My serpentine belt on my 91 300 E 4-matic started squeeling during start up-when it is cold out-it usually stops in under a minute-I was able to con my ex into taking a look at it..He is a jaguar collecting, Audi driving, MB hater-HOWEVER-for a stock broker he is a darn good mechanic.....He feels the belt is loose, and told me if I could find out where the tightening point is, he would fix it for me (we were in my cold driveway, not his heated garage) Question-where is this part located? AND I did a search under belts-does my car have a belt tensioner-and if so, should I have it replaced at this time? car has 211,000 miles on it...Thanks for any help you can give me!

Yes, your car does have a tensioner. It's located right next to the water pump.
First of all, check the condition of the belt. To check the tensioner the first thing you should do is remove fan shroud (much easier). Next, locate the main bolt in the middle of the tensioner (19mm, I think). Loosen the bolt about I complete revolution. Now locate the tensioning nut (13mm) on top of the tensioner, next to the water pump (it sticks out vertically). To tighten tensioner turn bolt clockwise to loosen turn counterclockwise. Pay attention to the tension indicator on main bolt and the lines on the tensioner housing. Make sure not to over tighten. Once you set the tension tighten main bolt.
If you can't tighten the belt you tensioner is probably busted. The rubber in between the middle and the tensioner housing breaks loose and no matter how much you tighten nothing happens.

Good luck,
Vin