vsawmike
+1y
The evaporator should not freeze up period. Even if you put a jumper across the low pressure switch it still should not freeze up. Most tool trucks used to sell a jumper made to use on GM cars where the pressure switch is out by the accumulator. It turns the compressor on and keeps it from cycling too fast until you get enough refrigerant in there.
On an older system like this using a different refrigerant than it was meant to have you can disregard the amount it says to put in by weight. Just get the low side to 40 on the scale matching the refrigerant you are using, NOT THE PSI SCALE, and it will blow cold and not freeze up. This is assuming there is nothing clogging the system and the high side does not get too high.
Understand that the basic way an air conditioner works is:
When refrigerant goes from a liquid to a gas (in the expansion valve and evaporator) it rapidly sucks in heat. When refrigerant converts from a gas to a liquid it rapidly releases heat (in the condenser).
The optimum pressure for this conversion is what the expansion valve is designed to get it to. There may actually be a different expansion valve that futs the lines in your vehicle that works better with R-134a.
Did you know that an expansion valve will work if installed backwards? A GM expansion screen will, and a Chrysler block type will not but an actual old school expansion valve will.