Hi Jim
I am wondering also and this is one of the reasons I'm still questioning all our understanding.
As I mentioned, it operates to MB spec. This means when unlocked the fan follows engine speed /2. When locked (I haven't measured the temp. of the rad), it follows engine speed until >=approx. 3000rpm when it releases. Then will lock again under around 1500rpm.
I measured the rpm by using an rpm gun and reflective tape placed on the blade housing - see here for a previous post quoting figures:
Viscous clutch/fan repair
At engagement the coolant temperature as reported by my gauge is 82-85C. The coolant will not increase over 88C - ambient temperature here at the moment is 15-20C.
I've also replaced the coolant sensor just to ensure it's reading correctly - and measured it and compared with it's quoted transfer characteristics. All OK.