2001 F-250 7.3L DIESEL - BATTERY OR ALTERNATOR?

When my battery warning light came on, I figured it was time to get new batteries. I drove the truck for one or two days. Then when I tried to start it the next morning nothing...Dead battery, warning light lit & battery gauge reading low.

I bought two new replacement batteries & installed them.
The battery warning light still remains lit, but the battery gauge reads normal.

I am able to start, drive, use the headlights & normal features of the electrical system.

When the truck RPM pings above 2000, the warning light goes out.
From what I understand an alternator does not increase voltage as the RPMs increase, so I should notice no difference.

I checked the serpentine belt, it appears to be snug & in good shape.

Any suggestions before I visit the repair shop?

Alternator probly has a bad or weak diode in it which will cause the light to come on.:)

start with this you should have 12 volts at the altenator larger red battery wire and you should have 12 volts on one of the other wire in the connector with the key on if you do not it is in the wiring alot of times the wires corrode off right at the connectors seen a few times on trucks but much more common on focus hope it helpsand if it doesnt also seen when you remove regulator look at the brushes sometimes grooved befond repair.

go to a corporate shop, they will run a battery tester on it for free, and that will tell you whether it is the battery or alternator

Just realized this is a double post [&:]

Was posted in the general tech section:)

hmmm..i just saw the yellow icon and clicked it.

My 1991 350 van did the same thing, The belt looked good, Thought it was the alt. But the next day the belt came off. ;)