Ok got in this morning and see the battery light on and ScanGauge reads 11.9v
I hadn't had any charging issues previously.

Had to go somewhere so I swung into parts store to test the battery and alternator. Parts store test said battery was bad but he also said he couldn't test the alternator.

I pulled the battery to see if they could charge it and they said their charger says bad battery.

I didn't want to buy their crappy battery so I put it back in and drive over to Costco. I noticed my volts dropping and by the time I got into the Costco parking lot, it was at 8.9 and the dash clock light looked dim.

I would have thought if the alternator was good, the clock light would not be dim, but at the same time, I see that my dash tach dropped to zero even though the truck was still running fine as seen in the picture.

Now I don't know if it's the alt or battery. Thoughts?

Engine is running in the pic below despite what appears to be zero RPMs.

If the battery is testing bad, then buy a battery. Once you have a new one see what the voltage is putting out.

Was the dash battery light illuminated anytime before today with the 4runner running?

FYI:
Alternator & Charging System Checks (Alternator Testing)

Both could be bad. You should not be under 11.9v. Sounds like a cell has died. Start with the battery like stated and move on from there.

could be both, but, if you cant test the alt, put in a new battery then see what the voltage is

No it has never come on other than normally at start up. I also monitor my voltage closely and it never strays too far from ~13.7v.

Alternator I replaced about a year and a half ago. Parts store crap cause I had no choice at the time.

Battery is three and half year old POS Duralast.

Going to head into Costco and grab a new battery. It's due anyways and the alternator should be under warranty. If the battery doesn't cure my ills, I'll pull the alternator.

I should also add that I already checked the connections and ground and found nothing loose or unusual. So I can rule that out too.

If their test says the battery is bad, it probably is. But that doesn't mean the alternator is good.

Start the truck. Put a voltmeter between the charge stud on the alternator and the negative battery post. Anything under a high 13v is trouble, but if you're reading near 14v the alternator is testing good.

Like an idiot, I left my meter on my work bench instead of in my truck.

Well just threw a new interstate battery in. 13.8v reading now on the ScanGauge but it has dropped to 13.1 — 13.4 at idle so I'm not sure if I'm just reading new battery voltage or not.

When I return home I'll check the alt with a meter.

How about the voltage with AC on full and headlights on? It should be about 13.5V or above at idle. And it should rise to around 14 V with 1000 or so RPM.

I always check the alternator with full load (headlights, AC full, fog lights on)

Alternator just tested at 12.3 volts so I'll be replacing it today. I knew something was up beside the battery since my dash light was dimmed out even though the truck was running still.

Now my 3 1/2 year old battery may well have been failing too and the shitty Auto Zone alternator just finished it off — so I'm still happy I bought a new one (can't beat $91 out the door for a new Interstate 24f at Costco).

Since the alternator is under warranty for a free POS replacement, I'll throw another one in and start looking for a decent remanufacted OEM alternator at my leisure. I figure I'm probably good for at least 6 months with a new Auto Zone alternator.

I don't know if they will do it, but I'm going to ask them to test the new alternator at the store before I even leave with it. Don't want to be playing games with the parts store all day.

Check to make sure there are no drops across the wires. but being in cali I doubt you have rust issues, you said wires looked fine.

I suppose poor alternator could wear you battery prematurely

Just remember it could have left you stranded somewhere. Get a good alt in that beast!!

I have noticed when I run a good OEM alt I see 14-14.5v charging. Non oem I have seen 13-13.8v.

I have had a bad starter kill a three year old battery. It happens and can hit the wallet hard when it is a one two financial punch.

As @JayRolla stated, you should see 14~14.4 volts at startup. If not, as well as chalking it up to the POS AZ alternator, check all the wiring and connections along with belt tension.

I asked the store to test the new one and they did, but only said it works fine. So who knows.

I'll be curious to see what the new one puts out. First though, i have to figure out a way to get this piece of shit Auto Zone alternator back into the truck. Already the tolerances seem slightly off since the alternator doesnt want to slide back into the top hinge bracket. Seems to wide by a hair. Last one i put in slid right in and back out just fine.

Not sure if i want to take this one back too or try and figure a way to force it in...

Anyone know of any tricks to getting a tight one in?