OK an alternator output is always connected to the battery.........the main output stud.
How it works........
The ignition switch sends power to the VR off the same circuit that powers ignition. This IS ALSO THE SENSING wire that the VR uses to "figure out" what to do.
The relay in the mechanical VR or the circuitry in the electronic one "shall we say" MODULATES the current going to the field connection. More field current= more output. As the VR senses the battery is low, it sends more field current to the alternator........higher output. As the battery "comes up" the voltage at the VR ignition terminal goes up and it "cuts back" and SHOULD normalize at nominal 14V
By jumpering the wires, you are simulating "full field" "completely dead battery" so to speak.............Again, watch your meter and don't allow RPM to bring the voltage up past 16
Your statement that the alternator will put out 15V does not mean much. An alternator that is DISCONNECTED from a load and that is spun up, with full field current can put out OVER A HUNDRED VOLTS!! (This is however hard on the alternator and diodes) IN FACT back "in the seventies" there use to be silly switchboxes sold that allegedly allowed you to get 110V (DC) out of your alternator. What this switchbox did was to.............disconnect the alternator from the VR and full field it............just like the test...........and also disconnected the alternator from the vehicle system........and connect it to a common outlet box!!!
The point? ALTERNATOR OUTPUT voltage depends on the FIELD CURRENT and the LOAD THAT IS ON the alternator output. If you have a battery that is fully charged, and no loads in the car (no fans, no big stereo, no lights etc) and you "full field" the thing, the voltage can go WAY over normal, up over 20 -25V until it damages something.
