Ok so the other day on my way home I started losing power. All my gauges quit then the windows wouldn't go down and about 6 miles later truck died... I towed it home and took the alternator off and took it to the parts store to have it tested, and it failed. I got the new one got it home and installed at still won't charge. I've now when through 3 alternators hoping I was getting bad ones. ( the one on there now currently tested fine before I left the store) I've got 12v on the pig tail going into the alternatorwith the key on and .02 coming out of the alternator on the terminal while running.. I'm at a loss.. help!!! 1998 f150 lariat 5.4

 

Should be a fusible link on the alternator. I don't recall if yours is between the solenoid and alternator or not. But you're looking for a fusible link that is fried.

 

The original alternator likely was faulty but has caused an external issue you have not caught up with.
The field fuse. If blown no alternator will charge.
This is what happen when part are replaced.
The second alternator not charging should have been a signal to look elsewhere for additional issue.
And knowing how the system works is very helpful.
If the charge lamp is on, it tells you the regulator is putting out a ground to light that lamp as it should.

Not going off when motor is started tells you the alternator is not outputting.
The only issues can be a blown field fuse.
The plugup at the alternator is faulty.
The regulator is faulty.
The alternator is faulty.
Or on rare occasions a ground issue.
If you see battery voltage at the alternator on the heavy lead the big fuse and lead is good.
There is no more to this simple circuit.
Good luck.

 

Did you recharge the dead battery first before starting up the engine with the new alternator? If not, you likely blew out the replacement alternator.

 

Yes actually I bought a new battery the last time I went to switch alternators

 

Did you CHARGE the new battery first?

 

Yes it's fully charged