Hi, I've looked at other similar posts but I figured I'd give details on my exact problem.

I have a '98 F150 4.6 4x4. Last week on my way to work (I work 3rd shift) the battery light would come on and go off several times and the headlights would dim when the light came on. It continued to get worse until the light just stayed on all the time. I thought it might be the alternator, so I had the battery checked and they said every time they put a load on it, the voltage(?) dropped and that it might be the alternator.

Replaced alternator and belt and the light was still on. Voltage at battery with ignition off, 12.4v, with truck running, 14.3v. The first couple of days, when leaving for work the light would remain off for 15-20 min. and then come on and remain on and lights would dim. In the morning leaving work the light was on. Two nights ago it would run normal, light on, and even over charged a few times with the volt gauge going way up and lights getting super bright. Last night the volt gauge slowly started dropping until it was dead and I lost my lights but truck continued to run.

Today I went and jumped it, started fine, volt gauge was showing normal charge but battery light was still on. I cleaned the battery terminals and checked connections but everything looks ok.

EDIT:
Oh, one other thing. A note that came with the new alternator says if the battery is putting out less than 12v, the alternator will shut down to prevent damage and therefore, you must make sure the battery has 12v after installing. It is also a 130amp alternator.


I plan on checking it more carefully this weekend but any suggestions would help. Thanks a lot!

 

You have obviously a very peculiar situation here.
It could either be a circuit problem or the regulator in the new alternator (which would not be too unusual on a reman), or possibly a collapsing cell in the battery. Here is a diagram of the charging system on your truck. Check all the wires, and fuse connections through out the circuit. Let us know how you progress.

 

Ok, I checked the wiring, mega fuses, resistance, etc. all looks good. I checked all the fuses in the fuse panel in the cab and all of them in the power distribution box under the hood. Those seemed to have corrosion or oxidation on them so I cleaned every fuse with an eraser. Put everything back together and tried it again. It was barely charging and the volt gauge needle was headed south. Went and had the battery checked again, this time at the garage next door, said battery was good. So I decided to take the alternator back and have it tested/replaced. Borrowed a car and took it to the store. They tested the "new" alternator and it was bad. Gave me another one and asked them to test it. It is good. So I am going to take my battery to work tonight and have the shop charge it overnight and then install everything in the morning. Hopefully the faulty alternator is the only problem!! Thanks for the wiring diagram.

 

That was it. Put in the new alternator and everything is great!