Good battery, good alternator, still won't charge

I’ve got a 99 F150 5.4L. w/ 188k on it.

It started doing something weird. The truck stopped running off the alternator and was draining the battery till it just died on the highway, necessitating a tow.

I had the alternator checked out (8 months old) and it shows that it is good). Same with the battery.

I’m at a loss as to what to check next. I looked at some fuses, but they don’t look like they are broken. Any ideas?

There are many wires, connections and switches involved with the charging/starting system. If you wish to diagnose and repair it yourself get a wiring diagram and a multi-meter.

It might be a charging fault, or, it might be an electrical short somewhere else. I think you need a mechanic skilled in electrical troubleshooting.

First backyard step is turn the key on and make sure the little red BATT or ALT light on the dash is working. If the bulb burns out the alternator will not charge.

If the light is on, then leave the key in the ON position and touch the tip of a small screwdriver to the front of the metal pulley on the alternator. You should feel a magnetic attraction in the tip.

If the light is on, there is a magnetic attraction at the pulley, then you need to inspect for a burnt fusible link between the battery and alternator.

That’s not the scientific approach but can work.