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Thermostat LCD is blank and a/c does not run--believe problem is loss of 24 vac from unit transforme. where is 24 vac transformer physical location and how do I check it with a voltmeter?

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  • mtsuo Dec 11, 2009

    this is a Goodman CLT48-1B central a/c unit with a Honeywell T8001C thermostat. In its configuration, there is no internal battery for the Honeywell thermostat, rather, this type draws it power from the 24 vac transformer on the system----somewhere???



    I need to locate the 24 vac power source---I think it is on the condenser--and check to see that

    the voltage is being delivered there.



    Can you help me by telling me where the pick-up point for this 24 vac source is physically located?



    And how do I measure its output using a multimeter(I know the basics of using one)



    thanks

  • mtsuo Dec 11, 2009


    jyackle5

    according to the troubleshooting T8001 chart, NO DISPLAY indicates NO POWER TO THERMOSTAT. It says check that TERMINAL C is connected to system transformer

    I need to know where that 24 vac transformer is physically located.

    thanks

  • mtsuo Dec 11, 2009


    jyackle5

    i took a multimeter to the thermostat mounting wallplate wired connections---there are 4 18 guage leads, and I measure the ac voltage across each pair of leads---each pair read 0.2 mV ac. so I don't think the thermostat is receiving power.

    i have to locate the 24 vac transformer---can you help me?

  • mtsuo Dec 11, 2009

    I've locatd a GOODMAN wiring diagram for remote condensing units. In it there are shown connections to a 24VAC power source for both the outdoor control box and thermostat.

    Specifically, it says to connect the above to appropriate control circuit having min 40VA 24 volt N.E.C. Class 2 transformer; etc.

    I am seeking the location of that transformer.

  • mtsuo Dec 12, 2009

    well, I finally got an a/c tech out here and they located the problem and showed me where the 24vac transformer was located. and fixed a circuit with a poor connection---that did the trick.

    thanks for your more thorough explaination, I think I could have done the same as the tech, with it.

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Hi, if indeed it is the transformer and not the batteries in the thermostat, check first, Depending on if your furnace is a split system, or a package unit, the transformer is located just after where you're main or high voltage enters the unit. In a package unit as on the roof, it would be in the main control panel just after the Contactor.On the split system furnace and condenser split up, it is located in the furnace control panel where the 120.volts comes into the furnace. It is normally a square looking silver solid looking box with electrical wiring connects to both sides.The split furnace will have 120 volts going into it, 1-black and 1- white.The other end will have 24 volts coming out. It may have markings such as R, C, G, Y, and W, just like the thermostat. This is the 24volt side supplies voltage to stat and other components. Set meter to above the voltage you want to check.Check to see if you have 120 volts coming in across the black and white. If there is power to the furnace, you will have 120 volts to the transformer. Now check the the other end to see if you have 24 to 28 volts from Red to C. If you do have between 24 to 28 volts a/c coming out, then the transformer is good and you need to look for a small 3 to 5 amp fuse. Newer furnaces will have them plugged into the board. Check to see if it is just the fuse blown. Another thing to check is, some transformers will have a reset on them. If no power out, no reset, transformer is faulty. Most package units will be 220 volts into the transformer, stepped down to 24volts. Check the same way, volts in from main power would be 220 volts or above, and 24 volts coming out from the red wire to the white or black or Red to most other combinations of the stat wires or control wires. Very easy to check. Always look for the reset on the transformer, and a split furnace will be 120 volts in, and 24 out.Nothing out, faulty transformer. A roof top package unit, make sure there is power to the unit first, then check the high voltage in which is called the primary volts. Coming out is called the Low or secondary, 24 volts. I hope this has helped you in finding and checking the control transformer. Very easy to check, and to replace.Just remember, power coming in with nothing coming out 24 volts, no reset, bad transformer. Just follow my steps to the incoming power supply, and you will see the transformer. If you need more help, please contact me, and I will be here for you.Take you're time and it will go smooth for you.Please let me know the out come. Sincerely,
Shastalaker7

Posted on Dec 11, 2009

  • 1 more comment 
  • Robert Horton
    Robert Horton Dec 11, 2009

    Hi, if this is a split which has a remote condensing unit with the furnace / blower unit elsewhere, it is located in this indoor unit not the remote condenser.Where ever this furnace is, and the power coming into it, you will find the 40va, class 2, 120 volts X 24 volt transformer.If no batteries in stat, then yes it is powered by this transformer. I tried to give you the best description I could, just follow main power into the control section of the indoor unit.The outdoor condensing unit has a contactor, compressor, run cap.Transformer is always in the indoor unit on a split system with a remote condenser.Make sure there is power to the indoor unit.It will be mounted on the down end of main power. It is hard to give you the exact location, but you will see the 4 conductors from the stat coming from the 24 volt end. Red, Yellow, Green and, White. Check from Red to ground and it will show 24 volts if it is good and not open.If you can't see it, look at the diagram on this indoor unit and it will point it out. !20 volts in, primary side stepped down to 24 volts on the secondary side.Follow the 4 colored stat wires and they will take you to the transformer, as that is where they will terminate on most all furnaces. Could be terminated to a circuit board also, but look for it on the main voltage in side. That's where you will find it, not outdoors, only if it were a package unit all in one.I have faith you will find it, it could be setting atop a fan center with a cover over it. But, it will be easy to find if you will indeed follow the main conductors in, and they will take you to it as this is where they step down power to 24 volts, from the 120 volts in. I know you will find it. Please keep me posted. Sincerely,
    Shastalaker7
    PS, Focus on the indoor unit, as it is there :)

  • Robert Horton
    Robert Horton Dec 11, 2009

    Hi, I just wanted to mention that the outdoor unit is supplied with 24 volts from the same transformer to control the contactor coil for cooling. This 24 volts closes the contacts energizing the compressor and outdoor fan.It ir fired by the thermostat. So once you locate the transformer and replace or check, you will be set for both heat and cooling. Remember to check for the fuse I mentioned. Good Luck and keep me posted.
    Shastalaker7


  • Robert Horton
    Robert Horton Dec 12, 2009

    Good morning, I am sorry that you had to call a tech out, and I know that in the search for the transformer you would have found it and been able to make the repairs, I was confident you would and could have. Thank you for you're ratings it was very kind of you. I hope you will continue using Fix ya anytime you may have a problem, and I look forward in helping you in the future.Happy Holidays to you and you're Family, Sincerely,
    Shastalaker7


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In side the thermostat there is 2 AA or AAA battery's that run the display. You might want to replace thous first before you go further. If you give me make and model # I will be able to help you more.

Good Luck

Posted on Dec 11, 2009

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