How can you tighten the serpentine belt on a 1986 Mercedes Benz 300E-6 cylinders?

Answer 1

Generally, most serpentine belts use a tensioner to maintain tension on the main belt; if it seems loose your tensioner may be weak. The tensioner is usually located near the top of the belt path and has a pulley attached to it. If the belt you are asking about simply drives one accessory, then it most certainly has to have a means of tightening it. Check on or under the accessory that it drives for the tightener.

Another Answer:

Here's how you tighten the belt. The above is correct but vague. You need more information. Take a look at the top of the engine, close to the front. Find the water pump pulley and water pump. Close behind that is a thing sticking up at an angle that looks like a bolt head, only the head is long. This is the tensioner adjusting bolt. I believe it's an 11 mm fit, but it may be a 13 mm. Slip the socket on this little bolt and turn it clockwise to tighten. Be careful. Turn it in 1/4 or 1/2 turn increments. If you get too tight with it, you'll break off the attachment down below on the tensioner, and the tensioner itself is hard to remove. After each adjustment, check the belt tightness. Again, don't overdo it or you'll be taking it to the dealer...or replacing the tensioner yourself, which isn't fun.