Naaaw... l had limited time and saw the bad ground so l replaced it. I didn't expect it to fix the headlight weirdness, but it couldn't hurt and it took less time than removing the alternator and taking it to get checked... probably do that next weekend.

 

Is the truck driveable like this? If so, don't remove the alternator. Drive the truck to the parts store and let them test the complete charging system as installed. No point in making more work for yourself.

 

Yep, it's drivable. I've never had an alternator tested while it's on the vehicle... l always just took it in.

I'll give it a shot.

 

BUMP UPDATE:

Got the battery retested and it was junk... apparently the first guy was wrong.

I bought a new battery and, as far as l can tell, the issue has ceased to be an issue. I haven't driven it at night w the lights on, but the tattletale emissions light now glows steadily at all rpm.
Hot Damn!

To keep me in check, my power steering has decided to start whining. I've always been (falsely) proud of my very quiet PS pump, a rarity on Ford trucks. Mine have pretty much always been noisy.

Time to start another thread, l guess...

 

Fantastic to hear you got to the root of the problem. Were you given a printout of test results either time? I'm curious what they found (or didn't find). One oft-overlooked function of the battery is to act as a big honkin' capacitor to smooth out the normal AC spikes from the alternator. This ability degrades over time, but the battery often succumbs to other problems before this is noticeable.

Do you remember what type of tester was used? Most places nowadays use a capacitive tester. The older style is a resistive tester. Each style has their pros and cons. One type can catch problems the other will miss, and visa versa. The resistive style has fallen out of favor to some extent, but I like to use both for a thorough test.

A new-fangled capacitive tester below. Note the sealed case, small leads, and digital menus. Commercial models often have a printer:




Here's an old-school resistive tester, aka toaster. Inside the vented case is a big heating element, to put a heavy load on the battery. You hold the switch for X number of seconds and read the needle at the end. There are digital versions out there, but they are pretty basic with no menus, and you still hold the switch for a certain amount of time. Note the big fat cables and clamps: