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  1. #1

    Cruisair SMX II Thermistor test

    Anyone know how to test the thermistor on the subject control system? My salon a/c control is giving me some weird numbers. Showing inside temp at 19 but set at 76. When I try to do the factory reset and adjust the offset it will only let me adjust it to 40 or something. Almost like its reading Celsius for actual and Fahrenheit for set point. Control is set to show Fahrenheit. Im thinking the control board in engine room is bad but before I replaced it wanted to rule out the thermistor. Yes I have checked all the connections and visually all look good. Thanks for any help

  2. #2

    Re: Cruisair SMX II Thermistor test

    Check the RJ11 phone cable connection at the condensing unit since it’s open and subject to condensation. If no issue there then swap out a known good thermistor and cable just down in the engine room and see if you get a proper temp reading. The thermistor varies resistance with temperature so you can check a few points. I have a chart somewhere for these thermistors in my worm hole of records. I can look for it if your interested. Alternatively, you can generate your own chart with a known good sensor as a reference.

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

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

  3. #3

    Re: Cruisair SMX II Thermistor test

    I can’t edit my post on my IOS device so in addition:

    If the sensor is bad you can splice in a new one near the evaporator to avoid pulling a new cable but you have to solder it and shrink wrap it properly.

    If the control board is bad you can have it rebuilt by Flight Systems. Myself and others on the forum have used them with good success.

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

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

  4. #4

    Re: Cruisair SMX II Thermistor test

    Thanks George. Connections look fine. I was thinking of putting the thermistor in an ice slurry but not sure what resistance should be. If you can find chart easily it would be good to know but if not dont worry. Im leaning toward the control board being bad as nothing happens in heat mode. No fan, compressor, etc. I think thats probably going to be the issue but would like to rule out thermistor while I am here on the boat. thanks for the info on splicing also. I will probably send the board to flight systems once I can go a few weeks without a/c. I got the inside key pad from them and it seems to be properly done.

  5. #5

    Re: Cruisair SMX II Thermistor test

    What connections does it have.

    They're not hard to replace
    Scott
    41C117 "Hattatude"
    Port Canaveral Florida.


    Marine Electronics and Electrical Products Distributor.

  6. #6

    Re: Cruisair SMX II Thermistor test

    My thermistor is hard wired at the evaporator coil end. Other end goes to the control board in engine room and has a two prong pin (female) type. The keypad has a phone jack style on the SMX2 keypad and on the control board has a female 4 pin connector. Both plug to pins soldered onto the circuit board

  7. #7

    Re: Cruisair SMX II Thermistor test

    The phone jack plugs are super sensitive. unplug and give them a light cleaning, sort of fragile.
    How about the SMX control itself, a constant source of trouble for us, always carry 2 spares.
    CRICKET
    1966 HAT50C101
    Purchased 1985 12v71Ns
    Repowered 1989 with 8v92TI
    Repowered 2001 with 3406E

  8. #8

    Re: Cruisair SMX II Thermistor test

    What Cricket is saying is pretty true, so let me offer this.

    Many problems with electronics are from external factors, namely inputs and signals related to physical environments, in our case high-salt, high-humidity air. Contacts oxidize, which is manifested as a high(er) resistance layer on connector pins. The RTD thermistor (if that’s what they went with) is 100’s to 1,000’s ohms and may now be is series with a few hundred ohms of contact oxide.

    Connector designers concern themselves with gas-free contact regions and wipe force cleaning.

    Easy to eliminate by cleaning as Cricket said, but quick test is to just unplug and plug in all connectors a few times. You said they “look clean” but oxides are microscopic and invisible.

    If still bad, suspect the board next.

    DAN

  9. #9

    Re: Cruisair SMX II Thermistor test

    Quote Originally Posted by Cricket View Post
    The phone jack plugs are super sensitive. unplug and give them a light cleaning, sort of fragile.
    How about the SMX control itself, a constant source of trouble for us, always carry 2 spares.
    It has been unplugged couple times but I will clean. Would it be ok to use di electric grease after cleaning? The SMX control, (im assuming the keypad?) in the salon is new from flight systems. It is doing same thing old one was so I dont think that is issue

  10. #10

    Re: Cruisair SMX II Thermistor test

    If it is a corrosion problem cleaning is the key. After you can apply a protective layer.
    Scott
    41C117 "Hattatude"
    Port Canaveral Florida.


    Marine Electronics and Electrical Products Distributor.

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