I took 55A 1G Motorcraft to a repair shop because it makes only 13.5V charge at no load, and drops to 12.5V or so at load (all lights, gauges = 13A + el.fan =10A+charging system), and eventually (40 miles ride) discharges the battery. They tested diodes and said alternator should work. I am wondering what else, except bad diodes, could cause the alternator is unable to produce full 14V charge?