MY 1975 corvette is having an issue with the AC (R-12), I have 12 volts to the low-pressure switch and maybe 1 volt on the other side going to the compressor, I ohm the switch and it shows it’s closed, but no voltage flows through the switch.  I was going to replace the low-pressure switch but the replacement switch is printed on it that it’s for a R-134a, will this work with R-12?  If so, is there a check valve in the system to allow me to change it and not lose my R-12 that’s in the system if any?

I also checked my compressor clutch by applying 12 volts directly to the compressor and the clutch in gages as it should.

Thanks Jeff

I don't have an answer for you, but bumping this back up, hopefully someone else that can help will chime in.

Verify the switch is bad by jumpering the two terminals on the switch connector. If the compressor kicks in, the switch is bad. You will loose a minuscule amount of refrigerant when changing it. There is(or should be) a shrader valve under the switch that will contain the refrigerant. 

Thank you for your response, I will do that.

I just read this thread and hopefully you got your AC working. Just a FYI, I also have a 75 and I am the original owner of it. I converted mine to r134 in 1998. I had to replace a ceramic type seal in the compressor up behind the clutch. Never changed anything else and the unit still runs and the air is cold on the original compressor. The car will celebrate it's 50th Body Tag Birthday this year. I'd be interested in your results.