The fan clutch I was talking about is the one bolted to the plastic main fan. The electric fan has no clutch. If your electric fan is working when you apply battery voltage to it, then it's ok and DO NOT REPLACE IT like you mechanic say. I don't much about your specific model. I have a feeling that the problem is related to a switch, relay, or the viscous fan clutch. The viscous fan clutch should have slight resistance when the engine is cold and higher resistance to spin when the engine is hot as the viscous fluid inside of it get thicker when temperature goes up. If you don't see a difference when you spin it hot or cold then it's bad. The fact that you have normal temperature reading while you drive make me think the viscous clutch is a possibility. The eletric cooling fan is related to the coolant temperature. It comes one at low and high speed. The high speed comes on for a few minutes only and when the temperature drops it either stops or spin at low speed. I have a lot of experience with BMWs and most of what wrote is based on experience with BMWs. However, MB and BMWs are not much different or even other cars when it comes to cooling. Also, one thing I strongly suggest is to check the overflow tank cap. take it to a shop and have them test it for leaks. If it doesn't hold presuure, then it's bad. You may just go to any parts store and get one for about $5 and see what happens. In my 190e, I couldn't figure out my heating problem for days and finally it was resolved when I changed the cap. From what you have described, I believe your system is working fine and it's just not holding pressure.
1993 190E 2.6
Advanced Diesel Systems Test and Research Engineer