300SD: what temp is A/C temp switch set for?

Some mechanic in the past removed the connections from it and bypassed the auxiliary fan control circuit with a jumper wire. This switch is located at the top of the receiver-dryer, has a red body, black heat-shrink on the two leads, and male spade-lug connectors.I was told that it is a pressure switch, but I think that the fan is really controlled by temp. So if anyone knows at what temp the switch is calibrated to close (its open now at room temperature), I could heat up the sensor, and using my thermocouple temp meter, see if it is really bad or not. I am converting to R-134, but I can't see how the temp. for fan control would be different from what would be needed for R-12. Thanks in advance.