Watch the pointer on the tensioner as you loosen it -- if it doesn't move at all, or only goes part way back to the beginning of the little "ramp" mark cast into the tensioner body, replace the tensioner. Otherwise, you will break the rod when you attempt to tighten the dead tensioner you have.

Bad design, although it works beautifully so long as the rubber holds up. The "spring" in the tensioner you "tighten" with the rod is rubber vulcanized between the tensioner parts, and it does a very nice job of keeping the belt tight and dampening vibration, but just like the rubber bushings in the rear suspension links, it dies of old age. Causes rapid belt wear, and broken tensioning rods if you don't notice it.

Peter

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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!