Need Electrical Help!

[patmac6075]

I have a '98 4.0l ohv with about 200k on the clock. This truck has always been bullet proof, that is until I let my kid start driving it to school in January. The lights have been left on three times and this last time (about two weeks ago) the battery finally gave up the ghost....if you tried to jump it, it would start and immediately die once you took the cables off....so I went out and got a new battery just thinking the battery was dead. My son drove to school and back a couple of times and one night he calls me saying the car just died while he was driving, again it wouldn't take a jump....I brought the battery home and charged it and the car started no problem...took the alternator out and brought it to the parts store and had it tested...failed....said the Alt is toast. Brought the alt to a local rebuild shop...they said the rectifier was shot and the brushes were completely worn, so I had it rebuilt....charged the battery all day (at 2a), installed the new alt., new tensioner, idler pulley, and belt. Now the car starts just fine, but the dash battery light is lit and won't go out.....
I'm a "parts hanger", not a diagnoser, and to compound things I'm not very savvy when it comes to electronics....need some help.
What's wrong with my truck?

 

[SoHK_alumni]

Think I'd pull the alt, drag it back to the parts house for a test.

Seems like the rebuild missed the mark and an independent test is probably called for.

 

[jimmyznutz]

^^+1. If the voltage regulator is faulty your battery light will illuminate.

 

[patmac6075]

Ok, I found the issue.
So I pulled the alternator this morning and brought it to the parts store to have it bench tested....passed. So I reinstalled it, but the dash light was still on...went back to the parts store had it tested on the vehicle...failed "bad regulator"?!
Got out the continuity light and started poking around. Turns out there is a 175a fuse on the side of the power distribution box....I had power going in, but not coming out. Apparently this fuse had been blown (perhaps during one times it was jumped....my son admitted that "maybe" someone "might" have put the cables on backwards).
Changed that fuse....dash light out...alternator shows it's charging.
Case closed!

 

[Flash]

my son admitted that "maybe" someone "might" have put the cables on backwards).

You'd really like to think that by the time they get old enough to drive 2 tons of your property around on a public road that they could be honest with you about these things.

Congrats on finding the solution.