2015 4Runner, 95K miles, that is on its 4th alternator since May 2023. Alternator 2 and 3 were O'Reilly remans that went bad within a few weeks. #4 is a Toyota reman from the dealership. I've installed them all myself, so it could be me.

Only thing besides stock on the electrical system is a Renogy 20A DC-DC charger to charge my trailer battery that I've used 4 times. Last time was 2 weeks ago, now I'm getting intermittent no starts and AutoZone test says it's the charging system. Charger stays unhooked until I use it.

What else can I check? Parasitic draw?

No misalignment issues on the belt, I replaced it with alternator #3. Am I just getting bad luck with the remans? They are the only option I have since it's 10 years old.

You need to see what the actual charging performance is with a load tester

basic load tester

That many alternators in 2 years is a lot. Not sure if the problem is a drain or a charging issue.

I would also recommend staying with non agm batteries as they have been known to cause multiple issues on 4 runners and tundras.

Thanks Fixer. I ordered that load tester. I consider myself pretty well versed in automotive electronics, but this has me stumped.

Any correlation between using the DC charger and the alternators failing soon after?

Also is the charger hooked up to a solar panel?

As far as I can tell there's no correlation with the DC charger and alternator failures. I load tested the alternator last night and it dropped to 12.1 volts, way below what it needs to be. Parasitic draw was 180 mA, which is above the recommendation of ~50 mA, but not outrageous.

So the alternator is definitely bad, but I can't figure out why they keep going. Bad luck or ? I did install an Accessorides remote start before I started having these issues. But I have to remove it because it was't working properly. I've got a note in to them to see if it permanently changes the ECM.

What was the charging voltage with lights, a/c, and stereo on?

Fixer, it was 13.3V. So not great again...

How old is the battery and how many cca ?

13.3 is fine fully loaded.

Like jp76 asked… I'd start looking at the battery.

Might measure voltage drop directly off the alternator then again at the battery. Have seen cases where corrosion runs up inside a wire but it looks fine on the outside. Can rule that out before you get too deep.