Actually, the "low" setting of the auxillary fan and the electromagnetic fan are both triggered by the freon pressure switch on the A/C drier. The 'high" setting of the auxillary fan is triggered by the temperature sensor in front of the engine. If you are running your 16v without A/C, both fans have to be triggered at temps above 100 degrees C.
*(100 deg C for the Elect. mag. fan and 110 for the Aux fan @ high: Red 3 prong switch for 16v)
*(105 deg C for the Elect. mag. fan and 115 for the Aux fan @ high: Green 3 prong switch for the 8v)
There are times that having your aux fan run at least on "low" actually prevents the temp from reaching above 100 degrees C to trigger your "highs".
I agree that 75 degrees C is a bit too low.
1986 190E 2.3-16