Alternator - 700-800mV on the positive side between alt/battery - Page 1
Forum section: electronics / repair

author: alank2
author_rank: Super Contributor
timestamp: August 07, 2023, 02:28:46 pm
content: 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 I have a newer version of this car, but the same generation, and it runs 200mV for the same measurement (B to batt pos.).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?

author: floobydust
author_rank: Super Contributor
timestamp: August 08, 2023, 02:40:40 am
content: I have this wiring charging system diagram for the 2008 xB. Does this match what you have?I think you are seeing higher than usual resistance between the alt(+) output terminal and the battery(+).Measuring 600-700mV at 40A, do some math and that is around 15-18mΩ or 24-28W of heat which is quite high and enough to melt/burn up a bad connection or blow a fusible link. It doesn't seem to make sense to have such high resistance.The actual alternator setpoint moves around, the ECM changes it. I find the battery does need a good top up charge to 14.4VDC then float at 13.8V as Toyota is chronically undercharging, like many car makers, to get better fuel economy.

author: SMdude
author_rank: Frequent Contributor
timestamp: August 08, 2023, 11:18:21 am
content: Check all of you connections, especially crimped lugs or terminals between the alternator and battery.The offending connection might be a bit discoloured and will also warm up fairly quickly, so you should be able to tell by feeling how warm the connections are. Just do a volt drop test on each joint.If the fusible link has been subject to moisture, corrosion can cause a poor connection here too.

author: alank2
author_rank: Super Contributor
timestamp: August 08, 2023, 02:32:32 pm
content: Thanks SMdude, I'll check that out!