"What you want" at the regulator IGN terminal (which includes the ground) is to be as exactly the same as 'at the battery' as you can get, within about 3/10 (.3) of one volt
You can either carefully measure battery voltage, carefully measure regulator voltage, and subtract the two, or "do it the easy way.........................
Turn the key to run, engine off. Set your meter up for low voltage, the lowest scale on the meter Stab one probe of your meter directly into the top of the pos. battery post.
Hook the other probe to the VR IGN terminal. "Rig" this so that everything, ignition, VR, etc, is still all hooked up
With the meter hooked this way you are directly measuring voltage drop in the hot side of the harness. You want a reading LESS than .3V
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To check the ground, run the car, fast idle, battery "up" and normal. Run this test twice, once with extra loads shut off, and again with heater, headlights, etc on
Stab one probe into the top of the battery NEG post. Stab the other directly onto the VR mounting flange. Once again, the lower the reading, the better, Zero is perfect, over .2--.3V means you have a grounding problem.
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"WHY" this is so. This power not only powers the VR and the field circuit of the charging system, it is the SENSING line which "tells" the VR what the system voltage is doing, so it can regulate. The VR causes the charging system to regulate "to it's setpoint" based on this sensing voltage.
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"What you'd like" at the battery. With system warm, battery "up" and normal, and running fast idle / low cruise, you should see no less than 13.5, no more than 14.5. "Optimum" is 13.8--14.2
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WHY it overcharges. Depending on the year, there can be "lots of stuff" on the "ignition run" line, smog goodies, electric choke, ignition, the VR and alternator field. All this draws current, and if the bulkhead connector or the connector on the ignition switch, OR the switch itself has a bad connection, this voltage will DROP
The VR senses this and says "HEY" (no it doesnt) "My voltage is LOW, let's JACK IT UP. So the VR jacks up charging voltage until the "low" voltage at the sense line "comes up" to 14V or whatever.
BUT there's VOLTAGE DROP in the ignition switch. This means system voltage is NOT 14, because in order to get that "low" point in the system up to 14, the REAL system, at the battery, BEFORE YOU GOT TO THE VOLTAGE DROP is now too high, maybe 15 or whatever.
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The exact same thing happens in the GROUND circuit. If the ground is "not the same" as the battery, the sensing is wrong. Same deal as above.
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OTHER problems. Sometimes the VR IS wrong. You can have 'zero' voltage drop and the charge voltage is wrong. Replace the VR
Sometimes the battery can develop interesting problems. Rare, but it does happen. Many times, a bad battery won't crank, so the problem is "self solving".....you replace the battery, and the problem is solved LOL
