, and still no 24 volts! Did a resistance check, all wires have some resistance. So now Im stumped. If it has voltage coming in, how is it not coming out!? What should I check next?
     
  2. Mar 6, 2013 #2

    nealtw

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    Switch transformers from the other unit, maybe you got a bad one.
     
  3. Mar 6, 2013 #3

    kok328

    kok328

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    Your on the right track but, nealtw has the most probable answer. What are the odds?
    Unfortunately, most of the time, electronic parts are not refundable as the seller suspects that you did something wrong to burn up the component.
     
  4. Mar 6, 2013 #4

    jayrod

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    Thanks guys. I have switched transformers this morning. I took the new one I just bought that didnt fix my downstairs unit, and installed it on my working upstairs unit. It didnt work! Same thing, power coming in, but no 24v coming out. So I took it out and put it back like it was and it works still so all I know is I did get a bad transformer or either something shorted it out right when I hook it up. but it doesnt smell or hum or anything. Oe thing I did notice is the working uptairs unit only requires the common and the 240v wires of the transformer to be hooked up while the downstairs unit requires the common, the 208v and the 240v wires to be hooked up. (Common goes to XFORMT1, 208v goes to T2, and 240v goes to T3) But thats how the old transformer was wired so I wired the new one the same. I just didnt think it was very likely to get a bad Transformer.
     
  5. Mar 6, 2013 #5

    jayrod

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    Wait, I believe I should have been measuring across the terminals to get 240v or 208v. I aint geting either when I measure across. I think there must be a short or a safety switch keeping the 240v from coming in. The contactor outside has 240 going in and 240 going out when I manually press it in. T2 and L2 have no voltage on the contactor.
     
  6. Mar 6, 2013 #6

    CallMeVilla

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    Standing by, with rapt attention, to see how this resolves .... :)
     
  7. Mar 6, 2013 #7

    kok328

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    Ok, now you've got me confused. You originally said that you have a 120V primary and 24V secondary transformer. Now your saying it's 240V or 208V. Did you buy the wrong transformer and smoke it when you hooked it up to a higher voltage?
    T2 & L2 are opposite sides of the contactor but, are the same leg.
    Sounds like you blew a fuse on the L2 phase.
     
  8. Mar 6, 2013 #8

    jayrod

    jayrod

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    I dont see any fuses on the outside part of the unit. Its the right transfomer, I just didnt undersand it. Its a 240v setup. It says 240vac on the little circuit board inside the handler.
     
  9. Mar 6, 2013 #9

    jayrod

    jayrod

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    Should T2 and/or L2 have any voltage when its not turned on?
     
  10. Mar 6, 2013 #10

    kok328

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    is this a heat pump unit?
    L1 & L2 is LINE power, T1 & T2 is the LOAD.
    On a split system, the air handler/furnace components would be 120V and the compressor unit outside would be 240V.
    Where exactly are you working (on the 120V or 240V side of the split system)?
    If your working outside on the compressor unit where I would expect to find a contactor, then the fuses would be in a service disconnect box. Follow the electrical lines from the compressor unit to the side of the house where you will find a metal box that houses the fuses.
    You may have to remove a piece of cardboard to see the fuses (make sure you turn the breaker off to the unit before digging into the box).
     
  11. Mar 7, 2013 #11

    jayrod

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    The metal box has a quick disconnect thats not fused. It says "disconnect not fused" right on it. Its 240 volts outside and inside on the air handler. I measured the main wires going into the handler and they measure 240v. I checked the main breaker and it has 240v.
     
  12. Mar 7, 2013 #12

    keepinitcool

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    Get a qualified person to service this unit. Electricity is nothing to mess around with and it will save you from damaging other components of your system and possibly costing you more money in the long run.
     
  13. Mar 8, 2013 #13

    nealtw

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    This may be good advice but kok328 has already determined that the op has some knowledge and is trying to work thru it with him. It is a diy site.
     
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  14. Mar 8, 2013 #14

    wireless

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    Bad fuse on the load side of the transformer.
     
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  15. Mar 8, 2013 #15

    kok328

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    Can we get a inside pic of the cabinet so I can see what I'm doing?
    Being able to see is most benefical. Blindfold a tech and ahe will at least be able to feel his way around, we can do neither.
     
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  16. Mar 9, 2013 #16

    jayrod

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    Sorry for the late reply It didnt notify me by email like it was and I thought you guys gave up on me. I should be fine working on it. I have fixed it numerous times over the years but this time Im not sure what the problem is. Heres pics of the handler inside the house. If you need more, let me know.

    ac 001.jpg

    ac 002.jpg

    ac 003.jpg

    ac 004.jpg

    ac 005.jpg
     
  17. Mar 9, 2013 #17

    kok328

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    Ok, now that I can see what I'm dealing with, I now know that I'm working on an electric furnace not a gas furnance.
    Pretty sure you have a multi-tap transformer that takes only 1 of 3 combinations of primary power (120V, 208V or 240V). The secondary power goes directly to the board.
    What are you using for primary power (120V, 208V or 240V)?
    Also, can't tell if the fan wire is just hanging loose or is attached to the Y2 (second stage cooling).
    Can I get a pic of how the primary wires are connected to power?
    At this point your saying that you have primary power but, no secondary power and that the transformer from your other unit works in this unit and the unit heats and runs fine with the transformer from the other unit, correct?
     
  18. Mar 9, 2013 #18

    jayrod

    jayrod

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    Yellow is connected to Y2. The green one is hanging loose because I tried to jump it with R way earlier.

    I never tried the transformer from my working unit in this unit for fear of shorting it out and then being stuck with no working units! I tried the transformer from the non working unit in the working unit and that made the working unit non-working, if that makes sense. Really you just had it backwards.

    By primary you mean the common? The common plugs into the XformT1 terminal in the above pic. It was unhooked in the picture. When I measure across the XforTt1 terminal and one of the main incoming wires, I get 240V. Wouldnt that mean its losing secondary power somewhere along the way in there? Thanks alot! I'll get another pic soon.
     
  19. Mar 9, 2013 #19

    jayrod

    jayrod

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    If I measure across XformT1 and any of the wires on the front of the heat sequencer I get 240v as well.
     
  20. Mar 9, 2013 #20

    kok328

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    If the two units are identical, then this will make troubleshooting a breeze.
    Are the primary transformer wires in the working unit connected to 120V, 208V or 240V?
    What voltage are you readings on the connections on the primary wires for the working unit?
    Are the same voltage readings on this unit?
     

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