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Did the results of a load test on the old battery indicate that it was likely contributing to your perceived voltage fluctuations and due for replacement?
The load test did indicate that the battery was due for replacement, as we could tell from the low voltage. Unfortunately, the new battery had no effect on the voltage fluctuations.

Old battery/new battery:
 

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After driving for a few days to get the battery fully charged, would you mind repeating the engine-off and 2K RPM voltage measurements?
 

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Sure.

Was going to go ahead and order a replacement Denso under warranty, but they are out of Densos. Anybody heard of/tried a TYC alternator? https://www.rockauto.com/en/moreinfo.php?pk=10369512&cc=1503708&jsn=419
Let me make the same measurements on my low-mileage '14 and see if your voltage fluctuations are present.

If you are fixated on replacing the alternator again, consider a Denso alternator rebuilt by Denso; see densoproducts.com

TYC/Genera is Taiwanese
 

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Let me make the same measurements on my low-mileage '14 and see if your voltage fluctuations are present.

If you are fixated on replacing the alternator again, consider a Denso alternator rebuilt by Denso; see densoproducts.com
To clarify, I did not have voltage fluctuations with my original alternator. I replaced it because voltage at idle was too low. The fluctuations started at some point after I installed the reman Denso.
In retrospect, I should have just replaced the brushes.
 

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After driving for a few days to get the battery fully charged, would you mind repeating the engine-off and 2K RPM voltage measurements?
Here are my numbers:

1a. Battery voltage after the car had sat for 18 hours: 12.44v
1b. Battery voltage immediately after a 10 min drive: 12.77v
1c. Battery voltage after 1b and having had headlights on for 1 min: 12.34v

2. Battery voltage @ 2000 rpm: 13.80v

Battery is a 3-year-old wet-cell.
Sorry, took a little longer.

With a new AGM battery (no voltage booster):

1a. Battery voltage after 4 days rest: 12.69v
1b. Battery voltage immediately after a 10 min drive: 12.92v
1c. Battery voltage after 1b and having had headlights on for 1 min: 12.69v

2. Battery voltage @ 2000 rpm: 13.80v
 

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No. From motor to body. The alternator is where your source ground comes from when the vehicle is running and the motor is isolated from ground through the motor mounts.
 

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If you're still getting momentary voltage drop it has to come from your connection from alt to battery then to front fuse box. I would check the resistance on that cable metering from alternator to fuse box it should be reading near 0 ohm.
 

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If you're still getting momentary voltage drop it has to come from your connection from alt to battery then to front fuse box. I would check the resistance on that cable metering from alternator to fuse box it should be reading near 0 ohm.
Resistance between alternator (+) post to battery (+) post: 4.3 ohms (engine off).
Voltage drop from battery (+) post to battery clamp: 0.0 mv (engine idling).
Voltage drop from alternator (+) post to alternator clamp: 0.0 mv (engine idling).
Voltage drop from alternator (+) post to battery (+) post: fluctuating between ~9 mv and 56 mv (engine idling).
Bad alternator cable?
 

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So the resistance measurement between alternator and battery I got last time was with a hot engine. Measured it again with cold engine: 0.0 ohms. Same between battery and fuse box.
Started the engine, voltage drop from alternator (+) post to battery (+) post was stable at around 45 mv. As the engine warmed up, the voltage drop slowly crept down until about 5 minutes later when it started fluctuating as before.
Voltage drop between the battery (+) post and the fuse box post was fluctuating between ~10 mv and 30 mv, after the engine warmed up.
The voltage at battery posts was fluctuating between 13.46 v and 13.66 v.
After turning the engine off, the resistance between alternator (+) post and battery (+) post was still 0.0 ohms.
 

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You’ve got to be very careful about making a “DC resistance” measurement on a live circuit.

Any small current (even milliamperes-level) can grossly distort the resistance measurement. Road dust, carbon powder from brushes, diode leakage current, etc. can all create a small parasitic current drain through the alternator, even with the engine off. With the engine running, changes in charging current, AC ripple superimposed on the alternator output, etc. will give wildly inaccurate "resistance" measurements.

Next time you make the alternator-to-battery resistance measurement, disconnect the battery negative clamp so there is no possibility of any low-level current distorting the measurement.

Actually, making voltage drop measurements is a more sensitive and repeatable method of determining resistance, unless you have specialized test equipment.
 

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Voltage readings would be hard to conclude unless you can read it while the anomaly occurs. I personally have multiple meters running in my vehicle due to a large stereo system. With all the time you've already spent on this, I would suggest hooking up a volt meter to your battery with display in cab and another meter running off of your cigarette lighter. My guess is when your lights dim, the cigarette voltage would drop and the battery would be stable. If thats the case, bad cable connections.
 

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Voltage readings would be hard to conclude unless you can read it while the anomaly occurs. I personally have multiple meters running in my vehicle due to a large stereo system. With all the time you've already spent on this, I would suggest hooking up a volt meter to your battery with display in cab and another meter running off of your cigarette lighter. My guess is when your lights dim, the cigarette voltage would drop and the battery would be stable. If thats the case, bad cable connections.
The voltage fluctuations are present most of the time, and can be measured at the battery terminals. I haven't noticed any lights dimming.
 
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