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  1. #1
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    Jul 2011
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    Confused Lennox RTU LGA120SH1Y LOW PRESSURE TRIPS

    I've got a lennox unit that is giving me a code 24 or low pressure trip on circuit #2. THis is wired into the A57 board directly. I've tried about everything from changing the low pressure switch, changing the boards, jumped the switch right at the board wiring, but nothing seems to stop this from tripping the unit with this 24 code on the A57 input. Any help would be appreciated.
    Last edited by mturan; 08-01-2011 at 01:14 PM. Reason: Wrong Board number

  2. #2
    Join Date
    Nov 2006
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    Southeastern Pa
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    What is the actual low side pressure?
    [Avatar photo from a Florida training accident. Everyone walked away.]
    2 Tim 3:16-17

    RSES CMS, HVAC Electrical Specialist

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  3. #3
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    Jul 2011
    Location
    Montana
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    6

    Actual Pressure

    The actual pressure was 68 psi and it never droppped less than 60 psi the entire day when it was going out on the low pressure trip.

  4. #4
    Join Date
    Apr 2005
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    where the beer flows like wine
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    the wiring/harness could be bad. you could set ECTO 4.14 to 8 counts (max) to disable compressor lockout due to LPS

  5. #5
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    Nov 2006
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    If you placed a jumper at the board inputs and you still get the code, you could do as suggested above and adjust the ECTO parameters, but I would replace the IMC.
    [Avatar photo from a Florida training accident. Everyone walked away.]
    2 Tim 3:16-17

    RSES CMS, HVAC Electrical Specialist

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  6. #6
    Join Date
    Jul 2011
    Location
    Montana
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    Low Pressure Trips

    Ok thanks for the advice guys I will try those both and see if this works for us. Have you ever seen this situation before on this unit? Not really familiar with the Lennox units. Thanks again!

  7. #7
    Join Date
    Nov 2006
    Location
    Southeastern Pa
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    Quote Originally Posted by mturan View Post
    Ok thanks for the advice guys I will try those both and see if this works for us. Have you ever seen this situation before on this unit? Not really familiar with the Lennox units. Thanks again!
    What I typically see is when there is a leak, the stage will be locked out on the low pressure switch opening three times. Look on the left inside door panel for the code list, and you will see that code right after the code you mentioned.


    Reset the code by holding in the black square buitton for three seconds. Run the unit and take a voltage reading at the two wires where they enter the terminal strip at the A55 board. You will see that one side of the connector body next to the wire has a little more room than the other side, and that's where you can slip in a slim test probe. Monitor that voltage, which should be near zero, and if a code sets while monitoring a zero voltage, you will know that the pressure switch and wiring is good, and that the board CPU or some ancillary device on the board is failing.

    If, however, that voltage goes higher and then the code sets, you could have a bad pressure switch.

    This problem you are having is unusual, so take your time and verify what you are doing. Make sure the sw # mentioned in the code list is indeed the switch you are looking at on the stage where you are taking the pressure readings. If you confirm that the circuit is closed at the board connectors, yet you have a code shown, it's time to replace the IMC.
    [Avatar photo from a Florida training accident. Everyone walked away.]
    2 Tim 3:16-17

    RSES CMS, HVAC Electrical Specialist

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  8. #8
    Join Date
    Jul 2011
    Location
    Montana
    Posts
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    Low pressure trips

    Thanks timebuilder for taking the time to help me with this. I will check on these items when I can get back to the site and let you know what I found on this. Thanks again for the help.

  9. #9
    Join Date
    Feb 2006
    Location
    USA
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    op u said u jumped it out at the board & issue repeated tho .. id say that pretty much sums that up . . no?

  10. #10
    Join Date
    Jul 2011
    Location
    Montana
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    Yea I jumped the switch at the A57 input cause its on circuit #2 and still got the diagnostic so i eliminated the switch and the wires. I have also changed out the A55 and A57 boards on this. Not sure what the IMC is but i can look into the ecto settings and try and do something there. I just appreciate the dialogue on this as I'm not that familiar with the lennox unit. Thanks everybody.

  11. #11
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    Nov 2006
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    For someone who is familiar with the unit, yes, that would sum it up. But, I have seen new guys who believe they have taken steps such as those, and yet, made some error along the way.

    My idea is to double check the work before a pricey board gets replaced.
    [Avatar photo from a Florida training accident. Everyone walked away.]
    2 Tim 3:16-17

    RSES CMS, HVAC Electrical Specialist

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  12. #12
    Join Date
    Nov 2005
    Posts
    1,350
    I would data log the suction pressure, DAT, and RAT, and possibly duct static, so I would know when the fault is happening. Possible low load, low air flow, etc. It sounds like you may be going down a alternate path.
    I STARTED WITH NOTHING, AND I STILL HAVE MOST OF IT!

  13. #13
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    I have worked in hundreds of LG's, I'm yet to find the first bad A55 board, cant say It doesn't happens but the boards are pretty much bulletproof.

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