A serpentine belt controls all of the accessories as well as the cooling fan and alternator on your vehicle. Because the serpentine belt is negotiated through multiple pulleys, it is important to keep the proper tension on the belt. Most serpentine belts have an auto-tensioner that automatically keeps the proper tension on the belt. However, some belts have an adjustment screw that applies tension to the belt. If the serpentine belt is too loose, your alternator will not properly charge your battery and your accessories may not work properly. To correct the problem, tighten your belt.
M/F Open the hood and locate the adjustment screw that controls tension on the serpentine belt. This screw is typically a wing-nut located near the alternator and is in plain view.
M/F Place the socket end of the socket wrench over the wing-nut and turn clockwise to apply tension. It is likely not possible to adjust this by hand, so check your socket set for a socket that will fit snugly over the wing-nut so that you can tighten the nut, thereby applying more tension to the serpentine belt.
M/F Check the tension of the belt periodically. To do this, grab any area of the belt that is between two pulleys with your fingers. There should be approximately 1/4 inch play in the belt for most vehicles.
M/F Do a secondary check of the belt tension. To do this, turn on the vehicle and examine the belt in motion. Listen specifically for whining coming from the alternator. If you notice any whine or "pulsing" sound from the alternator, your belt tension is too tight and will damage the alternator. In this case, turn the adjustment screw counterclockwise to loosen the belt and recheck the tension.
M/F Do a final check. You can also check for continuous power from all your accessories. Turn all your accessories on at once. If you notice any abnormalities, then your belt tension is incorrect. For example, if your headlights pulse, then it is because the alternator is not delivering a constant charge to your battery. This is normally because your belt tension is too tight. Check the heater fan, air conditioning, etc. to make sure that every component is functioning normally.