R134 and R12 use operate at different pressure. Your refrigerant might be low, R134 should be filled at 80 percent in R12 system. My 1989 was retro fitted with R134 and the I had to turn the screw in the pressure switch 1/4 turn cc to keep the compressor from cycling too early.
Like I've said previously , on auto the AC runs all the time summer or winter , but the AC shuts off on ECON . If the window is down the AC freezes up summer or winter when set to auto . Recently I found a used reasonably priced controller on eBay for $35.00 , I installed it myself but it operated the same as the original . Not a big deal $35.00 and a couple of hours work , at least I know now that'* not the problem . In the winter I unhook the wiring to the AC under the hood because if I forget to turn the ECON button on the AC goes on . It'* not the end of the world but I would love to figure this out .