99 3.9L, 2wd auto:
Battery went dead. When jump started it'll only run when given fuel, doesn't want to idle. Runs fine with a fresh charged battery. This is the second time it's happened within maybe a month in between. The first time an interior light was left on which ran the battery down. Not sure if something similar happened again, but either way my thinking is alternator isn't charging at low speed.

Sound reasonable or should I be looking elsewise?

Thanks,

Mike

 

yes that's usual with a low battery. Only way to tell for sure is check the running volts, as well as the charging amps with a clamp-style ammeter. When battery is low, it will take the alternator's idle output plus some to keep the truck running. I have an underdrive pulley and with headlights on mine draws more than it will charge by a noticeable amount.

 

It wasn't the alternator, it was the voltage regulator.
I took the alternator to a local rebuild shop. They tested it, said it was fine. Apparently the voltage regulator is inside the computer. The shop put an aftermarket, self exciting regulator in the alternator that turned it into a 1-wire alternator. She charges strong now.

For what it's worth, the guy at the shop said those alternators are pretty darn tough and more often than not they see voltage regulator failures on dodges of this era. At 18 years and 118,000 miles they said an alternator failure would have been premature.

I was curious and also found a YouTube video where they were taking an older dodge external regulator and wiring it on to one of our alternators.

I was told if you have a battery light, it will illuminate with this fix however. Mine has a gauge and it just reads voltage, everything else seems normal.

Hope this helps out the next guy...

Mike