The purpose of the pointer is to measure the angle through which the rubber torsion spring in the middle of the tensioner is turned. You don't need to measure the slack in the belt.

To get the belt off, loosen the big bolt then slacken the tensioner. Watch what happens to the plastic pointer as you do this.

When it's all back together and the belt completely slack you set the plastic pointer to the end of the cast triangle marking (can't remember which end but you'll soon work it out if you noted what heppend when you loosened it) then tighten the adjuster until it points to the other end. Then tighten the big bolt.

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Mick J
'08 Chrysler 300CRD (MB OM 642 engine)
'95 E220 estate
'89 230TE (R.I.P.)