2000 F-150 V-6, about 75k miles, factory battery and alternator.

For several months, I've noticed that the output from the alternator drops from around 13.8V down to about 12.5 any time I go above 3000 RPM or so. At about 4000 RPM, the battery light comes on briefly.

I'm a ham radio operator and have a mobile radio in the cab with a voltage display and I can see the drop. And now with more night driving, I can see my headlights go noticeably dim when I accelerate and go ovr 2500 RPM or so.

I've tried the "simple" tests in my Chilton's to check the alternator and battery, and they're fine at idle RPM. The guys at the local parts store checked the battery and said it might be marginal, but since they couldn't get to the battery directly to read the amp rating, they had to guess at the rating to enter into their testing device. So I don't trust their answer.

Before I run out and buy a new alternator, can anyone tell by the symptoms if the alternator is really the problem?

Thanks for any advice!

 

Just went through this today with a 2000 F150 4.6 V8.

At about 3K RPM the output voltage would drop from a normal reading to 12.3V and the ALT LIGHT would illuminate.

R&R'ed the alternator with a 100% new unit which took care of the problem. An inspection of the old alternator showed one brush worn down past the limit and the associated ring burnt. May be able to salvage it by turning it in my lathe.

Seems as if the voltage drop around 2500 to 3000 rpm is common as it's talked about many times in the Ford forums.

Rolynn

 

Thanks for the reply - I replaced the alternator yesterday and all is well. Solid 14.3 volts from idle through 4000 RPM.