Since pressure and temperature of the refrigerant follow a relatively continual relationship in the range of the P/T curve where we are interested, either/or has been used to measure when to "throw the switch" to the aux fan(s).
The original receiver/dryers had an orange switch with a 6mm thread. These measured refrigerant temperature (corresponds to pressure, remember
) and were NOT in contact with the refrigerant. You could replace the switch with no refrigerant loss; but most have these have been replaced with the newer R/D that has a red switch. There is a green switch also, but you don't see many. They were used for new R/D with old R12 for a short time, I believe.
The new switch is a 9mm thread and goes into the refrigerant and measures refrigerant pressure to turn on the aux fan. Remove this switch on a charged system and you get a surprise. I'm going by memory on the dimensions, but I'm close. Should be verified.
In 1985 (I think) another switch was added to the thermostat housing that works on coolant temperature, as well, to activate the aux fan. One reason for the Klima relay? in this year. Maybe
?