Recently I've noticed my battery indicator light will come on at about 2400rpm and then go back off after dipping below 2400rpm. The longer I drive it seems to come on at lower rpm (2100rpm after 30mins driving). The dash gauge shows a good battery charge level. Anyone know what could be causing this? Alternator not meeting demand at higher rpm?
Battery is approximately 1yr old.

Thanks in advance!

Forgot to mention...1999 F250 LD.

 

I just did a quick test with a voltmeter. Truck off, battery shows 12.8V. Idling it reads 14.1V. RPM range when light comes on it reads 12.3V.
Alternator?

 

I would make sure the battery is good, but I would have to say that your alternator is on the way out. It only takes about an hour to remove and replace. Should be about 14VDC when the RPMS are increased.

 

Your alternor is bad

 

Could be an intermittent open rectifier diode or even bad brushes. In any case your in need of a new alternator.

When you idle and there is no problem, no intermittent problem then you should read around 14 VDC.

With your truck not running the battery reads 12.8 VDC.

When running and you see the the light come on and the voltage falls below 14 VDC to less then the 12.8 of the battery then you know the alternator is NOT charging.

Your battery voltage will be less than the "not running" battery voltage because your drawing current from the battery to run the truck and not putting any current back in. So the alternator is not working during the times you see the light come on.

Not too confusing I hope...

Good luck.....

 

I bought a new alternator and serpentine belt and swapped them out Tuesday night. No problems since. Runnin' like a champ again (knock on wood).
Thanks for the replies.