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Old 04-04-2016, 06:43 AM   #1
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Coleman roof AC thermostat

My roof ac cools great but after running a while it will freeze up then the air flow slows because the evaporator is iced up. The filters are clean and the coils are clean and the fan is on high. The thermostat (manual dial on the inside ceiling cover) does not turn the compressor off no matter where you set it. The compressor just seems to keep running. Does this mean the thermostat is bad? How can you test it? It's a 1988 Holiday Rambler Class C.
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Old 04-04-2016, 12:04 PM   #2
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There should be another sensor in front of the evaporator coil or near them, that should cycle the compressor off if the it senses the the unit getting too cold. Check that it is not bent out of the way or defective.
I am not sure on but it may also be located up top under the upper cowl cover..
This can also be caused by poor airflow over the evaporator coil in the first place. Are the coil and fan clean?. Is the divider between the hot and cold side of the unit intact so that you are not driving the cold air around in a circle?Also you may want to check that the unit is draining properly.
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Old 04-04-2016, 12:58 PM   #3
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Yes everything is clean and all joints are taped up. It's like the compressor just never cuts off. The manual thermostat knob has no affect. The small copper sensor from the thermostat is clean and straight next to the vent box. I guess I need to pull the air box cover off and see if something is disconnected or out of place by the evaporator coil from when I Cleaned the coils not long ago. Since adjusting the thermostat knob from max cool to all the way off and nothing changes I thought that may be the problem. I cant find any info online for checking the manual thermostat everything seems to refer to a wall mounted thermostat.
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Old 04-05-2016, 10:24 AM   #4
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On high inrush current devices such as compressors. I have found the contacts weld themselves together, in this case thermostat, and will not de-energize. With many cycles off operation contacts arc and have jagged adges that tend to reduce curret flow and are apt to weld. Does the main fan/ cool switch shut the compressor off? If it does the thermostat is probably defective.
Ken
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Old 04-05-2016, 11:18 AM   #5
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I agree also with Electrowood regarding the relay not de energizing.
Did this start after you cleaned the coils? If yes than as you say something may have been disturbed. It does not sound like you need to replace the unit, but you may have to get a refrigeration mechanic to look at.
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Old 04-05-2016, 12:00 PM   #6
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Yes the main fan switch will cut it off. I took the thermostat out and wiggled all the wires and put it back in. It now will click when it gets to the compressor off on setting but it doesn't seem to be a consistent setting. It's not freezing up anymore and if you let it run a while you can hear the compressor cycling. Every now and then you can hear it kick in. It's cooling good but it seems I have to turn the thermostat up more to the max cool setting more to keep the temp cool. It is warming up to near 80 today so I guess it is working harder. I may replace the thermostat. I'm sure it's been rotated a lot of times since 1988 so maybe it's just worn out.
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Old 05-19-2016, 06:10 PM   #7
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My roof air is now too cold. A while back I added some R22 and it has been working good. Now the last two nights I woke up freezing. It has been about 80 outside but my inside temp is 50. My compressor is staying on and not kicking off when it reaches a set temperature. Is this a bad thermostat? Usually I can here it click at the high and low settings but I don't here that click now when I turn the dial. When I turn the ac on the compressor is already pumping.
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Old 05-21-2016, 08:54 PM   #8
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These units do not come with access valves. Did you or someone else add a tap valve on the process line? If refrigerant charge is low, hi pressure liquid will not flow out of capillary tube at steady rate, evaporator ( inside coil) will freeze up.

Turn power "off" to unit and move thermostat through range and listen for mechanical click as it opens and closes. As I stated earlier, if the contact fuse or weld together they probably will again.
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Old 05-21-2016, 10:53 PM   #9
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Yes it had a service valve added. I figured out I could bump the thermostat with a screwdriver and it would start clicking at the setting and working correctly. Guess I need to replace it.
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Old 06-13-2016, 04:41 PM   #10
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New thermostat wires

Got my new coleman mach 3 thermostat but the terminals are marked differently. There is a terminal 2 and 3 and one terminal that looks to be a negative sign. The original thermostat had two white wires going to the terminal marked com the blue wire went to terminal 2 and a yellow wire to terminal 3. My ac unit has the heat/cold manual dial thermostat mounted on the inside roof panel
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Old 06-15-2016, 07:54 PM   #11
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I figured the wiring out. The two white ground wires go on terminal labeled 2. The yellow wire goes on terminal labeled 3 and the blue wire goes on the terminal that looks lke a 1 (more like a minus sign). AC s working great now it was just a bad dial thermostat.
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Old 06-16-2016, 09:25 PM   #12
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Good job.
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