need more info- aux fan should engage only with AC on or gross overheating. if you have electromagnetic clutch I'd be amazed if it was bad. Did you change your $21.39 fan switch (on top of the water return from the radiator)??? also, is it overheating while driving? if you get over 100C on the road, you need a new radiator (fans work only at speeds below 40 mph, after that the flow of air is sufficient to cool it down). I had the same prob-non MB mechanic told me my clutch was bad (idiot). then 2 MB certified MB mechs told me my temp was normal. Then I found out my fan switch was bad (so called 100-110 switch- kicks in the main fan at 110C and runs until temp falls to 100C).
1985 190D 2.2l Sold-to Brother-in-law
1996 Mustang 3.8l -"thinks it's a sports car"
1988 Grand Wagoneer - Sold (good home)
1995 Grand Cherokee Ltd -"What was I thinking??!!"