So why was the spring replaced? Did you rebuild the tensioner assembly while you were in there, or just slap a spring in it? The stock alternator will not break the spring. The only time the spring gets stressed is when the tensioner shock wears out and allows the pivot to rapidly vibrate. It destroys the pivot bearing, idler pulley bearing, tensioner spring, and if left unchecked can let the belt off or crack the front timing cover.

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Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]