diagnosing potentially borderline alternator and replacement alternator

2004 Toyota Camry, 4-cylinder
5-6yr old battery replaced summer 2023 with EverStart Maxx from Walmart (not their cheapest battery). Jan 2024 noticed weak start and had Autozone use their diagnostic tool which showed: 

• battery: pass
• starter: pass
• charging system: fail
  • voltage regulation: pass
  • diodes: pass
  • charging current: fail

Battery voltage with car off: 12.45V
Battery voltage with car on, at idle: 13.85V 
Battery voltage with car on, headlights, radio, fan all on: 13.36V

Connection check (voltage drop):
Battery neg terminal to alternator case (car on, lights, radio, fan all on): 0.018v to 0.026v depending on which part of case; seems within spec. As comparison, catalytic converter heat shield to battery neg terminal is 0.016v, so I think ground connection from alternator to battery is good.

Battery pos terminal to alternator B+ post (car on, lights, radio, fan all on): 0.19v
Battery neg terminal to alternator B+ post (car on, lights, radio, fan all on): 13.55v, compared to 13.36v across battery terminals, as expected given the 0.19v drop from B+ post to battery pos terminal. I think 0.19v drop is within spec; I believe spec is 0.2v.

So I think the connection between alternator and battery is ok.

A few days ago, I replaced OEM Denso alternator with a remanufactured Denso. Battery voltage car off 12.6V. Battery voltage, car on at idle is 13.5V (slightly less than with the old alternator). Autozone diagnostic test shows pass. But given that first start the next day seemed weak, I took to another Autozone and their diagnostic test shows fail.

• battery: pass
• starter: pass
• charging system: fail
  • voltage regulation: pass
  • diodes: pass
  • charging current: fail

At idle, under 'max load' (high beams, rear defrost, flashers, radio, AC on high): battery voltage is 12.45V. My understanding is this should be at least 13V.
At 1800-2000 RPM, battery voltage:

• accessories off: 13.6V
• radio, headlights, fan on: 13.5V
• 'max load': 13.4V

Seems like we should be getting at least 14V at 2k RPM, but we are well shy of that.

Serpentine belt tension seems ok. Alternator pulley - no whining or squeaking, and is not slipping.

Battery voltage this morning after sitting overnight, car off: 12.6V. First start of day is not confidence inspiring, but it does start.

Took the original alternator to Autozone for a bench test (since it was out of vehicle, my thinking was this would eliminate any potential confounders), and this was a pass:


The simplest explanation I can see is that both alternators are borderline bad - they may pass or fail a diagnostic test on a given day. But this seems like a low probability outcome - i.e., two Denso alternators with the same behavior - charging, but inadequately. 

I can swap alternators (put the old one back in the car) and have the remanufactured alternator bench tested, but not sure what that would prove at this point. 

Suggestions on how to get a definitive diagnosis on the remanned alternator, or the old alternator for that matter? Or am I possibly barking up the wrong tree and the problem is elsewhere?

Do you have access to a labscope?
 

Chad wrote:

Do you have access to a labscope?

 

I do not.

Some resistance and diode measurements of both alternators (battery disconnected):


Checked battery voltage after car has been sitting several hours (ran an errand in the morning, but sitting since then). 12.70V. Alternator (remanned alt) voltage (car off) is the same - 12.70V.

I don't notice symptoms of low power like flickering lights. Just the weak starts, and of course the observation that battery voltage seems too low when car is running.

So if I'm reading the 'zone battery/starting/charging system test results correctly, the only reason for the failing result is the 'charging current', correct? When the charging system test was performed, was a current clamp connected to one of the battery cables? If yes, did the test results include current measurements?

If no, then their tester could be producing failing results solely because the current measurement wasn't taken. Maybe the person running the tester didn't know any better. Maybe someone lost the clamp. Either way, the failing result may be misleading. Because the alternator seems to be performing well otherwise.

If you want to do your own current testing, you can do it with something as simple as a clamp meter:

www.amazon.com/dp/B0188WD1NE/

Specified max current output varies. Toyota service info indicates 90A. Denso shows one part number at 80A, the other at 100A. Put the clamp around the alternator output wire, turn on all the loads and raise the engine speed to 2,000 RPM. I'd be happy with 80A or more.

As for your weak starts, are you talking about slow cranking speed? If so, that doesn't necessarily indicate an alternator problem. With a healthy battery installed, you're more likely looking at a weak starter (failed motor segments, resistive brushes, ect.) or voltage drop to the starter. 

The Autozone diagnostic tool connects to the battery terminals, that's it. No clamp. They make it idiot-proof so they don't have to do extensive employee training to use.

I have tested with a clamp meter. So for the 2k RPM measurements I posted earlier, I'll add in amperage:
At 1800-2000 RPM, battery voltage:
accessories off: 13.6V, 13.6A
radio, headlights, fan on: 13.5V, 43A
radio, highbeams, fan on: 13.5V, 53A
'max load': 13.4V, -- (was not getting a reading, perhaps because it was over the max measure for DC amperage on this DMM, so I tried various combinations of loads, and highest reading I got was 58A)

Both the original and remanned Denso alts are the 100A version.

Yes, the weak morning starts have me concerned. But you're right, could be something other than alternator. But seems strange that battery voltage is only ~13.5V at ~2k RPM under 'max load' (defrost, high beams, .AC on high, radio, flashers). I was expecting this to be at least 14V - is my assumption in this regard incorrect?

Sounds more like a lazy starter or battery going low over night, possibly due to reduced charge capacity.
Ideally I would scope the battery during this lazy start to see how low the battery voltage is dropping, but you can do the same with a multimeter that displays MIN-MAX Volts.
I don't like to see them dip below 9v while cranking.

This morning, after sitting overnight, battery voltage was 12.61V; alt voltage was the same (B+ to alt case).



Here's a youtube video of battery voltage at the first start of the day. Battery voltage does not drop below 11V. And it actually stays above 14V for a few minutes. What I've seen on subsequent starts is battery voltage can reach 14V, but very quickly drops down to 13.5V. So maybe what's happening is the first start is the hardest (most friction to overcome), and this start depletes battery the most, and so alternator charges the most after this start. And maybe the 'low' battery voltage I see under load at ~2k rpm of ~13.5V is normal because the battery is already near full charge, and so the system logic is to have alternator provide current to run all the loads, but don't bother to charge battery by providing more voltage than 13.5V because battery is already at/near full charge.

So in this scenario, the initial Autozone diagnostic test with charging current failure, was a red herring, and my original alternator is fine (and it did pass the Autozone bench test the other day). And the concern that led me to have Autozone run their diagnostic test, the weak start, is not related to the alternator at all. And given what we are seeing with battery holding charge overnight and not dipping below 11V on cold start (though temps were mild this morning), the 1yr old battery is probably fine as well. So @Noah, to your point, maybe there's something going on with the starter I should further investigate.

Do you guys interpret the various datapoints in this thread the same, or is the lack of 14V+ battery voltage under load at 2k rpm problematic?

The Autozone diagnostic tool connects to the battery terminals, that's it. No clamp. They make it idiot-proof so they don't have to do extensive employee training to use.

Yeah, that's what I was worried about.  Definitely misleading. Maybe not intentionally misleading, but still.

radio, highbeams, fan on: 13.5V, 53A

Interesting. Any more loads you can add in? 53A isn't terrible, it'd just be nice to have that 110% confidence in the alternator with a higher reading.

 Battery voltage does not drop below 11V.

This is more of a red flag than anything else. Suggests that the starter isn't drawing enough current. Or excessive voltage drop.

Do you guys interpret the various datapoints in this thread the same, or is the lack of 14V+ battery voltage under load at 2k rpm problematic?

Not problematic at all, IMO. If this was a customer car (or mine, for that matter), I'd send it. I'd probably leave the Denso alternator on there, just because genuine alternator failure is halfway common on that YMM of Camry.