I have a 1998 f-150 5.4 4x4 with 249,000 miles on it. My battery light started coming on while pulling my boat. It would come on when the engine was at high RPMs and go off went they dropped. I installed a new belt because I thought it might be slipping. Well today it would come on and go off when passing someone while the truck was unloaded. I traveling at highway speeds and would speed up to pass the light would come on, when the RPMs went back down it would go off. I checked the battery, alternator and both checked fine. I am now stumped at what might be my problem.

 

you had someone check your alt ?

 

it may be time for a new belt tensioner if the alt checks out good

 

Your alt may have checked out fine, but I would check the voltage regulator on the alternator. Its an internal regular, the wires associated with the regulator are on the outside of the alt connected via a clip with some 18ga wires protruding. If your regulator isnt working properly it could be sending 15+ volts to your system thus causing the issue. The alternator is regulated by that connector and will only allow ~12-14.4 volts to pass from the alt back to the battery and starter solenoid. Without the regulator the voltage is could go way up causing possible hard to wiring and internal electronics. (since they are only rated at 12v DC).

Le me know if you need a pic of what I am talking about

 

It's time for a new alternator. Most likely this is the original unit. Could be just a regulator problem but with that kind of mileage, complete unit is the way I would go. I have seen this many times with the ford vehicles. Higher rpm's the light comes on, lower rpms the light is off.

 

Thanks for the suggestions. I checked the tension on the belt it is fine. I thought about the voltage regulator, but I changed the alternator a couple of years ago and since they are one unit I didn't think it would be the problem. Now I think I am getting a new alternator. Thanks again everyone.

 

Put a voltmeter on and see how many volts that alternator is putting out