I have recently been experiencing a problem with my battery light coming on. First thought was the alternator was going bad and I replaced it. The truck is a 97 F-150 4x4 extcab with a 4.6L. I have owned the truck for 4 years and put over 100k on it with very few problems. I have had the battery checked (4 mo old interstate) and as stated a new alternator. Both of which check out fine. The battery light first came on and the voltage gauge showed a significant drop before replacing the alternator, after which I have noticed the light coming on occasionally and the voltage gauge fluxuating apparently to compensate for what seems to be a floating short in the wiring system. Any clues or suggestions on where to look for the problem. Any suggestions would be greatly appreciated. Thanks in advance.
dc

 

There is a possibilty that you may have got a defective alternator. I would also check the terminals in the wiring connectors at the alternator, make sure those are clean & tight. Loose/ corroded wires can cause low voltage problems.

 

The alternator is not outputing when the battery light is on, therefore the voltage decreases. Check the wiring connectors (single pin and three pin) to the alternator to verify that they are free of corrosion.

 

Turns out it was the alternator. Note for anyone who is looking for the answers to the same problem. Get the higher amperage alternator because it is about $20 cheaper as a rebuilt. Thank you everyone for your input and replies it gave me an idea of where to look.
dc