How to Tighten the Serpetine Belt on a Mercedes Benz 190E

Visual inspection. Serpentine belts are supposed to be self tensioning. You will need to visually inspect each of the items the belt comes in contact with to see why it is loose. Also inspect the belt. Visually check each device it passes over to make sure nothing is broken.

Unlike older type fan belt systems where you had a separate belt for each device the serpentine belt turns all your systems. AC, power steering , alternator, water pump etc. it also passes around the main pulley on your engine where it gets it's power. It is really a clever design.
In this drawing I have let the alternator be the culprit because it is the one I have seen the most. It is common for alternator mounts to crack. You have to look closely but your eye will adjust and you will begin to see and realize more and more. A cracked mount can cause the belt to be loose. There are many other possibilities but don't let this overwhelm you. Things will begin to appear as you work. Take a break, go in the house and drink some water. Things will look easier when you come back.

Take a few pictures with a digital camera so you can see how things were before you started taking them apart.
There should be a diagram of how the belt goes somewhere under the hood.
If it's not there and you can't get a shot of all the pulleys with your camera don't be ashamed to take notes.

It is always a good Idea to have someone helping when doing a job like this.
Locate the automatic tensioner, it is different from anything else because it is smaller and has a dogleg shape hooked to it. If you look at it closely you will realize that if it is moved a certain direction the belt will be loose. You have to place the long wrench over the nut (the one on the pulley down by the belt) You won't be loosening the nut or removing anything but the belt. When you pull on the wrench (it will be hard to pull but it should move smoothly)The belt will start to sag and your helper will be able to lift it right off the pulley. Make sure they keep their fingers clear and do not let off on the wrench until they are clear of all devices. With the belt off you will be able to physically inspect all the pulleys.

Now that you have the belt off you can turn almost all the pulleys by hand. Make sure you are wearing gloves. Each pulley should turn smoothly and with little or no sound. The one on the bottom of the engine will not turn as it is the power source and is connected to the inside of the engine. None of them should wobble or rattle. Don't go spraying everything with wd 40 . It is a great product but if you get it on your pulleys your belt could slip. Look at the old belt. After doing all this don't reuse it. Go ahead and get a new one.

You have checked all the pulleys and nothing seems wrong. They all spin with no rattles and everything seems ok. Lets go back to the auto tensioner, the one you put the wrench on to take off the belt. Pull it and make sure it is a good firm pull. Check it for wobble. Check it for loose mounts. If all ok check the mounts on all the other devices. If all ok place the new belt on the car using the reverse procedure of how you got the old one off. If loose go back and recheck. If everything is still ok make sure the old belt and the new belt are the right size and model for your car. Don't worry you haven't wasted your time. You would need to inspect all these points anyway. If you are sure you have the right belt and it is still not tight make sure the alternator mounts are properly adjusted. Your last resort would be to replace the auto tensioner. Before you do make sure it is adjusted and mounted properly.
After replacing defective parts install new belt and happy driving.