2002 Deville 4.6L
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Make sure your vehicle is turned off and the parking brake has been applied.
Rotate the tensioner pulley to give the belt enough slack to remove it.
Use a half inch ratchet and an extension to rotate it inserting the square end of the extension in the whole in the tensioner pulley.
Check all the pulleys on your engine and make sure they are all free of any debris and make sure they are dry and there is no oil or other fluids on them.
Route your belt around all the pulleys according the the diagram provided leaving the tensioner for the last one. Be sure to keep tension on your belt to make sure it does not slip of another pulley.
Using your ratchet, rotate the tensioner pulley enough to slip the belt over it.
Inspect the belt and all pulleys and make sure they are all aligned correctly.
You can rotate the tensioner pulley a little to make any adjustments needed.
Read about tensioner pulleys.
How to check your belt