The Coleman Mach is one of the most common thermostats in RVs and campers. They are inexpensive, have a simple design, and are reliable (mostly).
Sometimes, something does go wrong with them, and you will need to do some troubleshooting. Common troubleshooting issues are the temperature settings, wires, blown a fuse, or dead batteries.
This post reviews the common issues and solutions to your Coleman Mach Thermostat, including:
- Thermostat Setting
- Battery
- Fuse
- Air Conditioner Setting
- Furnace Setting
- Wire
This post reviews troubleshooting tactics and the best solution for each fix.
Table of Contents
RV Thermostat
The Coleman Mach is an analog thermostat and not a programmable thermostat. That means that all the settings need to be done manually and that it has no way to adjust the set temperature by the user automatically.
It is not uncommon for an RV manufacturer to have a manual thermostat in their camper. They are less expensive and, being of a simpler design, are less likely to break. They serve the purpose of managing your HVAC temperature and are easy to use. No need to mess with programming and settings. All you do is set the temperature you want, and you can walk away.
Although the Coleman Mach is a manual thermostat, it is still higher tech than the old mechanical thermostat, which has a flat coil to measure the temperature and just a couple of wires to set the motor fan on.
The inside of the Coleman consists of a modern-day circuit board and electrical components. There are capacitors and circuitry that help it function your complete HVAC system. There is even a fuse that will protect the circuit board and can be replaced if blown. More on that as we continue to troubleshoot.
Thermostat Setting
As you troubleshoot, let’s first look at the thermostat settings and their function to see what the issue could be.
Temperature
As you look at the thermostat, you will see a setting for the temperature. Note that this is the thermostat setting to the desired temperature and not the actual temperature of the Rv.
In a Coleman Mach, the room temperature can be set but is not displayed. This is a challenge when troubleshooting as you can’t tell what to set the temperature on compared to the camper’s room temperature.
Solution
The first step with troubleshooting is to confirm what your thermostat is set at relative to the RV room temperature. If you are looking for heat from our HVAC system, you must confirm that the thermostat temperature is higher than the RV room temperature.
Since this thermostat does not have room temperature readings, this is a guess. The simple way to confirm is to put the temperature setting to the higher reading. This will force the heat on and should start the fan motor.
The same is true for the reverse if you want cold air from your air conditioner. You won’t be able to see the room temperature, so you will need to force the air conditioner by bringing your temperature bar to the lowest temperature. This will force your air conditioner to turn on and blow cold air.
If you were at the correct settings and still not getting the desired heat or cold air, continue on for more troubleshooting.
Coleman Mach Thermostat Troubleshooting
If your temperature settings are good and the HVAC system is still working, let’s explore the common issues that could be causing you a problem.
Batteries
Although the Coleman Mach is a manual thermostat, it still requires batteries. It makes sense since we reviewed that it has modern-day circuits and capacitors.
This thermostat takes two AA batteries for power. Before diving into more advanced troubleshooting, I always suggest the simplest fix before diving into more complex solutions.
Confirm that both batteries are fully charged. You should be getting at least 1.5 V of power per battery. Even one dead battery can cause an issue. If not, or you are not sure, just replace your batteries and see if that makes a difference in the function of your thermostat.
Solution
Replacing the batteries is a simple process. They are located at the top of your thermostat, just behind the cover plate. To remove the cover, lightly squeeze the side and firmly pull it towards you. This cover plate comes completely off with no wires or anything else attached to it. Just put the cover nearby, and in a safe place, so it’s not lost or accidentally stepped on (it’s happened to me).
Fuse
You can see the circuit board and capacitors with the cover plate off. You will also notice a fuse at the lower left corner. This is a 2 amp fuse to protect the thermostat should there be a spike in power. This could be the potential issue of your problem.
Solution
Inspect the fuse to see if it is blown. This is a glass vile; you should be able to tell if the inside fuse wire is damaged or broken. Sometimes, when a fuse is blown, you will get dark spots on the glass.
Get a flashlight and see if you can see the fuse is intact. If it’s not intact or looks blown, replace the fuse. This is a fuse that you should be able to find at any hardware or RV store.
To replace, pull out the blown fuse and insert the new fuse. If you remove and replace a blown fuse, it should fix the issue with your thermostat.
If your fuse continues to break every time you replace it, there is likely an electrical issue beyond your thermostat and more of an issue with your air conditioner or furnace. Let’s continue to see how we can troubleshoot the furnace and the RV air conditioner.
Air Conditioner
The great thing about the Coleman Mach is that it can operate both your furnace and air conditioner. Although, most thermostats can do this.
It can tell your AC unit to turn on the compressor, powered by a relay that gives 120V power. Since the thermostat is low voltage, it does not have the direct ability like a switch to power 120V. A relay on your AC unit does this to turn on and provide cool air.
If your thermostat is not working and the ac unit doesn’t turn on, lets first confirm the settings are correct.
Solution
The thermostat should be set to cool mode. Also, you should set your fan to the auto setting. This fan setting to auto will only turn the fan on when the ac unit is running. If the fan is in the on position, it will have the fan run continuously, regardless if it is not blowing cold air.
To check the thermostat, move the “system” slide to the off position and back to the “cool” position. The slide switch may not be properly seated into the desired position, even though it looks like it. So turning it off and back to cool should set it to the proper position.
If this does not work and the ac unit still does not turn on, you could have the potential of three issues:
- AC Unit relay defective. The relay is the switch that gives your AC 120V power to turn on. Even if the thermostat is sending the signal to turn on, a defective relay could cause the power not to go to the AC Unit. If this is the case, the relay will need to be replaced.
- Defective thermostat. This could also be an issue with the thermostat itself. Any one of the components and capacitors could get damaged and cause the air conditioner not to work.
- Check the circuit breaker. If the air conditioner is not working, the problem may not even be the thermostat. To confirm, check your circuit breaker for a tripped breaker. The electrical panel should be labeled as to which one is the ac unit. Also, since the air conditioner takes the most power in your RV to operate, look for the largest breakers; this is usually the air conditioner. Even if it doesn’t look tripped, reset the breaker. Sometimes it trips just enough to disconnect the power but not move the switch completely off.
Furnace
As you learned, the Coleman Mach also serves to function your furnace and get hot air from your heating system.
As a side note, the thermostat for your furnace operates slightly differently than the air conditioner controls. With your furnace, there is no 120-volt power. The furnace and the blower motor operate on propane and 12-volt power from your battery. This makes the control of your thermostat simpler since there is no relay to switch on 120 V power.
For more info on how your RV furnace uses battery power, visit my post on if you can run your RV furnace on 110V.
Solution
The fix for the furnace is similar to that of the air conditioner.
The thermostat should be set to heat mode. Also, you should set your fan to the auto setting. This fan setting to auto will only turn the fan on when the furnace is running. If the fan is in the on position, it will have the fan run continuously, regardless if it is blowing cold air.
To check the thermostat itself, move the “system” slide to the off position and then back to the “heat” position. The slide switch may not be properly seated into the desired position, even though it looks like it. So turning it off and back to heat should set it to the proper position.
If this does not work and the furnace still does not turn on, you could have the potential of two issues:
- Defective thermostat. This could also be an issue with the thermostat itself. Any one of the components and capacitors could get damaged and cause the furnace not to work.
- Check the Fuse. Since your furnace operates on 12 v battery power, it will be where your fuses are in your distribution panel. Check your furnace fuse to see if it is blown.
If your trouble is that your furnace fan will not turn off, visit my post on why your RV furnace fan won’t turn off.
Wire
Another issue may be the wiring from your RV to your thermostat. The Coleman Mach has 6 various colored wires coming from it. Each wire has its own specific purpose and function. For example, the blue wire is a wire that helps operate the air conditioning.
A loose wire wouldn’t be unheard of with your RV driving and bouncing down the road, causing a disconnection issue.
Solution
Inspect the wires that connect your RV to your thermostat. Check to make sure the connections are tight and secure. The wires should be tight with no loose wire connection.
Also, check the wire connection on the thermostat’s circuit board. There is less likelihood of a loose wire, but it is possible and can easily be checked.
Bad Thermostat
If all else fails above, the issue could just be a broken thermostat. With all the electrical components and slide switches, there could be a failure in the thermostat.
Solution
After all the troubleshooting, you could just have a faulty thermostat that needs to be replaced. These thermostats are not too expensive at around $75 to $100, and can easily be replaced.
The best practice to replace is wire by wire. Even though the wires are colored-coded, sometimes the RV has a different corresponding color. Doing wire by wire ensures you don’t accidentally connect the wrong wire to your RV.
Final Thoughts
Coleman Mach thermostat trouble can be straightforward due to its simple design. Checking things like temperature settings and batteries is the most common fix. At the same time, in-depth fuse and wire connections are easy to troubleshoot with this thermostat. Even if you have a bad thermostat, the Coleman Mach is inexpensive and can easily be replaced by reconnecting each wire one at a time.
Tony is an avid camper and RV traveler. He fell in love with camping on his first RV trip with his wife over 25 years ago. Tony loves sharing lessons learned and tips about RV maintenance and safe traveling.
