Sounds like the system isn't cycling after a while. Get a thermometer (preferably digital) and put it in the center duct and turn the blower down and the temp to "min". The temp should go down to somewhere in the high 30's to the mid forties. If one is driving on the road the incar temp drops and the ducts cool. The temp in the ducts can not get to 34 or so or the evapaporator will ice up. The cycling temp should generally be the same continuously.

It is important to use a digital as they react quicker. The time of a cycle can be so quick an analog thermometer will hardly notice the change. There will usually be a 3-6deg change of duct temp during a cycle.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician