They are extremely problematic. Get the connector and switch for a 1990 G20 van application. They make a R134a specific switch that has the older R12 thread. The older style switch is much more durable. It is also adjustable and can be used to adjust the compressor cutout to maximize cooling without freezing up the evaporator core.
For factory R134a trucks, I use the equivalent switch for a 1994 G20 van. When I replace the switch on any of the GM vehicles that used that small diameter junk switch, they get the equivalent larger diameter older style switch.
Edit, the R12, 7/16-20 thread R-134a switch is Four Seasons 36674. The connector is 37219. The wires are not polarity sensitive. I have used solder sleeves for the connections on many vehicles to make a reliable junction although now I use un-insulated barrel butt connectors and 4:1 marine heatshrink. Both work equally well. Remove the switch, screw the new one on, cut the old connector off, splice the new one on. Turn the screw 1/2 turn counter clockwise. Test drive the truck down the highway. If the evaporator core does not freeze up, go 1/4 turn more and repeat. When the cores starts to freeze up you will notice loss of cooling and airflow. At that point turn the switch 1/4 turn clockwise and retest. I usually finalize that adjustment on longer road trips. If the core freezes up, I switch to vent for a few minutes to let it thaw out, then adjust it at the next stop. Balancing that fine line between freezing the evaporator core up and cycling the compressor before it freezes into a block of ice is how I achieve vent temps like this.
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