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Old 07-24-2021, 04:40 AM   #1
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I hope it's the thermostat

The DGF's son is spending the summer living in the motor home. Suddenly the air-conditioner quit. I suspect the problem is in the thermostat. No voltage to the AC unit, no reaction to any thermostat controlled function, not the LPG furnace, fan or AC. The fuse in the thermostat is good. I didn't think to look for voltage AC or DC at the thermostat. I happened to have a brand new digital thermostat I installed in the bedroom two years ago, it never started the furnace, the AC or the fan but the original Coleman unit worked fine until a week go. Pictures attached. Please advise!
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Old 07-24-2021, 07:53 PM   #2
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The first check is to see if you have 12 volts at the thermostat. If it is there then the problem most likely is the thermostat. If it is not there you have to find where it is coming from. It could come from the furnace or from the fuse panel.

If the 12 volts is there you can jumper it to any control line to turn on that function. Jumper +12 to the furnace wire and it should turn on. Jumper +12 to the AC fan and it should turn on.

Since you have lost all function I am leaning toward a loss of the 12 volts to the thermostat.
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Old 07-25-2021, 11:58 AM   #3
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Thanks for the prompt reply Jim. I've got 12 VDC at the thermostat. I've tried to swap out the old Colman with the new Honeywell. No joy. I also tried jumping from the 12V+ to the other wires in the bundle. Nothing changed, (came on). The bundle connected to the old thermostat has an 18 (?) four conductor cable, red, white, blue and green. In addition there's a 16 gauge white and red and white 18 gauge wires. No combination activates anything. I'm going to try Honeywell again until I get a better idea or more information. Busy while standing by.
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Old 07-25-2021, 12:52 PM   #4
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Good news! The Honeywell thermostat isn't going to work. That's not the good news. The good news is that the Honeywell requires 18 to 30 volts. The good news is that, that's why it won't work. Amazon tonight.
Thanks for trying to help me Jim.

Best!
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Old 07-25-2021, 07:20 PM   #5
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A new thermostat may not solve your problem. Thermostats are nothing more than a switch that connects 12 volts to different output lines. If you can't fire up anything by jumping the 12 volts to different control lines, the thermostat won't be able to either.

The other component of the equation is the common or ground connection. I don't know if the grounds are routed to the thermostat ground wire or not. If all the grounds are tied together in the thermostat wire harness, that would be something to look at. It takes +12 volts plus ground to activate any of the control circuits in the furnace or AC.
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Old 07-26-2021, 04:44 AM   #6
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Hmmm... I wondered about that. When the old thermostat was connected I got 12+ between all wires and the single 12+ red wire. I could reconnect all to the old unit then try the jumper. Nothing activates between the 12+ and any wire with all of them disconnected.
What say you Sir?
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Old 07-26-2021, 02:34 PM   #7
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What is the exact model number of your old thermostat? The reason I ask is that I upgraded my son's 5th wheel thermostat with a new style like I had put in my unit. It wouldn't work at all. Turned out it didn't use 12 volts but rather some weird voltage (for some reason 8 volts comes to mind).

I ended up designing a conversion board for his AC that allowed it to operate with the new 12 volt thermostats. It was just a electronic relay board that took the new 12 volt control signal and adapted it to what the AC wanted.

I can't remember for sure but I think the old thermostat supplied a ground signal to the devices rather than a +12 volts. If that is the case, connecting your thermostat's ground to the control line to the AC may make it come on.

I was totally shocked when I encountered that thermostat. I think it may have looked like yours.
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Old 07-26-2021, 05:30 PM   #8
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Yesterday I found out from Honeywell that the thermostat I have requires between 18 and 30 volts. I looked on Amazon and found several units designed for RVs that operate at 12VDC. As you mentioned earlier a new thermostat might not solve my problem. I'm going to take another whack at it tomorrow morning. The information you just sent me is another good lead. Thanks and I'll let you have how it shakes out.
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Old 08-01-2021, 09:36 AM   #9
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Hi Jim!
It's me again! It's me!
I don't have the exact model number of my thermostat, no numbers on it outside or inside. Colman Mach, that's it. I took the cover off the AC unit to access the internal control module. The AC cable goes in and four 18 gauge wires, red, white, blue and grey come out. The AC cable going in is hot. There is also a six conductor sixteen gauge wire plug plugged into the control module. I've tried jumping all combinations of all wires in both wire bundles. Any ideas, suggestions, advice or desist orders?
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Old 08-01-2021, 10:08 AM   #10
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Could the capacitor(s) on the AC unit be bad and not providing power to start the compressor and/or fan? Maybe there's nothing wrong with the thermostat.
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Old 08-01-2021, 11:37 AM   #11
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If it were me, being an electronic tech and experienced troubleshooter, my next step would be to go to the appliance and try to make it come on from there. You would need to get the wiring diagram, probably, unless things on the board are labelled.

Since both your furnace and AC are not working, I would start with the furnace. These are strictly 12 volt devices so there will be a minimum of wires going to it. There will be 12 volts, ground, and a trigger wire from the thermostat. It may also have a wire that feeds 12 volts to the thermostat. I would first measure the 12 volts to ground to verify it is there. If it is missing then you need to go back to the fuse panel and measure there.

If there is 12 volts at the furnace you can simply jump the +12 volts to the trigger wire and the blower motor should start up. If it starts up then there is definitely a problem in the thermostat circuit (not necessarily the thermostat).

The AC is much harder to play with since it uses both 120 volts to run the motors but also a control voltage which is most often 12 volts. I say most often because I encountered my son's unit which was totally weird and didn't use 12 volts from a wall thermostat.

I did some research on these units and see that they were non-plenum models that apparently originally had the controls on the AC unit instead of a wall thermostat. I see that there are control boxes you can get for these that convert them to the standard 12 volt thermostats.

This experience will either make you very familiar with your wiring or drive you insane.
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Old 08-14-2021, 01:37 PM   #12
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I swapped my Coleman thermostat like yours for a Coleman Digital thermostat. It swapped out wire for wire in the front of my Greyhawk, my rear bedroom only controls the rear A/C so the new thermostat wire for the heater was not used it was a very easy swap. Now my room temps are a lot more manageable. Amazon has these thermostats…..
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