I assume that your vehicle is an early 997...say 2005-06? They had issues with the starter-alternator harness crimps causing excess voltage drop.
Fully loaded and idling: the voltage at your battery can't be more than .5v lower than the voltage coming out of the alternator. I use a curved pick tool and an alligator clamp to reach behind the alternator and contact the terminal with the engine running WARNING: NO ROOM FOR ERRORS here! The belt will eat you leads or sleeves, and probing for 30 power Where you can't see leaves room for arcing to ground (causing you to jump and contact belt).
99/100 early 997's have excessive drop here. Also, if any work has been done on the tranny/clutch there is a power connection on the transmission that can be loose. Good luck! On the otherhand, slot of dealers are goodwilling this repair if yer nice
