Battery light on, no codes

2012 4.0 manual. Battery light came on last night for the first time in the truck's life. Engine is making a whining noise that most people wouldn't notice. Multimeter shows 14.4ish volts at the battery with the truck running. Alternator was my guess, but multimeter says otherwise. Battery is Northstar AGM 24F, maybe 3 or 4 years old. I keep it maintained pretty regularly - put it on the charger overnight once a month, more when ive been winching a lot.

Any ideas?
Thanks!!

 

Unplug the alternator and see if the whining stops, if it does then the alternator is failing.
https://www.tacomaworld.com/threads/2nd-gen-information-diagnostics.784008/#post-28042155

 

Thanks for all the work putting that together!
I'm planning to buy an alternator to have it on hand when testing, since its the most likely culprit. Should I just get the Denso OEM replacement? Pretty sure I have the 130 amp, as I live in Montana and bought the truck new here.

Forgot to mention in the first post that I have the diode, or whatever it is, to increase charging voltage. Got the one from Australia. Its been in for several years.

I also have a Redarc 25 amp DC to DC charger for my Northstar AGM 27f house battery in the cab , which I use for a 16qt fridge and other little things. It used to be my starter battery.

 

First choice would be OEM if you can afford it

Second choice would be a Denso remanufactured

 

Lots of good choices for upgraded alternators, especially with you running dual batteries

 

From personal experience try remanufactured from the dealer. Will cost a little more but more than likely it will work correctly. If you can get a remanufacted Denso, ok, avoid Rock Auto though.
Good luck

 

Towing Package has the bigger alternator, see what size the main ALT fuse is. If it's 120A you have the smaller alternator, if it's 140A you have the larger one.

 

Rain/ snow mix out there today, so not ready to crawl under it in the driveway. A local Carquest is going out of business, so I ran up and grabbed a few spares - 100 amp alternator $10+ core charge, water pump $10, 2 front hub assemblies $20, and a whole lot more. Battery is on the charger for now and ill crawl under the old girl when I find a dry spot to do it in a day or two.

I try to use OEM parts whenever I'm not doing a true upgrade (bearings, sensors, brakes,etc) so most of what I bought today will be in the spares box.

 

Woke up this morning to winter. Found a warm garage and stuffed in my $10 alternator ($50 with core charge). All issues resolved. I'm not turning in the core, I'll use this as an opportunity to learn how to test and rebuild an alternator, and have an OEM unit as my spare.

Thanks for the help, everyone!