Alternator wiring
 

1984 K5 Blazer. Just wondering if anyone can tell me what the 3 wires coming off the alternator actually do? From what I can tell the main lug on the back should be charging the battery (via starter). The #1 connector on the plug goes to the gauge or dummy light (via fuse panel) and the #2 connector goes???
I have6 Alternators from 1978-1986 and am trying to make a test bench for them but using a volt meter, non seem to charge. Any help would be greatly appreciated.

Thanks
Polarboyz