Should have an automatic belt tensioner -perhaps it is defective I don't think there is a belt tensioner. Belt tension is adjusted at the power steering pump bracket and idler pulley assembly.
it has a manual tensioner
The 1998 Lincoln Continental belt tensioner assembly is held in place by three retaining bolts. Remove the retaining bolts. The belt tensioner will come off.
Put a ratchet on the tensioner pulley bolt... set your ratchet on "loosen" turn the tensioner pulley and assembly upword... hold it there and remove the belt... DO NOT LET GO OF THE RATCHET AFTER YOU REMOVE THE BELT slowly let the tensioner pulley move back to a resting position...
The serpentine belt is held by tention with a automatic belt tensioner. If the serpentine belt is loose, you will most likely have to replace the entire belt tensioner assembly. the assembly comes with a new pully. Please visit my bio for a link to my Saturn help site for pictures of the assembly.
Should have an automatic belt tensioner - if tension not correct replace tensioner
If you are refering to the belt tensioner. I have not seen one on a grand wagoneer with a 360.
by loosening the belt tensioner
Remove the serpentine belt. Tensioner is below Alternator. Remove the three mounting bolts on the tensioner baracket assembly. Replace entire unit.
Your belt is probably stretched or the tensioner is weak. Try a new belt, or replacing the tensioner. If it is still loose get a 1/2 inch smaller belt.
first the spring puts tension on the belt, then you tighten the tensioner assembly and it hold there.
this should be spring loaded self adjusting if not tensioner is bad.