Are you using the proper size (lengthwise) of belt.

I once went through 3 alternators (bearing issues) and one bent tensioner pulley because I was using the wrong belt.

I installed a belt for the 350sdl engine. I should have put on the belt for a 300sdl. It was one inch shorter and enough to damage the bearings.

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Ben
1987 190d 2.5Turbo