On the 10si type GM alternators, one the two smaller connections are not quite as described above...
The 3rd wire is not field power, it's the "sense" line -- the alternator measures the battery voltage with it. This compensates for voltage drop in the fat charging wire; the alt will raise it's output as necessary such that the voltage at the sense terminal is the set point voltage (14V nominal, varies with temperature I think).
The alternator would work fine if you just jumped it to the big terminal, but if you had voltage drop in the wire(s) between the alt and the battery, the battery wouldn't get full charge.
The sense wire should be as close to the battery "+" terminal as possible, electrically-speaking.
