Evaporator freezing

[RossL]

I have a vintage air system in my 55 Thunderbird. I think the evaporator is freezing because air flow seems to be restricted after running the unit at max cool.

I was trying to get the unit as cold as possible (there are a lot of leaks around my convertible top), I had the thermostat on max, vent temps were 38.

What is the lowest temp at the vents that I should dial in (with thermostat knob) to avoid the freezing?

[Tim]

1/4 turn from full clockwise.