What causes our new belts to break after about 2 weeks? on 1992 Mercury Capri

When the alternator went bad, the belt had to be replaced.
About a week after the belt has been replaced, the car belt
squeels when starting and the battery guage drops to low
then goes back up to high. Then after about a week of
squeeling, the belts breaks. We have replaced the alternator twice and the belt 4 times. Our mechanic
cannot figure out what is wrong. 92,000 miles. First
time belt has been replaced was with the alternator
replacement.

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3 answers
The replacement alternator has the wrong pulley on it there was a recall for this issue and they make a pulley replacement kit had the exact problem on my 91' replaced the pulley and problem was solved
Sounds like you have a pully out of alignment. Puts belt at a angle. Or poss. (but not often) tensioner keeping belt to tight.
The belt is not tight enough had the same problem with my 93 fought it for days. the belt has to be really tight snug will not do.