My 91 Volvo 740 seemed to have a lot of problems with the alternator belt. The belt had broken twice so far within a year. This time the belt was so loose (seemed to be stretched) that it was not charging the battery and the car was dead on the road suddenly without any sign. I managed to jump start it but it died shortly after driving a few blocks.
I could not see anything unusually. The belt seemed to be aligned properly and the tension was right.
Anyone can help?
If the belt was so loose it did not rotate the alternator you should have had a warning light come on.
If no light came on, but the alternator failed to charge, your problem could be worn out alternator brushes. Those are very easy to replace. Two screws on the back of the alternator. Get old unit out, bring it to a parts store. But new unit, install and tighten the two screws.
While at it, tighten the belt!
Loosen your alternator and move it side to side. If there is a lot of play it could be the bracket mounting bushings. These rubber bushings are used for power steering and alternator brackets, as they age they will affect your alignment of your pulleys and will cause the type of problem you mention with the belts. In some cases you will never get them to tighten correctly. As far as the alternator itself - you may want to have it checked.
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