Over the past 1000 miles my +Battery- light on the dash comes on when I accelerate at a high rate. Happens if I floor it from a dead start or while I am already moving. The light goes out after the RPM comes down when I let off the gas a little. The alternater gauge on the dash dips a little but not very much. The needle is normally at about 5/8. If I rev the engine while in park the light does not come on and the needle increases a tiny bit. Nothing else appear to be wrong..the engine does not falter. Starts just fine too. No trouble cranking the engine.
I replaced the serpentine belt and the same problem occurs. While I had the belt off I checked the pulleys and all seemed to spin freely (except the crank shaft!). None of them appeared to wobble from side to side.
I have a 2000 F150 XLT Supercab with 5.4L V-8, Automatic, A/C and 92,000 miles. I do not have a 200 watt stereo or any other extra electrical load. Happens with the A/C on or off. My next guess is change the battery since it is 4 yrs old and perhaps it can't hold a charge as well as it used to? Wanted to check here for any other ideas before I pitch the battery to the junk heap. Can I just ignore this light or is something about ready to fail?

 

Drive your truck in to the nearest auto parts store (AutoZone, Pep-boys) and let the guys hook up the battery/alternator tester to it. It doesn't cost you a dime and it will pin point the problem a lot faster then guessing. If you're getting a new battery, consider a new Optima (yellow top or red top).

Tony

 

Know this is an old thread but same thing happened to me yesterday. Went to auto part store, tested, said battery bad, alternator good. Original Ford battery with 140,000 miles on truck. Replaced, still happened exactly as above. While driving, hit pothole, scratchy radio reception suddenly got clear and battery light went away. Suspect loose wire causing light to come on when accelerating. This morning, battery light back.

 

Larry, mine did the same thing when my alternator was going bad at about 110k miles. I replaced both the alternator and battery, and that fixed the problem. Probably just a matter of time before the alternator goes and leaves you stranded

 

Larry Try this.

1. Disconnect the (+) battery cable
2. On the back of the alternator there should be a red wire held on by a nut. This is your charge lead. Disconnect this.
3. Test with test light or volt meter if available (note: It runs directly to your (+) battery terminal.
4. Clean connector with fine grit Sand paper, emory cloth, Steel wool, or SOS pad.
5. Clean bolt on alternator as well
6. Bolt it back on.
7. check that the 2 wire plug is making a good connection.
8. Hook (+) battery cable back up and see how it works.

Hope this helps you. If it dosen't then your only out the time it takes to do it.

 

The alternator controls the battey light such that the alternator turns the battery light on when the alternator is not outputing. If you have already checked the conections for corrosion per previous suggestion, then the most likely caused of the battery light is worn alternator regulator brushes since the vehicle was 92K miles. Severly worn brushes will bounce due to high rpm engine vibration causing the alternator to temporarily stop outputing. The alternator regulator brushes will continue to wear until the light comes on continuously.

Good Luck