Mod #97: Coleman Mach III Thermostat Modification

Mod #97: Coleman Mach III Thermostat Modification

Submitted on: 07/21/10

     Category: cooling, electrical
Mod Rating: 12345

(28 ratings)

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Mod Description:

This modification was designed and completed by ModMyRV member Stewartfan. This article carries a copyright. Permission must be obtained from the author before duplication or reposting.

Some of you might know the thermostat built in to the Coleman Mach III air conditioning unit is not really the best at keeping a setpoint within a couple of degrees. With this modification I was not concerned with turning the air conditioning fan on and off. I just wanted to have better setpoint control by controlling the compressor.

Mod Difficulty:

Caution! Disconnect any AC and DC power source to your RV before attempting this mod. This includes the shore power cord, batteries, inverters, etc. Otherwise you run the risk of electrical shock!

With this design I use 12 volts DC to energize a relay located in the air conditioning unit itself. My source of 12 volts DC is located under the refrigerator in my camper. You can source the DC power (positive) wherever is easiest on your camper.

With the thermostat, I make and break the positive lead going to the relay located in the air conditioning unit by terminals Rc & Y on the thermostat. The ground (or negative) lead gets wired directly to the negative side of the coil on the relay.

With the relay in place I do nothing more then complete the 120 volt compressor circuit on the normally open (NO) contacts the same way that the original thermostat does. All of the high voltage (120 volts) stays up in the air conditioning unit along with the relay.

For the heating side, I already had used this thermostat to control my furnace. For this you use the Rh & W terminals in place of the original thermostat.

Wiring was a challenge. There is really no way to run wires under the ceiling since there is styrofoam insulation. With that said I used surface raceways.

ModMyRV recommends these parts for this mod:

Hunter 44110 Set and Save Programmable Thermostat
SQUARE D Relay, 8 Pin, DPDT, 12A
Wiremold Raceway Track

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26 Comments For This Mod

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  1. brad

    i like this mod how did you up load your photos?

  2. stewartfan

    How did I load the photos to the website? Is that what you are asking? Then contact

  3. Jason


    I would like to do this mod, but I unfortunately have very little electrical competency.

    Was the relay already in the a/c unit or did you have to add it?

    Also, I assume that you still have to turn the unit on normally. If I understand correctly, you are running the fan constantly and just cycling the compressor as needed to maintain temperature.

    Would it be possible to also control the fan via another 12V relay?


  4. stewartfan

    Hello Jason, I did add the relay. This simply takes place of the dial thermostat that is located on the unit from the factory.

    Correct, I turn on the keep the fan running. As the temperature rises in the camper the thermostat “calls” for cooling and energizes the relay thus the compressor.

    Sure you could cycle the fan as well, even with the same relay since it has more then one set of contacts. I chose not to do this for a couple of reasons. First I think that the constant movement of airflow provides a better air mixture for the thermostat to sense. Second, when running off my generator (Honda EU3000) it does not put such a surge on it if the fan is already running. Third, I really like the white noise sometimes !!

    If you are uncomfortable doing this you might want to call a electrician (residential). With all the parts this only takes under three hours to complete.

  5. nadkaw1957

    Was a 12A relay really heavy enough to run the compressor? I have a Coleman Mach3 and the label says something about drawing 20 amps. I bought HVAC contactors to do this mod, but they won’t fit in the space I have. I’m worried 12 amps might not be heavy enough. Thanks for your reply.

  6. Jaime

    I need help. I was trying to do the digital thermostat mod but it wouldn’t run my compressor or my furnace, so I went ahead and bought the a new RVP thermostat. Now my furnace kicks on, but my compressor still won’t run. I downloaded a troubleshooting guide from RVP, and discovered that I do not have 115V to the 1& 3 pins on the ceiling assembly (these are for the compressor relay)

    But for some reason I am only getting 6 volts between the red (fan) and blue (DC-) wires, and 6 volts between the yellow (Compressor) and blue (DC-) wires. RVP troubleshooting tells me only to “restore 12 volts to the thermostat” if I am not getting correct voltage. How please, exactly, do I do that?

  7. Lingus

    I might be complicating this but the write up is very unclear on what you’re installing and what is existing equipment. This seems to be a t-stat swap and then you added a circuit to provide functionality to keep AC fan running constantly.

    Is this what you’re doing?

    In the schematic, what wires do the load side of the new relay connect to?

    Is the schematic for the circuit to keep the fan running constantly?

    Please clarify.

  8. Lingus

    Also, at the load side of the relay you’re shorting the hot and neutral according to the sketch. Please take a closer look at your sketch.

  9. stewartfan

    Lingus I did not add a circuit. I am using the relay to make and break the neutral for the compressor just like the old dial thermostat. Where do you see where I am “shorting” L1 and L2 ?

  10. Ron Day

    I have the 2DT39 relay. I have put 12 volts across the coil but I don’t see the relay pull in to switch contacts. What am I doing wrong? Thanks Ron Day

  11. stewartfan

    Ron Day, I believe the coil is polarity sensitive, make sure you have the positive going to the + and negative to the -.

    Sorry that I can’t look more for you I traded the travel trailer in for a motorhome.


  12. Ron Day

    Found The problem, there are 2 different relays listed! The one pictured is 2DT39 which is the wrong one (120 volt coil)! The correct one is 2DT53 which works fine. That was a $35 lesson. Ron Day

  13. Marcey

    We have the same camper, and where you put the thermostat is where our thermostat is for the furnace. Is the thermostat you installed only for the air or is it for both heat and air?

  14. Luis

    You did not feed any power to the thermostat at all, running on batteries only?

  15. stewartfan

    Luis, the thermostat runs on two AA batteries.

  16. Brett W

    hello, im trying to complete this mod on my 09 jay feather im not concerned with running a new thermostat i just want the fan to shut off when the compressor does so i would only need to run my relay off of the existing thermostat right ? any feed back would be greatly appreciated

    thanks, Brett

  17. avenger123

    stewart…new to relays so bare with me please.
    this sound correct?

    power + > new stat RC
    power - > - relay coil
    new stat y > + relay coil
    old dial stat w neutral > A side relay no
    old dial stat w > A side relay no
    old dial stat y > B side relay no

    do any wires get connected to the common of the relay…jumper wires?


  18. Danny

    I just did the same thing a much easier way. Simply get a thermostat control box for a ducted system and a coleman heat cool thermostat. You will leave the old controls for looks but they will have no function. Everything is controlled through the thermostat.

  19. Chuck

    I have a few questions after reading all these posts.
    Regarding the use of the relay, are you sure the contact rating of 10 to 12 amps is high enough. I think the compressor draws a little more than that, especially on start up. I was thinking of a relay that had contact rating of around 20 Amps.

    One thing you might want to do if using relays is to connect a diode across the relay power connections in reversed polarity from the way you connected the power. That way when the relay d energizes the voltage spike from the reversed magnetic field won’t go into your thermostat.

    If you want to use relays and control the fan speed you need more relays, one for high speed and one for low.

    I guess you could use solid state relays, but you will need to figure out the current limiting resistor size needed for the device so you don’t burn out the SS relay.

    Last question - someone posted that you can simply get a thermostat control box for a ducted system and the coleman thermostat. I found the thermostat, but I can ‘t find the control box. Can someone point me to a link for the control box?


  20. Bill

    If you want the fan to cycle automatically there’s a kit you can buy from a place called airswitchrv. I got one and it was very easy to install and works great. Also, I just pulled the t-stat capillary out thru the front vent hole and the t-stat is much better controlled by adjusting the capillary placement (near or far from the cold air outlet). Much better on and off timing (hysteresis I believe it’s called).

  21. Tim

    Nice choice of thermostat. I like the relatively small size, the adjustable span on temperature control and the cost. I have a 1987 Dometic-Aire A/C unit that is short cycling (build in thermostat no longer available) so I’m doing the mod. I have confirmed that all current (with exception of the fan) goes through the old thermostat so I will be using a 25A relay OMRON “G7J-2A2B-T 12VDC” (for .25″ spade terminals) or “G7J-2A2B-B 12VDC” (for screw terminals) by the spec sheet it appears to be designed for this type of application. It’s more readily available and has the current rating I need. You may have to get the W bracket for the relay to mount it ($1) part number is “R99-04 for g5f”. Most websites show the relay with the bracket attached (some state bracket included some state bracket not included with purchase). Tim

  22. Steve

    Danny, I read your comment about doing the same thing by replacing the thermostat control box in the unducted Coleman Mach III with a thermostat control box for a ducted system. Unfortunately, none of the RV parts places around here (I live 20 miles from South Bend IN, where they make 90% of the RV’s and most of the appliance makers and such have their factories so you would think they SHOULD know what I am talking about) seem to know what that part is. Could you include a part number or the part name or something for that? Or, maybe do your own Mod writeup about how you did that? Thanks

  23. Steve

    Chuck, I have part numbers for the conversion of the non-ducted Mach III to do it using the control unit, thermostat and a new ceiling unit to replace the one with the dial controls and such. The tech I talked to at Coleman/Airexcel said I could go with the old ceiling unit and the control unit & thermostat. I can’t give anyone any advice on how it works, I couldn’t really understand what the Coleman tech was saying, he just said it would be easy to do once I had the unit open I would see where to put the thermostat control box and I would see where to place the control box. I am hoping Danny or someone else with more knowledge than me can make a reply here and tell me where and how to place that thermostat control box. Here are the Coleman/Airexcel part numbers, I can’t guarantee that this is right, but go to this page and you can make your selections and it will give you the part numbers too :

    part numbers :

    Ceiling unit # 9430-4552
    Control Kit # 9330C755
    Thermostat # 8530A3451

    You can get the thermostat and the control kit on Amazon, the lady in the parts department of a local RV place here said she couldn’t find that model thermostat but she found me one that she thought was just a different color.

  24. krbjmpr

    Steve, hopefully you have completed your mod already, but just in case you haven’t, here is how everything ties together.

    The Coleman control box / relay box attaches to a couple of screws located on the evaporator cover, directly across from the evaporator coil but up high (full size models). Your box will likely include a pair of wing nuts to go onto the screws one the box tabs are in place.

    Depending on exact part number, there are going to be a few connections. The basic ones are going to be Y (Compressor), GL or G (Low Fan), GH (High Fan), and B- (Battery negative). These will connect to the Y, GL, GH connections on the thermostat. B- needs to find its way to battery negative. Keep in mind that the control box is little more than a collections of relays for 120vac that are controlled by 12vdc.

    The Coleman thermostat is little more than a collection of relays controlled by temperature and mode switches. The thermostat requires both 12v+ and 12v- to operate for all but the most basic models. If you see a circuit board, it will require both. You will need a minimum of 4 wires between the thermostat and control box, 3 if you have battery negative close by (like a light fixture). Personally, I prefer using 8 wire ethernet cable for future needs, but use whatever you like. Make note of what colors connect to what terminals on the control box. Then match colors to the appropriate thermostat connections.

    Depending on the model of Coleman thermostat, you may have a few extra wires available. The 2 blue wires are for the furnace connections. If the thermostat is capable of controlling a heatpump, there will be another wire though I forget it’s designation.

    Basically, here is how a cool only thermostat will operate in cooling mode, with fan set on high speed and automatic. When interior temperature exceeds thermostat setpoint, a few things will occur. Power will be applied simultaneously to both the compressor (Y) and high fan (GH). When interior temperature meets the setpoint, power is removed. If fan were to be set for low, then GL would be powered. If fan was set for on instead of auto, power would always be on the low or high wire. Power travels from thermostat to the control box activating the corresponding relay, turning on that part of the air conditioner. Power then returns from the control box back the battery on the B- wire. If the thermostat controls a furnace, a relay closes connecting the 2 blue wires. Note that the AC fan can still be turned on, but compressor will be disabled as long as in furnace mode. If you want to test your control box, applying battery positive to the terminals will engage the relays.

    There are 2 more connections on the control box that are required, but are not connected to thermostat. This is the frost / freeze sensor. The sensor is placed on/in the evaporator depending on model. It is basically in series with the coil of the compressor (Y) terminal. If the evaporator gets too cold, power from thermostat is interrupted until evaporator warms up. Most of the Coleman thermostats will have a compressor delay of a minute or two. If you find your compressor is short cycling (off, then on right away) the freeze sensor is primary suspect. Common with sensors that are mechanical (installed on evaporator) but the electronic ones (inserted in evaporator fins) are more resistant. I prefer the mechanical versions since able to troubleshoot easier and I can also use the function for load shedding if the control box is not equipped.

    Hope this helps you and others understand what happens behind the scenes.


  25. Steve

    Actually I hadn’t tried this mod yet, I was waiting until I ordered the 2nd roof air unit for the rear air, to mount it back in the bedroom. My brothers and I have done quite a bit of plumbing, electrical, carpentry, auto repair, appliance repair and things like that. We have even done some heating & air (simple) work in our homes. Nothing much with RV, my Georgie Boy motorhome is the first RV we’ve had in the immediate family.
    Your comment/writeup was good and I think we should be able to figure out how do do most everything now by following it.
    So I finally bought the rear air unit, another Coleman Mach III for the back like I have in the front. I have to order the control units, ceiling units and thermostats from the RV place and I will be ready to attempt it. But I have a question that is going to sound really newbie.
    I had the front unit installed at an RV place in the area, so I still haven’t even looked into the insides of a roof air unit. Seen the outside, when I clean the unit when it starts to get dirty. So anyway, I ordered the rear unit, and my brother and I un-boxed it and got it up on the roof. When we opened it up, there was a Styrofoam ‘piece’ inside it, shaped just like the chamber it was setting in. I told my brother “I think we need to take that thing out” - I had never seen a piece of Styrofoam meant to be left inside an AC unit or anything before. My brother said “I’m not so sure. My window unit AC has something exactly like that in it, they aren’t meant to be opened up (the AC unit) and things taken out before you put them in the window, and it works just fine and cools great. That may be meant to be in there in your roof air unit too.”
    So now at the risk of sounding totally clueless, I have to ask. Is that Styrofoam piece supposed to stay in the roof air unit like it is in my brother’s regular window unit - an intended feature - or are we supposed to break it up and pull it out? And it is molded exactly like the chamber it’s in, so you would have to cut/break it up and pull it out in chunks, if it’s not meant to stay in there. After everyone stops laughing at the newbie I hope someone takes pity on me and answers that one for me. Thanks

  26. Steve

    Sorry about my last post being all bunched up.

    I cut and pasted from a text editor, and for some reason it cut out the blank lines between paragraphs I had used to break them up.

    I didn’t catch it before I hit submit.

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