TyF Posted December 10, 2022 Share Posted December 10, 2022 (edited) Title explains a lot. Had new alternator installed thinking that was the issue for my overcharging. New battery as well because the old one was bad. New alternator still overcharges. Decided to disconnect PCM connector from it, drops from 14.5 at idle to 13.8. New alternator was installed by a shop, FoMoCo branded. No reason to suspect they did anything wrong. I told them to change it, not diagnose the issue...now I see I should have asked for their opinion. From my understanding, the fact the alternator drops to 13.8v with the PCM unplugged from it suggests that the alternator itself is likely working properly because it wants to output 13.8 normally. However, the PCM is requesting 14.5-16+ when connected. My question, what causes this? I suspect the alternator is ruled out, right? I have seen some blame the IAT sensor as lower temps mean the car will charge at a higher voltage to keep battery warm. Bad sensor reading low temp would, in theory, make it charge higher. However, the issue I have with this is that once or twice per hour or so of highway driving, generally back to back, the car will hit 16.5v turning off the gauge cluster screens. But then this won't happen again for awhile. Is the PCM capable of asking for that much power even if the systems obviously cannot handle it? Or is possible the PCM really just thinks it is so cold or that the battery is so bad that it needs to charge that high? Keep in mind, it will generally idle at 14.5-15v. 2000rpm will go around 14.5-15.5, but will rarely peak to 16.5 which is when stuff like Sync 2 will turn off climate controls and screens on dash will turn off. It does fluctuate while driving, but doesn't fluctuate at idle. This is all while PCM is plugged it. Just sits at 13.88v with alternator's PCM wires disconnected. Edit: Voltage was checked with multimeter at battery connections. Also, scan tool confirms voltage is the same. Also says ECU voltage is whatever the battery voltage is- everything seems to agree it is overcharging. Edited December 10, 2022 by TyF 0 Quote Link to comment Share on other sites More sharing options...
mozz Posted December 11, 2022 Share Posted December 11, 2022 Was the battery life BMS reset? 0 Quote Link to comment Share on other sites More sharing options...
TyF Posted December 12, 2022 Author Share Posted December 12, 2022 7 hours ago, mozz said: Was the battery life BMS reset? I attempted the KOEO 5 bright flashes, 3 brake pedal presses and nothing happened. Did it like 15 times. Door closed, did it quickly after turning on the key. Never got it to work. No battery light comes on- either with the key turned over or after attempting the reset. The light does work though because it will turn on with the PCM disconnected from the alternator. 0 Quote Link to comment Share on other sites More sharing options...
mozz Posted December 23, 2022 Share Posted December 23, 2022 Well then Forscan or dealer. 0 Quote Link to comment Share on other sites More sharing options...
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