A drive belt tensioner is a pulley mounted on a spring mechanism or adjustable pivot point that is used to keep tension on the engine belts. ... Both are used to keep tension on the engine serpentine belts so that they can drive the various engine accessories.
If equipped with a tensioner screw, back off the tensioner screw. Then, push the alternator toward the other pulleys, loosening the belt. To loosen an NAI tensioner, loosen the locking nut or bolt, then back off the tensioner screw. Push the pulley toward the other pulleys or accessories, loosening the belt.