I have a 1987 F-150 4.9 L. The battery light came on then the ammeter in the dash would only go up to "8" a few days later. The alternator was tested at an auto parts store and was found to be producing 4-5 volts. I installed a new alternator but there was no change. I had the new alternator tested at the auto parts store and it was fine. I checked the volts at the battery with the engine on and off. It was 12 volts both times. The battery was fully charged and the serpentine belt is new. I was told to make sure there was a good ground, but obviously didn't check it properly. I connected a jumper cable to the negative battery terminal and the other end on the hood. When I turned the key all the electrical power was gone. After that, I discovered a wire that was corroded and held together only by a portion of the insulation. This wire went to the starter solinoid. Any idea as to what I might have fried ?

 

Age and corssion! DC electrical systems and water (rain and humidity) will cause corssion and eat wires, connectors, screws and even plastics!

 

I put a different battery in the truck and the posts were smaller than the original battery posts so the negative clamp was a little loose on the post. I resolved that problem and now have power. As for the alternator not charging I discovered a melted fusible link where the alternator wires connect at the starter solenoid/relay. All is well. Looks like the truck might make it another 17 years......... maybe not.