2003 F150. I replaced the alternator about two weeks ago. The battery light came on and it was only showing 12 volts running. Everything seemed fine then today I cranked up the truck and the battery light is on again. It starts right up, battery is strong but only showing 12 volts again,

 

Where you seeing 12VDC ?

 

Are your battery connections nice and clean?

 

Did u replace battery?
did you replace alternator with NEW

Bad batts kill alternators regulators
Reman alternators are unreliable quality, used parts sometimes

 

 

True story. My Buddy's little Brother worked at a Starter/Alternator rebuild shop a few years back. They used to tell them that if it worked and tested good, leave it. Some items in an alternator or starter may only test bad under heat conditions, etc. For the past few years, I've just been getting junk yard alternators. No sense paying 150 dollars for the same thing you can get for 15, minus the paint job... lol.

 

Probably another one hit wonder brought in by that popup that shows up when you're on the forum without being signed in or having an account

 

Same here 2001 F150 5.4L. I bought a remanufactured alternator from O'Reillys 18 months ago. The battery light comes on occasionally but I've never had a problem with charging even in subzero temperatures. I kept the receipt to return it but it seems to work fine - not worth the simple effort to take it back out and bring it in for testing.

 

Check the inline fuse for the alt harness, prob blown.

 

Mike Berkemann?