Failure is usually when the rubber torsion bush breaks up. When you put it all back, you set the plastic pointer to one end of the triangle mark, and increase the tension till it is at the other end. Then tighten the large bolt in the middle of the tensioner assembly.
The adjuster rod breaks when either;
1. Someone tries to adjust it without first slackening the centre bolt, or,
2. It gets pulled up too far becasue the rubber has failed.
'08 Chrysler 300CRD (MB OM 642 engine)
'95 E220 estate
'89 230TE (R.I.P.)