My 2001 F150 battery indicator has started to come on when the engine RPM reaches about 4000 Rpm. Battery has 12.4 volts when engine off, 14.3 volts engine running. Replaced drive belt no effect. Any ideas?

 

If youve replaced the belt,and the problem persists,I suspect a defective alternator rectifier diode.Take it off,and have an auto parts store check it.
Keep in mind however that alot of auto parts stores (especially Autozone) will give you "bad news" just so they can sell you one. My personal opinion is if you have it checked,take it to two different places,and see if both agree.Of course if you take it to a reputable place like NAPA,you can pretty much rely that theyre giving you the "straight scoop".
lastly,if you do find its bad,do NOT get a reman from Autozone

BTW...Welcome to the forum!

 

Mine was doing that for a couple of years. I was just waiting for the rpms to come down when the light came on before I did anything.

Well, one morning went to start the truck and the guages all swept to the right, then back. Started fine.

Checked the battery that night and found it was going. It was origianal to the truck, 2001 and this was Dec 08. Put a new battery in, and the light no longer comes on.

 

you need to check it with a load tester. if its charging 14.3 your alternator is probably fine unless its cutting out at high rpms, check it while revving the engine. check the battery with a load tester while engine off. it shouldnt drop below 12.

 

While the load test is indeed a good idea,IF there is a defective diode in the bridge,it will STILL show the correct charging voltage.

 

While his issue could be the battery,I would think that dirty posts could be an issue,but perhaps not the battery itself. usually when a battery goes bad,it just wont have enough cranking amps to turn the vehicle over after sitting for a period of time.
Alternators generate AC voltage.There is an internal rectifier "bridge" that converts this(otherwise non useable) AC voltage to DC.It is then fed through the regulator,and to the electrical system. If there is a shorted,or open diode in this internal bridge,it will allow a certain amount of AC to be "inserted" into the electrical system of the vehicle along with the remaining good DC voltage.The load test however will recognize only the TOTAL amount of Voltage being output.Which in alot of cases will indeed be around 13.4-14.4 volts.The only way to properly diagnose a suspected defective alternator is a bench check,and proper equipment designed to measure AC and DC.The battery light is bascially a "clamped" circuit with a zener diode.This circuit is designed to illuminate the lamp in conditions of under/over voltage as well as sensing that more than a certain amount of AC is being generated by the system.

 

My opinion is you need an alternator.
Reason is the brushes are not following the comutator at higher RPM due to wear and age.
The internal regulator detects this and lights the lamp as it is supposed to do.
Some store testing may not pick this up unless they spin it fast enough to see the problem.
4000 engine rpm is not the same rpm the alternator spins due to the difference in pulley sizes between the crank pulley and the alternator pully.

 

I would say it is your alternator. I had the same exact thing happen to me with the battery light coming on with the higher rpms.

 

BG-7 hit it right on the head. It used to be a problem with the old Generators and regulators of older engines.
Alternators are much better but it still happens. Very likely the brushes cant follow well because they are worn, or the rear bushing is loose.
Either way, you should swap it out before one of the brushes come loose and ruin the shaft.

 

X3 on the alternator. I had a simular problem except my voltage pegged at 16 volts anything past 3-4K rpms....replaced the voltage regulator first with no results which lead to an alterntor replacement anyways.