Well that small rod can snap off.
Sounds like a dead tensioner to me....
M103s in the W126 go through accessories pretty quick if you drive on the freeway often, due to the low geared rear diff giving them high RPM.
If your alt has failed, replace it with a large unit from a W140. 143-150A is a great upgrade for today's batteries. I think the W1240 400E came with a 110-120A unit, which is also a nice upgrade. Allows for you to have everything on at idle... rear defroster, A/C on high speed, wipers, headlamps, etc.