I've changed the batt/alt. and they are good, took it to a dealer and they loaded it up and could'nt get it to fail, well driving to work the batt. light is on constantly,I think its either a loose connection or bad mga fuse,also I'm reading others responses and think I need to check the MEGA fuses, where in heck are they (i've got 2 fuse boxes interior/underhood) which one?, my chiltons says that its a 175 amp fuse that can't be can it? thats huge.

 

Pull the black cover on the firewall, passenger side. Don't think the fuse is the problem. If you have not changed the alternator and don't know what to do, I would change it. First, check all grounds and wiring connections.

 

I recently replaced the alternator on my '98 F150 4.2L. The charging light stayed on and the battery went down while driving. The alternator bench tested OK. On the test with it installed, it produced less than 12 V and the machine said check wiring. There is a fuse on one of the small wires connected to the harness on the alternator. Up near the battery fuse block, there was a fuse case about 1" square by ¼" that had a (blown) 20A fuse in it. When I replaced it, the light went off.