The refrigerant/temperature control is controlled by the low pressure switch, so if your freezing the coil, your low pressure control is not opening the circuit to the compressor clutch and allowing the pressure to go below freezing for R134a. I had that happen on my 94 2500 and it destroyed the evap coil. I would get it in the shop and not use it until repaired. When water freezes it expands and can destroy your coil the very first time it happens, however it can freeze up several times before it effects the coil.
Unfortunately the evaporator is buried under the dash and can cost a lot of money to replace. I had to remove the whole dash from my 94 and replace the evaporator, because I did the work myself it did not cost as much as it could have. You will need a good set of gages to properly check for this, the pressure switch is easy to replace, so I would replace it first if I did not have the tools to check it out. The control is set for the proper pressure and because of a check valve below it, when you remove it, you will lose hardly any refrigerant if any. Note: make sure that check valve exists before you remove it, it is there on the 94 trucks.
There is a remote chance you have a restriction at the refrigerant control (not to be confused with the pressure control) and could cause the over charged condition per the tool and freeze up as well. I am a AC contractor and would rather have an auto technician check it out first, but I can afford it and you know the old saying "time is money".
I hope this helps
Steve...
Attention: TDR Forum Junkies 
