[s-cars] Serp belt slipping

[Mthdealer at aol.com]

Hello All,
My serp belt is slipping on occasion. The volt meter will read 12 V and
slowly creep up to 16V, while the belt is squealing. After it gets to 16 V
the noise stops and it's fine. Won't do it all the time, but happens more
when more acces. are on. The belt seems tight and not worn. How can I tell if
it's the tensioner? Is this an easy fix?
thanks,
Dave O'Neil 95.5 S6