I have a 1996 E300. it's my second W210 diesel. Recently purchased, Been off the road getting alot of work done to it.
I had the car run in the driveway a long time after changing seals, lines, etc. It began to run EXTREEMLY hot, so it got the garden hose at the radiator treatment to cool it down. Conclusion, it needed a new mechanical fan clutch. That is now in and there is a huge difference. I strongly recomend anyone with a high mile car to change there fan clutch.
When I was thinking about it, the thought arose; Why did the electric cooling fans not kick on? I know that they work when the AC is activated. I thought when the engine got too hot they also turn on? I tried junping direct power to them and I got nothing. The electric fans were also dead.
I now have a new set of eletric fans in the car. NEW PROBLEM, when the engine is running the fans are on all the time. I unplugged them for now.

On a sort of related note. The climate control panel on the dash is no good. I can turn it on and raise/ control fan speed, but that is all. The display reads LO for temp and it always blows cold. I'm under the impression that the no good climate control is sending signal to the electric fan to constantly run?
Thoughts?

Could be one of the temp sensors has gone bad. Try unplugging the engine coolant temp sensor. On my OM606 (95 E300D), it's blue/green and located in the front of the valve cover by the radiator hose. With the car running, unplug it and see if the fans run.

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RIP:
2011 E350 BlueTEC, Schwartz 81k (totaled by uninsured drunk driver)
1964 Unimog 404.1 (wrongfully towed by the city)
1994 E320, Brilliantsilber 208k (transmission shuffled off after sudden catastrophic fluid loss)
1982 300D, Silberblau under a blue repaint 256k (sold for parts)
1995 E320 Polarweiss, 131k (sold for parts)
1995 E300D Polarweiss, 287k (totaled by drunk driver)

either that, or the relay switch

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1983 300D, the "Avocado"
1976 240D, 4-spd the "Pumpkin", SOLD to Pierre
1984 190D, 2.2L, 5-spd, my intro to MBZ diesels, crashed into in 2002