Heat okay, AC not
Results 1 to 8 of 8
  1. #1
    Join Date
    Jul 2004
    Posts
    3

    Question

    Hey guys, I have a furnace with a Honeywell ST9120C-4040 System board in it.

    My heater works fine but my A/C won't kick on (worked fine last year) anyway I'm a complete noob at this HVAC stuff, but I pulled out my meter and the signal from the transformer is 24V.. the diagram on the Carrier AC unit says "External Power Source 24V"... but the signal going over the 2 wires out there is only 5V (When the T-Stat is set to AC on)... So is it just that I need to replace this board? Or is it maybe something on the carrier not sending a signal back?

    Maybe it's that simple and it's just a dumb question but figured I would ask before dropping the $105 on the board.

    Thanks in advance

  2. #2
    Join Date
    Mar 2004
    Location
    SE Michigan
    Posts
    17,772
    Is there a condensation pump at the furnace? If its full of water, it will break the cooling circuit. Next, check 24v wireing at stat, furnace, and cond. Make sure all connections are good. Does the condensor have any safty switches?
    "Politicians are the lowest form of life on Earth. Liberal Democrats are the lowest form of politician"

    - General George S. Patton

  3. #3
    Join Date
    Mar 2004
    Location
    Indiana
    Posts
    665
    Too many posibilities to guess at.
    Drop the bucks and get the system evaluated and repaired.

  4. #4
    Join Date
    Jun 2005
    Location
    Chicago
    Posts
    2

    Smile

    Yesterday I switched out the same board as above, and without being deterred by the comment to seek a professional, I broke down and purchased a volt ohm meter. Would have saved me a bit on the switch board, but it wasn't brain surgery. Though I will comment that multimeters now have these new fangled international symbols-so there was a learning curve on that instrument. Anyway, my original problem was that my heil furnace quit working-no fan, not a thing was happening. I did use a test lamp to determine voltage-but that didn't tell me what my real problem was. My 24 V AC transformer wasn't working properly. For the price of a quart of beer, I'm now enjoying my A/C again and I still saved money over hiring a professional. See, living in Chicago costs a lot of money if you call in a professional. I am not rich and these devices are a lot simpler to troubleshoot than the fire control radar systems I serviced in the military. Thanks all for posting your tips!!!

  5. #5
    Join Date
    May 2003
    Location
    Dixie
    Posts
    1,344
    hey laust and chillinied....what do you do for a living?...i mean what is your line of work?

  6. #6
    Join Date
    May 2005
    Location
    Calgary, Alberta, Canada
    Posts
    92
    Sometimes you get lucky............
    Yes we have air conditioning in our igloos.

  7. #7
    Join Date
    Jun 2005
    Location
    Chicago
    Posts
    2

    Talking

    I'm a dot com tech refugee who is mainly a carpenter-but I'm willing to try to do anything to feed the kids. For instance today a client who had an air conditioner which wouldn't kick on. Seems some of the neighbor kids had swiped the 30 amp fuses from the outside breaker box. That's how I keep them fed today.

    [Edited by chillinied on 06-07-2005 at 06:46 PM]

  8. #8
    Join Date
    Jul 2004
    Posts
    3
    good to hear you got it goin Chillinied.




    I'm in Computers for the time being, until I get laid off for the 6th time since 2000. LOL, oh well, guess thats how life goes for some people. =)


    I found a set of 4 wires that go into a plug with 6 pins, if I change the pinout of these wires I can get my AC running now.. but then in the winter I have to change it back for heat. To be honest I just didn't spend much time on it after that, but I am going to follow up on it and try to get it all right.

    The previous owner must have changed it over after I viewed the house back in september, then swapped it to run heat on me in in oct when I closed.

    [Edited by laust on 06-07-2005 at 07:34 PM]

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