I need to know what make, model, year you are referring to, as well as what motor you are referring to. As much as I would like to help you, without this information it is very difficult.
On most serpentine systems, the belt tensioner is not adjustable, it is pre-loaded from the factory. However on some motors, the belt tension needs to be manually adjusted when removing and installing the belt - which brings me back to my first point. I'll be happy to help you once I have some more information.
The tensioner is spring loaded, it is not adjustable. When you install the new belt, the tensioner will self adjust to the proper tension. Hope this helps,
