charging problem

You confused me with part of your post

"Voltage drop is caused by resistance in a circuit. All circuits have some resistance, it's just that "usually" such things as wire, connections, and switches have such a tiny amount of resistance, that you almost cannot measure it. This is GOOD Otherwise, you might burn down your house (or even your car)

On a factory wiring harness, the voltage regulator IGN terminal IS not only power for the regulator, but the SENSING terminal, from there to ground

So "ground" must be same as battery NEG (voltage)

And hopefully, the IGN terminal should be the same voltage or as close as we can make it to battery + voltage

DO NOT try to measure resistance (ohms) Use low voltages scale on your meter. If you don't have "auto" ranging post a photo or the brand / model of your multimeter, and we'll help you set it up.

Picture in your mind the path the battery "might" take from NEG post to the regulator mount flange.

From the battery NEG terminal, the battery clamp, the main ground cable, to the engine block.

From the engine block there should be a ground jumper to the body

There might ALSO (or instead) be a ground pigtail from the battery NEG clamp to the body.

ANY of these points (additive) might have some resistance---rust, corrosion, loose.

The grounding right at the voltage regulator to the body can and usually is a culprit.

So what you do is...........

with the engine running "fast idle" make the following test first with all accessories off

and again with lights, heater, etc turned on.

Stab one probe of our meter right into the top of the battery ground post. Stab the remaining probe into the mounting flange of the VR. This is with the meter set on LOW VOLTAGE. You are measuring across all the connections, good and bad, on the "road" from battery NEG to the VR mounting flange.

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The same thing is going on down the "roadway" from the battery positive post, through all the wiring, bulkhead, connections, and back, on the way to the VR IGN terminal.

This is what you are checking following:

Engine stopped, Key in "run"

Put one probe on switched 12V ignition, the other probe on battery POSITIVE.

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In BOTH the above tests, you are hoping for a very very low reading on your multimeter, the lower the better. More than 2--3 tenths of one volt means you have a voltage drop problem

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If these above tests check out, I would RUN THEM AGAIN and read this again to make absolutely certain. Re--check your charging / running voltage.

If the above tests don't show anything, and the charging voltage, WARM, is more than 14.5, Replace the regulator. ( I guess you already have?)

If that doesn't cure the problem, I would try and swap in a known good battery.