First off - I'm not a pro.
Your 190E I believe has what's referred to as an M103 engine in it. There are various sizes, but if yours is an in-line 6 cyl, Single Overhead Cam(SOHC), this is what you've got.
Many M103s have a fan relay that runs off of the A/C receiver drier, near the A/C low pressure switch. The fan is activated(I think) when the refrigerant pressure reaches a given point or the engine temp reaches 102F(in the case of my M103). I suppose your fan relay could be faulty; then again, it could be an number of things. Try searching the archives.

Mike Murrell
'91 300-SEL