Please re-read Arthur's post
It contained *everything* that's pertinent to your problem.
Since only one fan worked, the switch is ok and a new fan is in order.
Re TEMPERATURE: although 100C is where it is designed to come on, in my car it is more like 105C. Probably the same for you. This is triggered by the Engine Coolant Temp (ECT) sensor as Arthur described. This is mounted on the front of the car near the water pump.
The sensor on the air cleaner is the Intake Air Temp (IAT) sensor. Has nothing to do with A/C or fans.
[Edited by JimF on 05-10-2001 at 07:54 PM]
Regards . . . . JimF
'94 S500 Cpe
Visit my Mercedes Web Page