Alternator issues after AUX battery fix / N3 fuse replacement

[FlatRonnie]

Need some help.... 2021 Mojave 56k miles

The other day I got the dreaded AUX battery failure. I was driving home from work and all of a sudden my car died. I was able to jump it but it went into limp mode and eventually came to a stop. I did the auxiliary bypass (pulling fuse 42/disconnecting and taping up the AUX Neg cable), but still had no power not even hazards were working. So I ended up replacing the Z fuse block array in the picture( the N3 fuse tested bad), and replaced the Main Battery. Everything is now working.

However I stopped by AutoZone just to make sure the old battery was really bad, which it was. My Alternator and Starter both tested bad

1) Alternator Voltage regulation: Fail
2) Starter RPM: Fail

Now the voltage on the dash was showing around 12.7v and voltage meter directly to battery while running showed the same. From what I understand this may be normal for a good battery and a smart charging system. I did take it for a test drive and and when I release the gas pedal and accelerate it does show the voltage jumped to 13.8v. I was just testing it on side roads. I wasn't able to really get up to speed if that matters.

So any ideas what's happening? Could this be connected to the whole start/top auxiliary battery issue? Or is it just a coincidence? Do I really need to replace my alternator and starter also?

This was posted in another group also. I just need all the great minds on this LOL!
Thank you!

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[Mr._Bill]

When the batteries are fully charged, it is normal to see a system voltage around 12.7 or 12.8 volts. The alternator is not a constant output, it is regulated to meet demand.

 

[swtrailboss]

Autozone tests on starters and alternators are performed by the young guys ( who usually got the D- in shop class ) or older gents who are in semi retirement (most the time??) and either are solely reliant on a piece of test equipment with a software for “Testing Upsell” . I would roll with your battery replacements only( both hope you did at the same time) and reset any DTC codes existing. Make sure both batteries have been put on a charger and fully charged before installation. Try his will give you your answer if anything else needs attention but likely not.

 

[FlatRonnie]

Autozone tests on starters and alternators are performed by the young guys ( who usually got the D- in shop class ) or older gents who are in semi retirement (most the time??) and either are solely reliant on a piece of test equipment with a software for “Testing Upsell” . I would roll with your battery replacements only( both hope you did at the same time) and reset any DTC codes existing. Make sure both batteries have been put on a charger and fully charged before installation. Try his will give you your answer if anything else needs attention but likely not.

Yeah what's interesting is that in the heat of all this testing, I forgot to turn off my trail lights and auxiliary lights. I ended up leaving them on for a couple hours with the engine off. When I went to start the car the voltage immediately jumped to 14V. It stayed there on my drive to work and then Zoe started to back down to 12.8V after a while. When I first installed the battery it would hover the 12.7V. So I guess that means the alternator is working, it's just doing it's smart charging thing?

I only replaced the main battery but I hopefully successfully bypassed the auxiliary battery

 

[Pancakes-n-eggs]

I’d put everything back the way it came from the factory (fuses, batteries, etc) and clear the faults to isolate the new problem. God bless you.

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