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  1. #14
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    I haven’t heard of the Low Z function. What is it?

    Quote Originally Posted by timebuilder View Post
    My suggestion is to clip one lead (I use the black lead, from my habits with DC) to COMMON, and I probe around with the red lead.

    This ensures that the voltage is at least present.

    My second level when doing this is to use the Lo Z function, which loads the circuit, causing an increase in the current used to make the reading, and that helps to reveal high resistances in the safety chain.


    I will typically pick a limit and look for voltage. If I have it, I look at the diagram to see where in the chain the voltage I just found is located. If that is the first break point from the board, I continue to check down the circuit to make sure it is never lost, and I find where it returns to the board, at its connector.

  2. #15
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    Quote Originally Posted by Achso017 View Post
    I haven’t heard of the Low Z function. What is it?
    Check your email. I sent you a couple pdf's.
    Instead of learning the tricks of the trade, learn the trade.

  3. #16
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    Southold, NY
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    Quote Originally Posted by rundawg View Post
    Check your email. I sent you a couple pdf's.
    Can I get a copy?

  4. #17
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    Quote Originally Posted by Achso017 View Post
    I haven’t heard of the Low Z function. What is it?
    .
    Attached Images Attached Images

  5. #18
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    Quote Originally Posted by rundawg View Post
    Check your email. I sent you a couple pdf's.
    Thank you. But now I have to buy a new meter. So thanks a lot


    Sent from my iPhone using Tapatalk

  6. #19
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    Quote Originally Posted by Achso017 View Post
    Thank you. But now I have to buy a new meter. So thanks a lot


    Sent from my iPhone using Tapatalk
    If your really good maybe Santa will surprise you!

  7. #20
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    Quote Originally Posted by VTP99 View Post
    .
    Thanks. Looks I’ll need to acquire a new tool.


    Sent from my iPhone using Tapatalk

  8. #21
    Join Date
    Nov 2006
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    Until you get that meter, you can take a contactor and put two jumpers on it. You will not exceed the current capacity of the board, but you can put one lead on common and probe with the other. Every terminal in the safety chain, including both board terminals, should pull in the contactor. If all of them do, then you do not have a limit issue...instead, you have an issue that is causing a limit to open...so it is doing its job.

    Of course, some RTU's have line voltage limit circuits, and this idea is not recommended for that kind of testing. This is only for 24 volt circuits.
    Last edited by timebuilder; 12-07-2019 at 12:20 PM.
    [Avatar photo from a Florida training accident. Everyone walked away.]
    2 Tim 3:16-17

    RSES CMS, HVAC Electrical Specialist
    Member, IAEI

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  10. #22
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    Aug 2018
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    Quote Originally Posted by pecmsg View Post
    If your really good maybe Santa will surprise you!
    If I saw Santa I would definitely be surprised. And if I saw someone in a red suit crawling out of my chimney I can promise you they wouldn’t stay “jolly” for long


    Sent from my iPhone using Tapatalk

  11. #23
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    Quote Originally Posted by timebuilder View Post
    Until you get that meter, you can take a contactor and put two jumpers on it. You will not exceed the current capacity of the board, but you can put one lead on common and probe with the other. Every terminal in the safety chain, including both board terminal, should pull in the contactor. If all of them do, then you do not have a limit issue...instead, you have an issue that is causing a limit to open...so it is doing its job.

    Of course, some RTU's have line voltage limit circuits, and this idea is not recommended for that kind of testing. This is only for 24 volt circuits.
    That’s...just...an incredibly great idea. My bank account thanks you as well.


    Sent from my iPhone using Tapatalk

  12. #24
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    Quote Originally Posted by pecmsg View Post
    Can I get a copy?
    Check your inbox.
    Instead of learning the tricks of the trade, learn the trade.

  13. #25
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    Southold, NY
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    Quote Originally Posted by rundawg View Post
    Check your inbox.

  14. #26
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    Oct 2018
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    Nc
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    Quote Originally Posted by BKHVAC View Post
    Hi. I was called out to work on Armstrong packaged unit pge10B36D075A-9B.

    After closing disconnect the furnace control board receives power, waits approx 20 seconds, and starts the inducer fan motor- without a call for heat. I checked voltage from c-w: 0v. Checked R/O and Hi limit circuit- maintains 27v back to furnace control board. Then checked pressure switch and it remains open until inducer fan kicks on, and then closes. No voltage across pressure switch unless c-w becomes energized and then voltage allowed through pressure switch, across aux switch on fan, and back to board.

    When the board receives power immediately LED flashes twice. The trouble codes are no longer on the furnace but from what i could find online seems to be - System lockout. Failed to detect or sustain flame. However, the gas valve never energizes and no spark is ever attempted.

    I've replaced the control board and removed the flame sensor from the board to see if the fault still exists, and still flashes twice. Any ideas?
    Have you check pressure switch and the tube to see if it have crack/leak? Or maybe check inducer assembly, sometime little crack caused similar issue.


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