Hi all - I recently encountered the above problem on my W202 and thought I would post the fix up here.
Symptoms. Coolant level warning light on, rough running, particularly when hot. Overheating. Checked coolant level okay. Checked engine fan (12v electric, not viscous): not operational at any temperature. Water temperature gauge read 80 degrees c even when overheating (water temperature was clearly higher than the indicated 80 deg c.)
All fan/cooling system fuses were checked and found to be okay.
Removing the connection to the water temperature sensor did not make the fan operate: neither did shorting the two temperature sensor contacts on the wiring harness.
Removing the connection to the fan control module made the coolant level warning light go out, but the fan remained inoperative.
The fan control module is in the nearside (UK) wheelarch behind the inner wheelarch lining. The wheelarch liner can be moved by undoing 3 small coachscrews - the fan control module is mounted to the chassis behind this and is secured onto two studded capscrews/mounting posts with nuts. The module was very dirty and was prised away from the chassis.
A replacement Fan Control Module was purchased via online auction for 30 pounds
Whilst the module was off, I pulled the wiring harness including connection up from the wheelarch and then mounted the Fan Control module in the engine bay to prevent any further debris/water ingress. There is sufficient space behind the nearside headlight.
Fitment of the module resulted in : Fan operating correctly, coolant level warning light off, healthy running. And a few beers to celebrate!!
I hope this will help others who may encounter a similar problem!