You need to check the resistance values of your coolant temp. sensor. If the sensor is out of whack, it will not provide the correct resistance value and the relay will not activate the fan.

Does anyone have the proper temp / resistance values for reference?

Also - on my car (1995 E420) I have added a resistor in paralle to the coolant temp. sensor. it makes the fan activate at lower temperatures. The car NEVER gets above 98 degrees C anymore. Hopefully I'm extending the life of the engine and wiring harness by doing this. I don't mind replacing electric fans if they wear out - probably much cheaper than replacing an engine.

Hope this helps.

Troy
1995 E420 93k