And the other thing is that, buried in some engineering books somewhere, are the RPM/ vs output for various alternators. If you drag race, or have a swapped system and therefore have mismatched pulleys, you may have slowed down the alternator to the point that it is below decent output RPM. "In general" most alternators do not put out full output until somewhere around 5-6K alternator shaft RPM. So figure your pulley ratios (just measure crank and alternator pulley diameter, it's a ratio) and estimate at what RPM the thing starts to work OK. If that estimate puts the alternator at 5K then you for sure have a problem.......too small, partially not working (1 or 2 bad diodes, bad stator, etc) and of course don't discount BELT SLIPPAGE as a possibility
