Ignition electrical issue. help?
You should check at a good fast idle, or simulated "low cruise."
EG if the engine is idling slow, the alternator is "just starting to" put out
So if say, the field is grounded, causing "full charge," and the battery is a bit down, you could misinterpret an idle reading for "normal."
But if you speed it up and the voltage climbs up towards 15, you know you have a problem in the field / regulator
The "usual suspects"
Voltage drop in the ignition harness feeding the regulator. That is, low voltage TO the regulator causes OVER voltage at the alternator output
Voltage drop in the ground circuit, IE VR not grounded to battery NEG
Sometimes, battery problems
The mentioned guess, IE grounded green brush causes "full charge" at the alternator
Bad VR, but actually, this is fairly rare.