An automatic belt tensioner is spring loaded in order to naturally apply the proper amount of tension to the serpentine belt, which winds through the engine pulleys and is used to power multiple peripheral devices. As such, if your belt tensioner is not adjusted correctly, components like the alternator, power steering pump and water pump will not function properly. The good news is, adjusting it is a fairly simple procedure, so you can save a good deal of money by doing the job yourself.
Disconnect the negative battery cable with a ratchet and socket, and locate the serpentine belt system near the front of the engine. Some automobiles will have the belt system on one side of the engine, and will therefore require removal of one of the front tires. If this is the case with your vehicle, you will need to lift the front of the vehicle with a jack, rest it on jack stands, and remove a tire with a tire tool.
Locate the adjuster bolt on the belt tensioner used to secure the serpentine belt, and adjust the tension by loosening or tightening the adjuster bolt with a racthet and socket while you simultaneously move the belt back and forth gently to find the proper tension. You will know that you have the right amount of tension when you can twist the belt about half a turn with your thumb and finger. Less than half a turn is too much tension, and more than half a turn is not enough. If you wish to remove the serpentine belt, simply loosen one of the pulleys with a ratchet and socket, and slip the belt off with your hands.
Complete the task by following your disassembly process in reverse.