190D22,

If your engine is a 601 series, it appears from my W124 manual (note this is not a W201 manual, and it does not specifically address the 2.2 liter 4 cylinder Diesel in your car, but it says it covers 601 series engines, in specific the 2.5 liter turbocharged units for these cars) that it has an electrically controlled magnetic clutch. It is normal for these to spin freely. At 100 degrees C a thermostat housing sensor, called S25/1, that has two electrical connections on it, sends a signal to the fan housing via a plug off the top of the water pump bearing housing. I think you can force it to engage by either unplugging it, or by unplugging it and shorting out the two connectors in the plug, as the switch inside is either on or off and it can be send the signal by either connecting the two or not.

Hope this helps, 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)