my 2000 f-150 4x4 xlt battery idiot light comes on above 3500 rpm or when I punch it. the system is charging around 14v at idle and above 3500 rpm it drops to around 13v. have not had any other problems other than door locs which seem to be working now. (I R+R them and shook them around a bit to free up the carbon dust which was causing some problems-shorting) I have a new battery the old one was an original 6 years+. anyway, Im thinking volt reg and or alternator. the belt seems tight enough. anyone see this or something similar??????

Look closely at the connections going to the alternator. Make sure they are tight. If wiring OK, I wouls suspect regulator.

definitely alternator. i had the exact same symptom. i checked battery voltage with a meter both at idle and when truck was off. everything looked fine(12.5 -14v). Eventually battery light was on constantly. Bad alternator!