Cruisair SMX II Thermistor test

[Briankinley2004]

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

 

[Reefgeorge]

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

 

[Reefgeorge]

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

 

[Briankinley2004]

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.

 

[Boatsb]

What connections does it have.

They're not hard to replace

 

[Briankinley2004]

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

 

[Cricket]

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.

 

[drburke]

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

 

[Briankinley2004]

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

 

[Boatsb]

If it is a corrosion problem cleaning is the key. After you can apply a protective layer.

 

[krush]

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/

 

[Briankinley2004]

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

 

[Briankinley2004]

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.

 

[Reefgeorge]

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

 

[Briankinley2004]

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

 

[madhatter1]

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.

 

[Reefgeorge]

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

 

[madhatter1]

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.

 

[Briankinley2004]

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.

 

[Reefgeorge]

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