Low Voltage / Voltage Regulator issue

Have a low voltage (12-ish max) problem, but cannon can't seem to find the source.

99 XJ
4.0
AW4
233,686 miles (~12K on the reman engine)
Basically stock, with a baby lift and 31s. And it's my daily/only.

Last week on the way home the voltage dropped to 12 or so for a couple minutes, then to basically nothing. Couldn't really tell if it was reading 9 or just pegged as low as the gauge went. I got a "check gauges" light as soon as that happened. Limped it to an O'Reilly's a few blocks away, hoping it was a battery and nothing more serious. Battery checked out fine, alternator checked out fine, voltage regulator failed the test. Learned that the voltage regulator is part of the PCM. Thought some unkind thoughts about the Chrysler Corporation for a few min.

Had to get towed home after not getting much farther down the road. I spent last weekend checking every ground, cable, and connection I could find, hoping it was not the PCM. I charged the battery, started it up, and while it never fell to 9, the thing still never made it past 12.16 volts while running. Nothing changed, even with replacing all the battery cables (Big 7 Upgrade, which I had already but hadn't installed yet), so I ordered a pre flashed reman PCM. It came yesterday, I installed it this morning, Jeep started right up, and nothing has changed. Still only reading 12.16V while running.

Chance there's just the same problem with the reman PCM, but surely it's not THAT likely... Right?

Could a faulty alternator pass just enough to fool the tester? Because I'm tired of this, and tired of Dallas Area Rapid Transit. DART isn't known for being the most reliable system around, but it's the only way I have to get to work without my Jeep, and I'm "essential" apparently...

TIA!

-alopeks

How did you check the alternator? The other primary cause is the field circuit wiring.

Just the standard handheld battery/alternator check tool at O'Reilly's. Which is part of why I'm not sure how accurate the results were...

Where would I find the field circuit wiring? I can't seem to get a conclusive answer, unless this is also the fusible link, in which case I've replaced that with a separate fuse as part of the cable upgrade.

-alopeks

Both PCMs are likely fine. If you have a volt meter, measure the voltage across the field terminal on the back of the alternator. You should be able to backprobe at the 2-wire connector by the alternator and see voltage across the wires if the PCM commanding the alternator to charge. You can disconnect the connector and apply 12-volts to the alternator field coil to kick the alternator into full charging as well (which in theory Oreilly's did if they claimed the alternator was good).

From the FSM:
Check the fusible Link between generator and PDC
Dark green with orange stripe from the PCM = Generator Driver
Dark green from the PCM = Gen Field Driver (+)

Had a hard time getting any consistent readings from the alternator last night, but decided I should at least try replacing it since I was running out of suspects. Wouldn't you know, that seems to have been the problem. Swapped in a reman from O'Reilly's, and even with all accessories, headlights, and A/C, it's still pulling 13.9V at idle. Unless y'all all agree that's still not good enough, then I think I'm done messing with this for a while. And I'm sending back the PCM. Probably need the cash before I'll need the spare...

-alopeks

Glad you figured it out. Not sure why O'Reilly said the alt was good.

Handheld alternator/battery testers are not terribly accurate. Alternators and batteries should be tested under load inside a test machine.

Simple rule of thumb, low voltage is typically a bad alternator, high voltage is typically a wiring/connection issue, PCM failure is very highly unlikely. Alternator output should be about 13.8 -14.4 volts.