If the 190E engine is similar to an M103 (and I think it might be) you have a bracket behind the tensioner that is y-shaped. The bottom of the y points to the passenger side of the car. If this setup is on your car, the bolt hole on the bottom of the y is through the cover into the engine. The manuals tell how you reseal it with the proper MB goop. I had a leak there also on my M103 and Permatex orange high temp silicon is performing an admirable job in lieu of the MB high priced goop. You have to take the tensioner off to get this bracket out. At your mileage it would be a good idea to replace the tensioner and its shock while you have everything apart.
If your engine isn't configured this way, I apologize for the waste of bandwidth.