The radiator that cools the engine coolant has a temperature sensor (Porsche calls it a "switch") that turns on the fans when about 87C is reached. The sensor is located down low at the front side of the radiator, at the driver side (eu).

Here are some pictures of this sensor 928 606 149 00.

 

COOLANT Temperature Sensor Values

C _ R (Ohms)

74 - 2300

77 - 2066

79 - 1931

85 - 1582

90 - 1330

95 - 1128