The drop causing this has got to be more than .3V somehow. The drop matches the amount of overvoltage. Another way to check is to check what v the VR is regulating at. To do that measure between the VR mounting ear and the VR IGN terminal "as close as you can get." (This is with engine running fast idle and battery normalized, engine bay/ engine/ battery warm. Once again, this is usually at the ballast. If it measures say, 13.8 at the ballast, and the battery is running 14.8 then you have a 1v drop in the harness and ground system combined
HOW DOES this work?
The VR only "knows" one thing and that is to provide field current up or down until system regulation point is "met." So the VR ramps up field current until it's IGN terminal (also the sensing point on these VR's) --until that point is at the VR setpoint. This is nominally 13.8--14.2 with VR warm.
But what is HAPPENING (assuming the VR is regulating correctly) the VR, because of the drop, must ramp system voltage AT THE BATTERY higher so that as the "drop" happens through the system, harness, and to the VR IGN terminal, the battery must be higher to compensate for the drop
BEAR IN MIND that in a common drop condition where the drop is "let's say" in the bulkhead terminal or ignition switch/ connector in the "IGN1/ run" line. there are various loads on that line. Depending on year, 70/ later they are
ignition system, whatever that might be
VR operating circuits
Field current through the alternator field
Electric choke if used
Various smog devices if used
Not all these loads are drawing the same current with engine on or off or various RPM, and if you disconnect anything during the troubleshoot, the current and drop through that line will change / lesson
The easy way out is a relay
