When it comes to "battery light comes on at high RPMs", I had the same thing with the third alternator I put on the SE. Somewhere above 2000 rpm, the damn alternator would quit charging - belt was fine, tensioner was fine, etc.

Replacing the alternator did it. I suspect that the ripple frequency of the alternator output confused the (obviously defective) regulator - I can't prove it, but that's what it seemed like. Personally, I can't imagine the tensioner in our cars being at fault - that is a HELL of a spring. I could see the spring breaking but that would mean NO tension to speak off, and the bet would be off the pulleys in short order, or so I would certainly think. But a "weak" tensioner? I doubt that.