Your question would be better answered if the type of belt was known, i.e. "serpentine belt" or "timing belt", etc. Also, one would need to know the size of your motor and the year.
routing for serpentine belt 2003 mercury sable
There is NO TIMING BELT on a 1995 Mercury Sable * Both the 3.0 and the 3.8 liter V6 engines in a 1995 Mercury Sable have a timing CHAIN
In a 1989 Mercury Sable : Both the 3.0 and the 3.8 litre V6 engines have a timing CHAIN
The 1997 Mercury Sable does not have a timing belt, it has a timing chain that normally requires no replacement and should last the life of the engine.
simply wear your seatbelt.
Right side lower belt driven accessoriy
It has a chain that requires no maintenance and should last the life of the engine.
The 3.0 liter " Duratec " V6 engine used in the 1996 Mercury Sable has ( 1 timing CHAIN to each cylinder head , so there are 2 timing CHAINS )
Try the links in the "Related Question" below regarding belt diagrams for Taurus/Sables.Don't know for absolute sure that company resources go back to 1992, but pretty sure they've got what you need.
You cannot adjust the tension on this, it has a spring inside of it. If it is weak, it must be replaced.
I would suspect one of two things. Either the replacement belt is not the correct size or the belt tensioner is bad. The squealing sounds like a loose belt which would also account for the battery light. possible bad battery causing the alternator to work too hard
There is no timing belt on a 3.0 DOHC. There are two timing chains which are inside the engine under the timing cover.