The system has a relay and a pre-resistor for the 2 fan speeds, as I understand it (my 95' S320 is set up this way)

Mine comes on at about 90 degrees centigrade (slow speed) and 110 degrees centigrade (high speed fan)

There is a temperature sensor that sends info to the AUX FANS circuit to turn them on/off through the switching relay. The A/C head pressure will also turn them on. On my 95', they had to replace the pre-resistors, as only 1 fan speed would come on (high, but not low)