Mine is a 1991 300d, so it's different, I guess. Mine has the belt tensioner and idler arm pulley.

Sometimes the belt tensioner (the thing that looks like a little shock absorber) loses its ability to pull the belt properly. Might have to replace that. If it's OEM, might last for 5 years (other people would probably know better than me), if it's a lesser brand, it might last for 2 years.

jeff