in response to a message on the Forum # 377870
My air conditioning system evaporator coil freezes up after 10-20 minutes

One of the causes of evaporator freeze occurs when the core cannot give up its cold charge. This happens when the air being blown across the cold fins is diminished. With time, the fins of the evaporator core become plugged from leaves and other organic matter that has fallen into the plenum and been sucked up by the blower motor and deposited upon the core's fins. If the evaporator core cannot loose its cold charge fast enough by having warm air blown over the cold fins - it freezes. If your output of cold air coming from the dashboard vents is weak, this is a further indication of a blocked evaporator core. Only remedy is to open the evaporator case and clean it out.

The evaporator core can also freeze if the blower motor craps out. Again no air is moved over the core’s fins to remove the cold charge.

If the AC is run in cold weather for an extended period, the evaporator core can also freeze. Same reason, cold air moved over the core’s fins will not allow the core to loose the cold charge.

If the evaporator case drain hole is plugged, all of the moisture given up by the warm air cooling collect inside the box instead of dripping on the ground under the car. If this cooled water builds up and cannot escape it can also cause evaporator freeze up. On 68-82 this drain hole is nearly an inch in diameter and less likely to plug than the 63-67 hole which is only about 3/8 inch I.D.

If you continue to run the AC with a frozen evaporator core, the compressor will burn out.