I'd pretty much figure on replacing the tensioner.
Correct procedure is to loosen the center bolt and then unscrew the adjusting nut to release the tension. You can leave it loose while doing the head, etc.
Going back on, leave loose and install belt except for the AC, then pull the belt out and push the tensioner pulley to relieve the tenstion -- the shock tends to extend if it's any good and shove the tensioner all the way tight.
I though I had the wrong belt when I did the water pump on the TE a while back, it pulls up so bad. Hans laughed at me, he forgot to tell me that little trick.
Once the belt is on, use the adjusting nut to pull the tensioner around till the pointer is near the top of the "ramp" cast in the tensioner body, then tighten the center bolt.
The bad part here is that the rubber is used as the spring in the tensioner, and looses is resiliecy (sp) with age. Chances are you won't be able to re-tension correctly -- tensioner should move back with the adjusting nut loose to the line in front of the ramp. If it doesn't, you need a new tensioner. Attempting to use the old one usually results in the adjusting rod going too far over on the exccentric and getting in a bind -- usually breaks.
Tensioner is about $150, I think. PITA to replace, too -- bracket in front has to come off.
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!