That's a no-brainer. Your main fan switch never starts. The reason is your auxillary fan always runs when your ac is on. Your main fan kicks in at about 110C and stays on until your temp goes down to 100C. So, when you turn your ac on your aux fan starts and cools the engine down. Now, since 190's use 100-110C switch ("starts at 110C stops at 100C") you actually never give the switch/main fan a chance to do it's job. If you don't believe me, ask any MB dealer mechanic.
Now, if at an idle your engine temp exceeds 115C, your switch is probably bad (it's about $22.00). I have a 1996 Stang, and the temp continuously oscillates between middle and about 2/3 of the gauge, so 110C for a 190 is an equivalent and it's normal, and, I believe, it's not "too hot".
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??!!"