I have a 2000 F-150 4.2Lt V6 5spd. The battery light came on one dark and stormy night. I know, this tale sounds familier. The battery was 6yrs old so I replaced it and the light stayed on. I then removed the org alt and found that the rear bearing on my alt had burnd out and was chirping. I went to autozone and tried to fix the problem with with two of their defective ones. Got my money back and went to Advanced auto. The guys from advanced auto put their rebuilt alt on the machine and pulled 15 volts out of it during a load test. I went home expecting to get at least 13-14 volts out of it when I put it back on. What do I get for my troubles, 12.5volts. and the battery light stays on. All of my connections are corrosion free and tight. The alt tested fine as did the new battery. What do I do? I even broke down and called the Ford Mechs at the dealerships and they were stumped. Is there a fuse that I missed? The main 180 amp fuseable link on the hot side of the battery is intact and since they no longer label the fuses in the power box, its difficult to find the right fuse and/or relay to check or replace. I have read most of the posts about alts and batts but no one has mentioned anything about the problem that I have encountered.

 

I have gotten 3 bad alternators in a row before. They do a crappy job of rebuilding them. I have had horrible luck with alternators, so it might just be that you got another bad one......

 

After I searched the web, I found out about a fuse that I did not know about. Its called the main field fuse for the alt. Its a 20 amp fuse in the power dist box under the hood in the number 11 spot. The fuse was BLOWN and I replaced it. After I started my truck, the alt started to sing like a rattle snake and I got 14 volts out of my battery and the dreaded battery light went off. Now I have a rough idle and a hesitation problem with my truck and the service engine soon light is on now.

 

Time to get the code... If it ain't one thing, it's another...