About three days ago my battery light came on at night and my lights dimmed than got bright again. I reved and no belt noise and everything seemed ok. I thought that was wierd and continued to work. I drive to work about 50 miles round trip and run my lights both ways, working 3rd shift overnights. Absolutly no problems that night or the next night. Today (daytime) I was just cruising and noticed my idle seemed to get higher and I looked down after a few minutes and my battery light is on again and is staying on! I went straight home. The gauge says above middle 3/4 (average place) even when I turn the lights on it doesn't drop much. It starts fine (so far) but something has to be wrong. I remember you can pull the cable off older cars and they should stay running on the alt. I tried that and it died. Does that mean the alt. is not charging enough?? 99 f150 4.6

 

At 6 years old, it's time for a new alternator. Brushes tend top wear down and stick/float.

 

The alternator is most likely dead, but have the battery tested as well. A dead (or dying) battery can kill even a brand new alternator in a matter of a week or so.

Also, on newer vehicles, don't pull the battery cable while the engine is running. You're likely to cause a voltage spike which could damage the computer or other critical components. On older carb'd engines, it wasn't a problem, but these new computer controlled engines are a whole other beast.

 

Your alt crapped itself. I just replaced mine and I had the same symptoms you described.

 

Thanks everyone. I figured it was the alt. I will have the battery checked tho.

 

My superduty did this same thing and I replaced the alt to no avail... Ended up being a corroded fusable link behind the battery. Mine is a 99 also. Might be something to consider if the alt does not fix your problem

 

Also check all the battery cable connections. If they are corroded on either end, that will happen too. And the chassis ground cable.

 

Thanks everyone! My alt. was only charging 11.5 so I replaced it (wow $145) and charged the battery. Checked the battery after and it was weak. Turns out its's the factory one. Not bad, 8 years old and 93,000 miles! New battery and alt. 210 bucks.