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.
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: