Ford F-250 1987 4x4, 5.4, standard 250000 miles
When I start the truck cold the gauge shows charging about 75%. After a few minutes the gauge drops to about 35% and the battery light comes on. The battery turns the engine real fast so I do't see it is the battery. I am thinking the alternator is shorting out after it warms up but is still working because it is a small short. Any comments before I spend money on an alternator?