I find that the engine driven fan with viscous coupled clutch in our 300TE (M103) works just fine. It only engages at higher temperatures and lower engine speeds. At times when every bit of engine power is needed to drive the car the fan would not be coupled anyway.
The engine driven fan in the 190E (M102) is an entirely different matter. It is coupled with an electromagnetic clutch which means it is either engaged and running at a speed proportional to engine speed or not running at all. Whilst driving at normal speeds it is not usually engaged. In traffic in hot weather the engine temp will rise to about 100 C and the clutch will engage. At idle the fan speed is barely sufficient to give any cooling and the temp will not drop or may even continue to rise until the electric aux fan cuts in (at around 105 or 110). When the traffic then begins to move the engine driven fan will roar as engine speed increases, considerably loading the engine. In all, no where near as satisfactory an arrangement as a viscous coupled fan. If it were possible I would fit such a fan and clutch. As an easy alternative, I am experimenting with swapped connections at the temperature switch on my 190E. This allows the electric aux fan to come on first at about 100 C, quickly dropping the temperature until the fan switches off again (at around 90, I think). The compromise mechanical fan (inadequate low speed cooling and too much high speed noise and drag) should only ever be required when things get really hot. It's winter here now so I'll have to wait for next summer to see if this is a better arangement but from autumn (fall) when I did the mod up until now it appears preferable. If it is the way to go I would also like to modify it such that only the electric fan operates with the aircon rather than both fans as is currently the case. This would prevent the mechanical fan roaring and causing noticeable engine loading as it cycles on and off with the aircon.
107.023: 350SLC, 3-speed auto, icon gold, parchment MBtex (sold 2012 after 29 years ownership).
107.026: 500SLC, 4-speed auto, thistle green, green velour.
124.090: 300TE, 4-speed auto, arctic white, cream-beige MBtex.
201.028: 190E 2.3 Sportline, 5-speed manual, arctic white, blue leather.
201.028: 190E 2.3, 4-speed auto, blue-black, grey MBtex.
201.034: 190E 2.3-16, 5-speed manual, blue-black, black leather.