I've got a Scion 2008 xB with 110K miles on it and the original alternator died. I replaced it with a new one by a company called Remy and the voltage fluctuation is a bit larger than I would expect (typically 13.0 to 13.8, but drops to 12.9/12.7). It doesn't matter if I am running a lot of accessories or not. If I measure the current on the heavy wire "B" lead coming off the alternator it is about 10A just engine running and about 40A with the lights/ac/interior fan full blast. I measured the alternator body to the ground connection on the battery and there was a voltage difference of 50mV which seems very reasonable, but the positive side of the battery to the alternator B lead is 600-700mV which seems like a lot. I have a newer version of this car, but the same generation, and it runs 200mV for the same measurement (B to batt pos.). I can see that the alternator does try to stabilize the voltage, if it drops down to 12.9 or 12.7, it will immediately come up to 13.5 on the next measurement (from a cigarette lighter plugin voltmeter). I looked at the schematic for the positive side and it goes through a wiring harness to a fusible link which is a 1" by 5" block with multiple fuses built into it. It looks like a pain to remove from the exterior fuse box, but essentially it has tapped holes that terminals screw into. I'm not sure what the fusible link is made of, but there is a clear window at the top that is presumably to see if any of the fuses are broken perhaps? Any thoughts on this? Could the fusible material just be terrible when more current is coming from the alternator which changes it resistance enough for the voltage to bounce around?