Alright, I got the belt on correctly. The only problem is now that the new belt is on it's way to loose. My tensioner has either gone bad, or I don't know how it works. Is it like the tensioner you would find on any GM car and self tightening, or do I need to tighten it myself to the desired tension level??? I'm guessing its the first, but hoping its the second.

Have a look here:
http://catalog.eautopartscatalog.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=1XC0ZKQX91ZA0YQ1VP&year=1992&make=MB&model=300-SE-002&category=G&part=Accessory+Belt+Tensioner
It suggests you pre-set the tension via a threaded rod.

__________________
2007 C 230 Sport.

Where is this so called "threaded rod" I am stumped here

The so called "threaded rod".



BTW, it is upside down in the photo.

__________________
2012 E350
2006 Callaway SC560

Can anyone explain in plain language exactly how you replace the tensioner and how it works. I assume from what I have read that the tensioner has some kind of rubber elastic material on the inside. It is bolted through the middle to the engine. There is an adjusting rod that slips over part of the tensioner. It appears that getting access to the tensioner bolt is difficult and the fan and fan shroud may have to be removed.

To remove the tensioner you first have to loosen the tensioner bolt, then loosend the adjusting rod. This releases the tension. You bolt the new tensioner (is it designed to be bolted on in only one way?), replace the belt, and then turn the adjustment rod (which direction?) which will pull on the tensioner increasing the radius so that it puts tension on the belt.

Is this basically correct? Any additions or corrections?

<<
Alright, I got the belt on correctly. The only problem is now that the new belt is on it's way to loose.>>

First, make sure you have the correct belt. They make 4 different lengths for 104 engined chassis and it is quite common to have the wrong length belt .
They come 2415/2440/2445/2475mm. You do not mention chassis #, but assuming it is a '93/300se/140.032, you need the 2440mm.
The adjuster linkrod can be seen between the PS res cap and the thermo housing. If it is there , it is the early style with the belt tension shock just below the Cam Position sensor. It is adjusted until the index marks line up with the pointer on the tensioner.
If it is the newer stlye , there is no adjusters/link rod/or shock.