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Old 06-15-2019, 09:42 PM   #1
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Coleman Furnace Cycling Problem

Hello, Iím having an issue on my rv furnace but canít seem to find any answers so I am hoping someone here has some insight so thanks in advance.

I have a 2009 Chaparral fifth wheel with a Coleman Mack furnace and air conditioner on a CM analog thermostat.

The furnace comes on and cycles every so often on itís own, regardless of the thermostat setting, even with the air conditioning running. It runs its dc fan and the igniter ticks. It will light and run if the propane is on. I can stop it from cycling by pulling the 10amp fuse or flipping the switch on the side of the furnace itself. Both pulling the fuse or flipping the switch also prevents the air conditioning from running. Unplugging the harness on the furnace also prevents the air conditioning from running so I am unable to isolate just the furnace and shut it off. I do not have a model number for the furnace and have not seen a tag. There is also no independent propane shut off valve. Has anyone had this issue before?

Thanks again

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Old 06-15-2019, 10:10 PM   #2
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I am not really an HVAC guy but that sounds like it is wired wrong if it really is the Coleman thermostat. What's it look like? I posted a picture of mine. If like this something is very wrong as there is no way to run the heat and ac at the same time.

I have heard the dometics do this and it is something to do with how you set it but I dont have one.

The analog thermostats are junk anyway with a 8 degree swing so I swapped mine for digital.Click image for larger version

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Old 06-15-2019, 11:35 PM   #3
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Remove the thermostat from the sub base. The wiring terminals for the t stat should be there. remove the white wire from the W terminal and put some tape on the wire so it dose not short out to other terminals. The white wire is for the furnace and should keep it starting. Put thermostat back together. If the furnace dose start the thermostat wire to the furnace is shorting together and needs to be replaced or find the short. If the furnace dose not start then the thermostat is junk and needs to be replaced. My bet is on the wiring being shorted.

Hope This Helps Tim
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Old 06-15-2019, 11:44 PM   #4
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I'd replace the analog CM T-stat with a digital CM T-stat. Wiring is like-for-like. So you can check the wiring when you replace it. Wiring should be:

R - Red (+12VDC)
Y - Yellow (power to AC)
W - White (power to furnace)
GL - Gray (power to AC fan - low speed)
GW - Green (power to AC fan - hi speed)
B - Blue (ground)

You'll be a LOT happier with a digital over an analog.

Here's one:

Here's another:
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Old 06-16-2019, 08:38 AM   #5
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Thank you all for your help. I’m going to order a digital thermostat, like you all recommend and I’ll update the thread with the result.
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Old 06-16-2019, 08:47 AM   #6
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Thanks Tim, I’m going to try that test first. Hopefully it’s not the wire, running a new one through the walls and ceiling will be tough.
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Old 06-16-2019, 03:44 PM   #7
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I didn't know Coleman made RV furnaces, just the thermostat.
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Old 06-16-2019, 04:35 PM   #8
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Coleman does't make furnaces. Find the fresh air return for the furnace and remove the grill. Inside you will see the furnace, on one of the sides or the top of the furnace you will find the tag for the furnace. My TT has the Coleman Mach air conditioner and the Coleman analog thermostat. My furnace is a Suburban SF-30Q. The tag is on top and I used my cell phone to read the label or you could use a mirror to read it.
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Old 06-16-2019, 06:42 PM   #9
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May just be a crazy stat. The heat and a/c are separate circuits in the stat. I would carefully disconnect the stat wires on each end and ohm the wires on the highest setting. Any continuity would indicate a short.

My suspicion is either a crazy stat or mice.

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