SMX A/C controller

[stimmy]

I noticed another recent post on these controllers and I hoped someone could help me with my problem.
My boat has 4 old SMX's that sort of work. I assumed it was their age that made them finicky and unable to hold a temperature, but after just replacing the 5th one with a new SMX 2, I'm not so sure.
I was working in the salon, where the new controller was installed a few weeks ago and I felt chilly, downright cold. I happened to have an independent thermometer in the salon and it read 68 degrees. I looked at the SMX 2 and it was set at 74 but the unit was running continuously. When I pushed the inside temp button on it, it read 77 degrees.
Where is the temp sensor (which I assume is just a thermister) located for the SMX? If it is on the board, as I suspect, it is inside a wall where there is direct sun on the exterior wall, which may make the SMX think it is 77 in the salon, even though it is actually 68 there.
The problem is similar to what the other 4 old units do; there seems to be little correlation between the setting and the value they hold. Sometimes a setting of 74 will chill you, and sometimes it will cook you. It seems that the temp is more closely held in the evening, when the outside temp is closer to the inside temp so the internal wall heating effect would be minimized.
I'm just guessing about all this, but it seems everyone with a cruise air system would have the same problems if it is as it appears.
Any ideas or fixes? An independent sensor in an area more representative of the actual room temp would go a long way if that is the problem.

 

[bobk]

On mine, the temperature sensors are located in the return air stream near the evaporator coil. I think these are thermocouples.

Bobk

 

[Reefgeorge]

On mine, the temperature sensors are located in the return air stream near the evaporator coil. I think these are thermocouples.

Bobk

Same here.

George

 

[stimmy]

Maybe all my thermocouples are bad? Where can I get some and how do you change them?

 

[bobk]

A thermocouple is a pretty simple device so it would be a stretch for all of them to go bad at the same time. They are basically dissimilar metal wires welded together. No moving parts, but presumably there is a potential for corrosion. There are many metal combinations used for thermocouples, so it you are going to replace one, you need to know what type the controller is calibrated for. The most common in our chemistry labs are J or iron/constantan, but iron may not be used in a marine environment.?

I would start by making sure the current TC is installed in the correct location. Did you say if this is a new issue?

Bobk

 

[Reefgeorge]

The thermocouple has a cable from the evaporator back to the condensing unit using a crappy unprotected telephone jack to attach to the controller box. Make sure that there is not corrosion there. Wiggle it aroud and see if the readings jump around. A known good probe can be plugged in and the wire run dragged into the salon/cabin to see what you have.

Thermocouples come with a variety of cable lengths and the telephone connector on the non business end. Call a cruisair dealer.
George

 

[krush]

Not to be that guy, but the sensors are most likely thermistors, not thermocouples. https://www.microair.net/products/sensor-air-temperature?variant=29194709131

 

[davidwigler]

The manual has a procedure to adjust the thermometer value. It must be calibrated to a known temperature.

 

[bobk]

Not to be that guy, but the sensors are most likely thermistors, not thermocouples. https://www.microair.net/products/sensor-air-temperature?variant=29194709131[/QaUOTE]


Not at all, I thank you for stimulating me to look up the differences between TCs and thermisters. So then I went to an on line Cruiseair parts catalog to see if the type of sensor was identified, and it was not.

Bobk

 

[krush]

So then I went to an on line Cruiseair parts catalog to see if the type of sensor was identified, and it was not.

As far as I know, MicroAir makes all the cruiseair controls. Call MicroAir up and they will help out.

 

[Reefgeorge]

This is pretty basic but just in case you missed something:

https://www.flightsystems.com/pdf/cruisair-troubleshooting-guide.pdf

George

 

[Tim Powell]

A thermocouple is a pretty simple device so it would be a stretch for all of them to go bad at the same time. They are basically dissimilar metal wires welded together. No moving parts, but presumably there is a potential for corrosion. There are many metal combinations used for thermocouples, so it you are going to replace one, you need to know what type the controller is calibrated for. The most common in our chemistry labs are J or iron/constantan, but iron may not be used in a marine environment.?

I would start by making sure the current TC is installed in the correct location. Did you say if this is a new issue?

Bobk

If you have the smx on continous fan mode they will be fine if lockated in the return. The compressor will be cut off and fan will be put in the slow mode. BUT if you have the controller smx on stop and start or as temperature changes the fan cuts off you may have to move the thermocupler. That is spelled out in the cruse air book. But i will tell you from experience. DOURING THE COOL MODE AND DURING THE HEAT MODE YOU WILL BE MOVING THE THING AROUND. Get a heat censer gun and shoot the location of the thermocupler now you will find out if the cupler is bad or not. Remember hot air rises and cold falls so if the cupler is locked in the return and the return is on the floor the season is going to dictate a change.
I have the problem with my salon ac/heat the return it at the top of the stairs and it sucks heat from below not from the salon it is hard to regulate, I moved the coupler to mid salon and i seems to maintain a better steady temp.

I noticed the tc in the web provided that is the new modle do not get confused if you see one that looks like a small red light its the old one. Both are just two wires does not matter which way the wires are connected that is if you have to cut the wire rather than running the full cable .

 

[Reefgeorge]

I moved my temp sensors out of the evap flow and moved them into the cabin on a wall. Then I set the fan to only run with the compressor like in your house. The improvement in comfort was dramatic. The humidity dropped down to 40% so we have really cool dry air now. We are morecomfortable and then boat smells better.

George

 

[racclarkson@gmail.com]

Glad it's working better for you. I seem to have better success letting the fans run on continuous--thermocouples are located in front of return filters. Since cool air settles, I was getting wide ranges of temps and short period of operation due to low airflow over the sensor mounted low in the cooler environment on intermittent. I have added a table top fan in the master that has helped dramatically. The evaporator and sensor are way out of the way, really out of the airflow pattern of the cabin overall. With the fan, I can set the t-stat up a couple of degrees and remain comfortable with temp swings that aren't noticeable.

FWIW, I had issues with getting my SMXII to come out of humidity and back to regular cool. When I finally got it back, I decided (when I'm gone) to just switch fan from continuous to intermittent at a higher temp. I only run the salon AC and at about 85 degrees, but leave a squirrel cage in bow compartment on as well. Also leave a squirrel cage on low in the ER all the time in warm weather. Seems to work for me.

 

[stimmy]

Once again, a wealth of info from experienced owners. You've all given me great ideas on how to fix the problem. Now I know where to look. Thanks!
It did get worse when I switched the fan from continuous to intermittant. The thermostat in your home must have the same problem; it seems the sensor should be in the room, exposed to the same temp as you are, so you can set it to be comfortable.

 

[stimmy]

I've had more time to evaluate the performance of my A/C units. I prefer the intermittent fan operation so I don't have to listen to the fans running constantly; also. it seems the reduced running time on them should extend their life and reduce coil clogging.
However, the temperature swings with the thermistors in the air returns is unacceptable. I often have to readjust the thermostats 3 or 4 times an hour which is a PITA.
So I will move the thermistors to a location that reflects room temperature, not return duct temperatures which are affected by the evaporator proximity in a fairly enclosed space.
Are the thermistors wired to the control panel on the condenser or into the SMXII controller? Running new wires in my 65 enclosed bridge looks like a nightmare; if I could just put the thermistor on a wall near the controller, it would be a snap.
I couldn't find a definitive wiring diagram that shows; anyone know?

 

[Reefgeorge]

On mine it's an RJ11 telephone plug on the side of the condenser control box.

George

 

[Reefgeorge]

BTW, I moved the temp sensors in all three cabins to the interior space and changed the fans to intermittent. This set up dramatically improved the indoor air quality because of the additional humidity reduction. It was a big help for my wife's asthma.

George

 

[stimmy]

BTW, I moved the temp sensors in all three cabins to the interior space and changed the fans to intermittent. This set up dramatically improved the indoor air quality because of the additional humidity reduction. It was a big help for my wife's asthma.

George

What did you use to cover the thermistors in the interior? My boat looks to be very difficult to run wires from the mechanical room, where all 5 A/C condensers are, to any of the interior areas they cool.

 

[tonytrakovich]

BTW, I moved the temp sensors in all three cabins to the interior space and changed the fans to intermittent. This set up dramatically improved the indoor air quality because of the additional humidity reduction. It was a big help for my wife's asthma.

George

George.

How does this reduce the humidity by turning fans to intermittent? Where did you mount the sensors so they would not be affected by sun load but still get a good sampling of the room temp? If I remember, the sensors are not very attractive to look at.