Powermaster tells us that loose V-belt tension is a common problem. An old or glazed belt will slip when expected to drive an alternator charging at 50 to 60 amps, yet it will not make any noise. To test belt tension, place a socket and breaker bar on the alternator nut and turn the alternator clockwise. The belt is the correct tension when the pulley attempts to turn the engine. Why is that important? Because your alternator cranking out 100 amps at 14.5 volts equals 1,450 watts, which converts to 1.94 hp. That means it’s going to take at least 2 hp to drive that alternator through the belt.