The most prevalent problem with coach A/C units is the filters not being cleaned. It is imperative that filters be kept clean. With unclean filters it reduces the air flow which can create freeze up problems, electrical problems, and efficiency problems just to name a few. One of the major undesirable results of a dirty filter(s) will be a dirty evaporator. An over simplified definition of the evaporator is the set of fins the air moves over to change air from hot to cold. Now with a roof mounted units you will normally only see the evaporator from inside the coach looking up into the unit when the inside ceiling assembly and /or filters are removed. When the filters become excessively dirty the excessive contaminants are pulled through the filters and will begin building up on the evaporator fins. This build up begins acting as insulation which reduces the efficiency of the evaporator. This can also create situations where the compressor can become overheated due to the use of more electrical power than normal.
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