老大厲害！ 網上找來的：

1) So why don't you replace the cycling switch? or at least test it by watching low side pressure with your gages (or a borrowed set). Put blower motor on lowest setting and watch low side pressure- if pressure goes below 20 psi or so-- your switch is not doing its job. The switch would be the first place I would look when dealing with evap freeze-up.

2) First thing I'd check is the condensate drain. If it's clogged, condensed humidity will accumulate on the evaporator. Refrigerant going through the evaporator will freeze the condensation on the outside of the evaporator causing it to ice up. If this happens, air flow out of the registers will decrease, and temperature will increase. Let it sit and thaw out...works fine until it ices up again. This scenario is very common in high humidity situations. Setting the AC mode switch on recirculate will reduce the tendency to ice up. The drain is located directly below the evaporator housing, accessible from under the vehicle... 1/2" diameter rubber hose pointing straight down, maybe 2" long. Stick a drinking straw inside and wiggle it around. Keep your face away, you might get an unexpected shower! Don't stick anything metal in the drain, you'll likely damage the evaporator.