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The typical voltage is 14.4v, with the amperage being whatever it needs to maintain that... 8 or so amps just to run the motor, then add whatever else is on.
A normal internal-regulator alternator has 3 connections (plus ground)... the main output wire, the dash light wire, and the voltage sense wire. Current through the rotor's field coil generates the output. The regulator's job is to ground out the low end of the field coil just enough that the sense line is at 14.4v. The dash light provides a small amount of current to the regulator and field coil to bootstrap it.
Since the main output will keep rising until the sense lead reaches 14.4, adding resistance in the main line will increase the load, at least until the voltage maxes out. Just try it... you'll probably get 18+v at the output lug. Even if you cut the output lead, you're not at minimum load, as the field would be at full current. Cutting the field wire would fix that, but then you'd have to deal with the inductive spike from the field coil.
The sense lead is the input to the control loop... if you want to control it, that's the one to tap into. Higher voltage means lower output. So if you put a voltage source in between the output and the sense line, it would reduce the output by that much. Current would probably be less than 100mA, but I haven't tested it. In any case, adding in extra voltage is a lot harder than taking it away, even at low current.
That's what makes the Honda alternator nice: the extra input is positive-going, so lower voltages lower the output.
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