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  1. #1
    Big Traveler boyscout's Avatar
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    Where should the internal thermistor for Coleman A/C be located?

    I've almost completed the A/C modification uncovered and reported by @Lampton here:

    https://www.mygrandrv.com/forum/show...l=1#post329705

    Great project, and I can feel the difference in airflow from bedroom ceiling vents after first modding the bedroom A/C.

    There is a thermistor (a small metal nipple on the end of a wire) inside these Coleman units. In the bedroom A/C that thermistor was hanging in the airflow on the return side of the unit, its wire wrapped around others such that the nipple of the thermistor was suspended in the airflow. The bedroom A/C always worked fine, not cycling too much (after we closed the wall-side vent on it) and doing a great job at cooling.

    In the main living area A/C the thermistor is jammed into what feels like a slot in a wooden board up inside the return chute. We've always thought that this unit worked far too much, roaring for ages before (I guess) achieving set temperature and shutting off, even in moderate outside temps in the low 80s trying to achieve mid-70s inside. There'd be a seemingly-brief quiet time and then it would be roaring again for a long time to bring the temp back down. I am hopeful about this mod improving the situation, but wonder if thermistor placement is part of the problem.

    So which placement of the thermistor is correct? Should it be jammed into that slot or should it be hanging free?

    TIA
    Mark - 2018 Solitude 310GK - 2017 F-350 diesel SRW short box - Pullrite Superglide hitch

  2. #2
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    Never noticed it but I would have it free rather than jammed in something. You could always jam it back later.
    David & Kyrenda
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  3. #3
    Big Traveler boyscout's Avatar
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    Quote Originally Posted by Lampton View Post
    Never noticed it but I would have it free rather than jammed in something. You could always jam it back later.
    Thanks for replying. Spurred by someone on another forum to take pictures, in the course of doing so I figured out that the "board" I was feeling high up inside the intake was actually the evaporator coil. It made sense that the thermistor should be inserted in it, instead of dangling in the intake.

    People on another forum have confirmed that its purpose is to monitor the state of the evaporator, particularly so that the A/C circuitry can shut off the compressor if the the evaporator might be starting to ice up.

    One person has reported that an RV technician told him that the thermistor should be at the left side of the evaporator, where the fluid enters the coil, rather than at the right side where mine is located. I have not confirmed this information.

    Here's the thermistor:

    Click image for larger version. 

Name:	Coleman_Thermistor.jpg 
Views:	11 
Size:	125.4 KB 
ID:	30627

    And here's the "board" I felt up inside, which of course is the evaporator coil:

    Click image for larger version. 

Name:	Coleman_Condensor.jpg 
Views:	11 
Size:	98.1 KB 
ID:	30628

    P.S. When replying to a post it's usually best to use the Reply With Quote link at the lower right of every post here. That not only copies the original post into your reply, improving context for anyone reading later, but also alerts the author of that post that you've replied to it.
    Last edited by boyscout; 11-11-2020 at 08:26 AM.
    Mark - 2018 Solitude 310GK - 2017 F-350 diesel SRW short box - Pullrite Superglide hitch

  4. #4
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    Quote Originally Posted by boyscout View Post
    Thanks for replying. Spurred by someone on another forum to take pictures, in the course of doing so I figured out that the "board" I was feeling high up inside the intake was actually the evaporator coil. It made sense that the thermistor should be inserted in it, instead of dangling in the intake.

    People on another forum have confirmed that its purpose is to monitor the state of the evaporator, particularly so that the A/C circuitry can shut off the compressor if the the evaporator might be starting to ice up.

    One person has reported that an RV technician told him that the thermistor should be at the left side of the evaporator, where the fluid enters the coil, rather than at the right side where mine is located. I have not confirmed this information.

    Here's the thermistor:

    Click image for larger version. 

Name:	Coleman_Thermistor.jpg 
Views:	11 
Size:	125.4 KB 
ID:	30627

    And here's the "board" I felt up inside, which of course is the evaporator coil:

    Click image for larger version. 

Name:	Coleman_Condensor.jpg 
Views:	11 
Size:	98.1 KB 
ID:	30628

    P.S. When replying to a post it's usually best to use the Reply With Quote link at the lower right of every post here. That not only copies the original post into your reply, improving context for anyone reading later, but also alerts the author of that post that you've replied to it.

    On my Solitude (Colman A/C), the thermistor is jammed in-between the fins of the evaporator coil on both units. They are just held in by friction. One fell out when I bumped its wires and I could see where it had been inserted in between the fins (the fins were bent). I believe the purpose of the thermistor is to signal the control board when the evaporator is starting to ice up and shut off the compressor.

    Chris
    Chris & Karen
    Fort Collins, CO
    2017 F-350 SRW 6.7 Lariat Value CC LB 4x4
    2018 Solitude 310GK - Sold 7/2023

  5. #5
    Rolling Along
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    Check with Coleman to be sure. It could have well been misinstalled by an unknowing worker. It looks like a temp sensor used to detect the temp of the return air flowing across the coil.
    On the Dometic marine AC systems I work on, the sensor is placed directly in the return air flow. I question jamming it in the fins of the coil as that would have it picking up the evaporator temperature instead of room air temp, which is what you set your thermostat by.

  6. #6
    Site Team xrated's Avatar
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    Quote Originally Posted by BobLandry View Post
    Check with Coleman to be sure. It could have well been misinstalled by an unknowing worker. It looks like a temp sensor used to detect the temp of the return air flowing across the coil.
    On the Dometic marine AC systems I work on, the sensor is placed directly in the return air flow. I question jamming it in the fins of the coil as that would have it picking up the evaporator temperature instead of room air temp, which is what you set your thermostat by.
    On the Coleman A/C units, the thermistor is used as a "freeze control" device and should be attached to the evaporator coil as stated. Temperature control of the room is done by the thermostat on the wall....or a temperature sensor on a wall if you have zone control.
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