I have a 2002 f150 4.2 with about 190k miles on it. Just replaced the alternator as the brushes had worn out. Have replaced the alternator with now 4 alternators from Advance Auto. I also replaced the serpentine belt. Problem I am having is that the battery light is staying on. Took reading and running at 12.3 and not 14 like it should be. When I turn on the power accessories the battery drains. Fuses are all good. Battery is from 9/2015 and had tested it to be good. I also replaced the alternator harness by taking one off of a '99 expedition ( same wiring harness). Any suggestions would help as I am scratching my head on this one.

 

Please read slowly and carefully.
The charge LAMP is powered through the ignition switch in start and run position.
It gets it's ground from the Alternator regulator to turn on.
So you know that circuit and part of the harness is good.
.
It is turned off by the regulator when the Alternator comes up to output after starting.
This is not happening in your case.
The Alternator has a separate 20 amp inline fuse to supply the field winding.
Have you checked for this fuse being blown?
Without field voltage, no Alternator will work and the charge Lamp will not go off as it was designed to do.
This is how the system tells you there is a fault of some sort.
To see this circuit, reference Haynes manual page 12-17 lower right in the diagram.
Good luck.

 

Thanks Bluegrass 7! I checked the 20A fuse and found out it was loose and burned on the connectors. Funny thing is it didn't pop but still that was the problem. My truck battery light turned off.

 

It could have just been very lightly broken, sometimes they do that. I've had a car where I've checked the fuse 5 times, and it looked fine, but there was a microcrack that broke the connection.