I have a 99 F150 XLT 4.4 with the 5.4L. About a week ago the battery light in the dash started to flash intermittently. Eventually it came on and stayed on. I tested the alternator by measuring battery voltage with the engine stopped and read 12.3v; with the engine running, this dropped to 11.7v. I took this to mean a bad alternator. I went to Advanced and bought a new one and installed it tonight. To my dismay, the battery light still illuminates. I checked voltage at the battery again, showed 12.3 not running and 12.4 running. The instructions for the alternator advised to to check voltage from the neg battery terminal to the alternator housing running, this showed .03v. It also suggested to check from the positive terminal to the alternator housing, this shows 11.4v. According to the manual this indicates a wiring issue from the battery. I visually inspected and cleaned terminals to no avail. I also cracked open the fusible link and cannot see any way to test that. It looks simply a splice with two smaller gray wires in between. I am starting to possibly doubt the battery however the truck runs great even prior to cahnging the alternator. Any ideas? I hate to think I just wasted $150 for an alternaotor but the truck has 115k on it so it probably isnt going to hurt that it was replaced. Any ideas? I am not a mechanic, just trying to figure this out. THanks!

 

You should also have the battery load tested, or at the very least charge it with a trickle charger. An alternator is not meant to charge a very low battery.

The dead battery could have caused the issue with the alternator, but if you continue on this path, chances are you could trash another one.

 

SScully,I did charge the battery to a full charge prior to the swap of the alternator. I messed with it some more this evening and I got it resolved. I was able to test the cables and verify that they were good. The only other thing that I could think of was possibly a fuse. I found one in the underhood fuse panel in position 11 labeled Alternator Field. This is a 20a fuse. I pulled it and tested it, it passsed multimeter test for ohms across the fuse. I replaced it in the panel and it started working. I am not sure what that fuse does exactly, I will search for it more. I am not sure if the fuse contacts were dirty or what but after reseating it, the lamp went out and I now measure 14.6v at the battery while the engine is running. Do you know what that Alternator Field circuit does? Thanks for the help!

 

An alternator needs to have power to make power. The field fuse supplies the power to allow the alt to work. (not the fuse it self, but the circuit it is on)

 

The alternator needs power to "excite" the fields, otherwise they will never start charging. They used to use a fuseable link in line and if that burned out, it was sometimes a real pain to even find it to check it. The easiest way if you ever have to is simply do a continuity test from where it plugs in to the alternator to where it picks up the + at the other end. It looks like yours had a fuse in place of a fuseable link. Makes a lot more sense to me than the fuseable link. PIB! That wire is full time hot. It is not terribly uncommon for a "good" fuse to lose continuity. That's why if the fuse for the correct circuit "looks good", it's always a good idea to "reset" it by doing just what you did, maybe even 3 or 4 times to make sure the contact is good.

 

This is why the test is with the fuse installed, reading VDC from the pins on the back of it to a known good ground.

It would have shown no VDC on 1 or both sides, and you would have known something was up straight away.

 

Thanks for the explanato guys, I am good to go