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On the 1.5dCi it's a bit of a pain. There's an idler pulley below the alternator that's adjusted with a metal quadrant held on by 2 T40 torx screws.
They are very difficult to get at as the space is...
You cannot adjust the belt tension on the alternator, it is automatically adjusted by the automatic tensioner located by the A/C compressor, if it's squealing check the tensioner for sticking, the...
http://autorepair.about.com/library/faqs/bl796e.htm It's probably similar to the Trooper since they share a motor (3.2L). On mine, you can see/reach it better from underneath. Loosen the locknut...