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  1. #11

    Re: Cruisair SMX II Thermistor test

    Call up Microair and ask for tech support (with your list of symptoms and results of your past tests). They have been really helpful to me. Also, I think they repair boards.
    Microair is the one that makes all these controls and boards: https://www.microair.net/
    FTFD... i drive a slow 1968 41c381

  2. #12

    Re: Cruisair SMX II Thermistor test

    Well I decided to clean the terminals on both ends. Used electronic contact type cleaner. As expected this didnt help anything but I did find I mis stated something above. The SMX2 keyboard has a 4 pin connector not a phone jack type as I thought. The same end is in the engine room at condenser. The thermistor has 3 pins not 2 as I stated. I cleaned both ends and checked for splices, bad wires, etc and found nothing. So I suppose I will start with a new thermisotor as I havent been able to find the test method. If that doesn't work I will send the board to Flight systems or somewhere

  3. #13

    Re: Cruisair SMX II Thermistor test

    UPDATE: I now have an answer to my original question thanks to Tyler at Flight Systems. I emailed them to find out the turn around time on rebuilding the power logic board. He does not think this is the problem based on the symptoms mine has. He thinks its the thermistor and gave me the following information to test it:

    "If you have an ohmmeter the resistance at 83 degrees is around 2.5K Ohms and at 73 degrees is around 3.5K Ohm"

    On further discussion there is supposed to be resistance only between 2 of the 3 wires. The third is a shield. He also said start with factory reset but I already tried that. He also suggested corrosion on the sensor itself could cause a low temp reading. I didnt look at it real close so that could be my issue. Next time I am at the boat I plan to trace this out but this is now the suspected problem.

  4. #14

    Re: Cruisair SMX II Thermistor test

    I found my notes. You measure the thermistor resistance between the green and red wires. This is a NTC (negative temperature coefficient) thermistor. 90F=2.2K, 77F=3.0K, 65F=4.1K.

    George
    Florida
    2002 Cabo 47
    MAN mechanical 800/8's

    "You can avoid reality, but you cannot avoid the consequences of avoiding reality"

  5. #15

    Re: Cruisair SMX II Thermistor test

    Thanks George. Your values seem to be proper interpolated with those I got from Fight Systems. I went ahead and bought another thermistor as they were not too expensive and it may save me a trip. I plan to go down towards the end of the week. Will let y'all know what I find

  6. #16

    Re: Cruisair SMX II Thermistor test

    Ice water test is a very accurate way to test the sensor. Must be crushed ice, filled with water, let it settle for 10 minutes, shake, and settle a couple more minutes for super accurate results. But to just see if its in the right ballpark you can add water, shake some and dip.

    In reality buying the new one makes the most sense and then metering back to the unit.

  7. #17

    Re: Cruisair SMX II Thermistor test

    An ice water test is good for sensors that measure freezing or near freezing temperatures like in refrigeration but this sensor operates in a roomish temperature range which is where it needs to be accurate.

    George
    Florida
    2002 Cabo 47
    MAN mechanical 800/8's

    "You can avoid reality, but you cannot avoid the consequences of avoiding reality"

  8. #18

    Re: Cruisair SMX II Thermistor test

    Yes that is true but the ice test is to calibrate the sensor to the controller because the water is almost exactly 32 degrees. Based on that it is deemed to be valid throughout the range. Thermistors nearly never go bad or out of range. When they do it is normally something else that forced the "smoke" out of them. Anyway since cheap replacement eliminates them as the issue. Then wires and connections. If that checks out then the board reading them.


    Shouldn't take OP too much to locate and fix the issue. Most likely minor corrosion on a connection as others stated. Hope so since anything marine and electronic is somehow big bucks.

  9. #19

    Re: Cruisair SMX II Thermistor test

    Well I am convinced in spite of what Flight Systems guy told me my control board is bad or maybe one of the solder joints on it is loose. Ohm reading on the new thermistor which I did not install is within a couple hundred ohms of the original. Pretty much at same temp one reads 2.55K and other 2.59K. Room is in mid 80's. Did factory reset again and it shows room temp at 13 degrees F. Unit still runs constantly with nothing happening in heat.

  10. #20

    Re: Cruisair SMX II Thermistor test

    One of the weaknesses of this design in my opinion is the RJ12 phone style connectors used to attach the SMX display and temp sensor to the board. They are open to the elements in a harsh environment and not protected and it's difficult to determine if the board is seeing 2.6K ohms after the connector.

    If you have more than one system the fool proof way to determine the status of the suspect board is to swap that one with a known good board from the other system and see if the problem follows the board.

    George
    Florida
    2002 Cabo 47
    MAN mechanical 800/8's

    "You can avoid reality, but you cannot avoid the consequences of avoiding reality"

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