190D22,

You may have an electric clutch on your car, which gets its signal from one of the thermostat housing sensors. If the sensor dies the fan clutch never gets a signal to engage. Or the clutch itself can go bad.

The fact that the temperature goes down when air runs through the radiator (highway speeds make enough ram air go through the radiator that the fan is not necessary to pull air through) is an indication the rest of the system is working ok. So, you most likely have a bad clutch or thermal sensor. How do the electrically driven fans work? They also run off sensors in the thermostat housing. They should come on around 105 degrees Celsius.

Hope some of this helps, but I know from reading threads here that changes in operating temperature trends can be frunstrating to fix. Good luck, Jim

__________________
Own:
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles

Owned:
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)