At idle your battery should be reading no less than 13.5 volts if you want to consider the alternator borderline, even. With the car off, and the surface charge removed a fully charged battery should register around 12.6 volts. A proper charging alternator will charge at about 14.4 volts.
The G3 alternator has a larger pulley than the stock alternator, which creates a sort of underdrive effect right there. The 1993 Cobra had an underdrive crank pulley from the factory, and it charged just fine. It could be a wiring problem, but more than likely, just a crappy regulator or alternator. The 130amp well more than doubles output at idle.
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