One of the 'accessories' is locked-up and not turning.
Could be the:
That happened on my Contour, and it was the alternator in that case.
Watch the front of the engine while someone else cranks it over - see what component is not turning - likely to be the culprit.
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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
A broken serpentine belt.
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.
To remove the serpentine belt from a 2000 Mercury Sable, first, locate the belt tensioner and use a wrench or socket to rotate it counterclockwise, which will relieve tension on the belt. Carefully slide the belt off the pulleys, starting with the easiest one to access. Make a note of the belt routing or take a picture for reference when installing the new belt. Once the belt is removed, you can replace it as needed.
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 )
You cannot adjust the tension on this, it has a spring inside of it. If it is weak, it must be replaced.
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.