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The AC will freeze for a number of reasons.
The coils need to be cleaned, the water will collect and not run off due to the crud that collects on the fins.
The freon in the AC is low. If you have an Armstrong, or duo-therm penguin, they have service ports that allow for a recharge by a home AC contractor.
If you are running the fan on low, in a high humidity environment will cause freeze up. The slower air movement allows for a higher amount of condensation to collect and it does not drain off fast enough, so it freezes.
WBCCI #5501 AIR # 49
1978 Argosy 28 foot Motorhome
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