A drop in voltage is, sort of, normal. The reason for this is the low idle. You can only check an alternator with an amperage check. If the amperage is low, replace the alternator.
You can try a set of brushes (regulator), but when they fail, the alternator doesn't work at all. If it is working about 1/2 way, the alternator is faulty.
1991 GMC Syclone
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