Question about Dometic Rooftop RV Air Conditioner

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Have a coleman mach unit that never cools correctly. If you run on low with thermostat halfway down it will keep running. If you run it on high, it acts like the coil is freezing up and barely blows air. I assume it is low on charge. Is there no way to add a charge? We have had our rv only 2 years and they replaced the unit once already.

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It may also be an obstruction in the ducting. When the forced air is restricted somewhere, it will freeze the coils, and compressor shuts down, until frost is gone, then starts back up and will repeat that all day. I'm thinking that the restriction doesn't affect it as much when fan on low. I'm going to assume this is a ducted system. If I'm right, first, make sure all ceiling ducts are open and if not, open them all up full and see if that changes anything. If it does, you've solved the problem. If it doesn't, shut unit down, remove air filter at ceiling, and then restart unit on high fan. Where the divider is between return air and forced air, carefully look up there with a flashlight and see, as well as feel, very carefully around the edges of that divider for cool air coming in to mix with return/warm air. If you feel quite a bit of cool getting through, shut the unit off again, and use foil tape to seal it up well, then retry it. If not a noticeable amount leaking through, remove the rest of the screws holding that ceiling assembly up, and lower it down as far as you can. Now look at the ducts that go into the ceiling for forced air and make sure they are wide open and no rubber roof hanging in them, or tape or anything else that may restrict flow at all. That is often where you will find that the rectangular opening has been restricted, and will cause that problem as well. If there's rubber roof , or similar in the way, carefully cut it out of there and apply foil tape over anything loose or even gaps into ceiling or roof area, and anywhere else. You want all the forced air going into a wide open duct and nowhere else. Hope that helps. Only other thing you may want to confirm first as well, would be that you do have 115-120 vac, hydro required, going to it. Only other test after that would be an amp draw test, that would require a clamp on amp-meter. My guess is that it's an obstruction in forced side. Keep me posted will ya?

Posted on Jun 25, 2010

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