Evaporator Coil Freeze-ups
I have seen many rookie technicians who automatically grab the refrigerant bottle when they see an air conditioning evaporator coil covered with white frost and ice. That's because the gauges indicate both the high side pressure and low side pressure are low.
But, usually freeze-ups have nothing to do with the refrigerant charge.
If the evaporator coil is running excessively cold, then the high side pressure and low side pressure WILL read low with the correct charge. Adding refrigerant to such a system will overcharge the system and could do damage.
Air Conditioning Evaporator Coil Freeze-ups are caused by:
(1) Low or incomplete airflow over the evaporator coil
(2) Expansion Valve Problems
(3) Low refrigerant charge
Make sure (1)and (2) are resolved before considering (3).
(1) Check for any reasons for low air flow. Check the condition of the belts, belt tension, dirty filters, dirty blower wheel fins, obstructed return-air duct or closed dampers.
(2) Check for proper operation of the expansion valve. (If applicable) Be sure that the expansion valve is not stuck open.
(3) Lastly, Check the refrigerant level. If it is low, start looking for the leak!