Moisture in the Refrigerant
A thermostatic expansion valve is the metering device for many automotive air conditioning systems. A sensing bulb expands as the temperature increases, causing the diaphragm to move a pintle, allowing refrigerant to enter the evaporator. As the evaporator cools, the pintle moves back to the closed position, blocking refrigerant from entering the evaporator core.
If moisture enters the air conditioning system, it freezes at its metering device. Notice the frost at the expansion valve's port above, where the refrigerant is metered into the evaporator core. (TXV) thermostatic expansion valve air conditioning systems contain a receiver drier that cleans and absorbs moisture from the refrigerant.
If the drier becomes saturated with moisture, it releases it at the hottest times of the day. This moisture freezes in the expansion valve and deprives the evaporator of refrigerant. This is why the customer could drive the vehicle for five or ten minutes before the valve froze and the A/C vents started blowing warm air.