Overcharging
The regulator controls current flowing through the green wire to ground. When the rotor needs to generate a stronger magnetic field, internally a direct connection to ground is made. In other, a switch is closed.
The voltage at I determines whether the current flows through resistance, direct, or not at all.
If the voltage I is 13.0 Volts, then it will allow current through even though you are measuring 15.5 Volts somewhere else.
If you are measuring 15.5 Volts at the Alternator output stud, and 13.0 Volts at a connection on the blue wires (say the ballast resistor or the bulkhead), that shows there is resistance to the current flowing between those two points.
If you then take the positive probe and place it on the output stud, and the other on the ballast resistor, it will show the 2.5 volt drop assuming its all in the feed. if there is resistance is in the ground, then it will be less)
To add to what 67Dart273 wrote, or to say it another way.
The regulator controls current flowing through the green wire to ground. When the rotor needs to generate a stronger magnetic field, internally a direct connection to ground is made. In other, a switch is closed.
The voltage at I determines whether the current flows through resistance, direct, or not at all.
If the voltage I is 13.0 Volts, then it will allow current through even though you are measuring 15.5 Volts somewhere else.
If you are measuring 15.5 Volts at the Alternator output stud, and 13.0 Volts at a connection on the blue wires (say the ballast resistor or the bulkhead), that shows there is resistance to the current flowing between those two points.
If you then take the positive probe and place it on the output stud, and the other on the ballast resistor, it will show the 2.5 volt drop assuming its all in the feed. if there is resistance is in the ground, then it will be less)