I'm with suginami, my 190 D runs at about 95-99C on freeway. in winter with heat on I don't believe t-stat opens at all (so-cool rad). the main fan (hydromatic clutch) kicks in at 110C and shuts off at 100C. the aux fan starts only with ac on or temp at 115C.
The way I checked both fans- turn off the heater/clime control completely. drive hard for 1/2 hours (or less), stop and idle. in winter it may take about 15-20 minutes. finally, open the hood and look at the fan (kinda like watch grass grow). your main fan should kick in at 104C and should stop at 96 C. And you will know when it stops.
I did not like how slow the temp dropped, so I slaved the aux fan to the main. Now the temp drops in about 10 seconds. My advise (and I was KILLED for giving it on this forum), leave the car alone. As long as the temp does not exceed the one where the main fan should start at (104C ?), you should be OK.
BTW I replaced 85-95 t-stat with a 80-90C t-stat. The result - the running temp in summer did not change, but in winter the car is running too cold (80 C instead of 95C) and it takes a while to heat up. I'm going back to the original T-stat.
1985 190D 2.2l Sold-to Brother-in-law
1996 Mustang 3.8l -"thinks it's a sports car"
1988 Grand Wagoneer - Sold (good home)
1995 Grand Cherokee Ltd -"What was I thinking??!!"