A new belt on a bad tensioner is a wasted effort. While the belt is off:
or long-handled ratchet is standard for rotating the tensioner to release slack. Pulley Inspection : Before installing new belts, check all pulleys for smooth rotation lack of play 2005 Cat C15 Serpentine Belt Diagram
The belt typically travels over a smooth idler pulley to ensure it has enough surface contact (wrap) around the water pump. A new belt on a bad tensioner is a wasted effort