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

(21 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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21 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

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