GMC alternator not working below 1000 rpm
 

Thought I would ask here to see if anyone has had a similar experience.
I have a 2010 GMC Savanna work van with the 6.0.
A few years ago I replaced the original 105 amp alternator with the larger 145 amp alt. Just recently I have noticed that any rpm below 1000 there is no output from the alternator. If you raise the rpm to 1000 it starts charging at a normal 14+ volts. I assumed that the alt had a problem and replaced it with another 145 amp alt from NAPA.
I still have the same problem as far as no charge below 1000 rpm however now as an added bonus the battery light on the dash is on too.
I took the old alt in to O'Reilly's and had it tested and it was fine.
There's a 4-pin plug on the alt however there are only 2 wires present. With the key on engine not running there is 5 volts on the orange wire.
FYI, After I first noticed this problem, I changed out the battery. It was 5 years old and probably ready to be replaced anyway.
I checked a bunch of fuses all were good.
I don't know if this is relevant but I had the 6L90 transmission replaced in late January of this year. This particular shop actually rebuilds the transmissions and also does there own programming.
I really don't know how long this has been going on. I look at the gauges often but usually while traveling down the road and like I said, all is good when above 1000 rpm. I have been driving it like this for a few weeks now with no ill effects.
Thanks for your help.

Did you use the original pulley (pulley diameter)?
Not that I know anything about the vehicle, I don't, but it sure sounds rpm dependent.

there should be an inductive pickup on the negative battery cable near the battery. it is determining how much draw is on the system and then the pcm tell the alternator how much to put out. there could be an issue with it and the higher amp alt may have caused a problem with it or the pcm as it can put out far more than the pcm is ever expecting to see.

https://cimg5.ibsrv.net/gimg/www.off...8608b7b4f2.jpg

that looks like it. the alternator does not have an internal regulator that just monitors output like the earlier ones. that sensor monitors the draw on the negative side and raises or lowers output. if you are driving 60mph and have nothing on then the alt output drops way down for more fuel mileage and vice versa. alot of people add amps, etc and run a ground straight to the battery without going through the sensor so the amp pulls a lot out of battery but because the sensor didnt see it the alt never kicks op output and the battery starts going down. not saying that sensor is the problem, just that is how it all works. not sure if the higher amp alt would cause a problem or not but would think if the output was at some time higher than the pcm would ever expect then maybe it could either hurt the sensor or confuse the pcm?

Great info ^
Here is more:
https://www.silveradosierra.com/thre...-volts.351434/

well that explains it far better than I ever could

I put the old 145 amp alt back on and now the battery light on the dash is gone. If you care to look, I have a video of what mine is doing. Not sure that this is the same thing the others are saying on the link SB shared. Mine drops to 12.5 - 12.3 volts and will not come back up until the engine rpm reaches 1000. I also don't seem to have a "battery current sensor". At least there isn't one at the battery. That schematic I posted does not show the 6.0. I have been told that 2010 was a transitional time for GM. I have a friend that said the same thing compedgemarine said that maybe I need to go back to the 105 amp alt. https://cimg5.ibsrv.net/gimg/www.off...9691a55f2b.jpg
https://cimg0.ibsrv.net/gimg/www.off...50d5d8acc6.jpg

Pq- your pics don’t show on my phone well.
Most gm’s current sensors are like this

https://parts.folsomchevy.com/oem-pa...ZtwcgfSt8dX_T5

That looks like the plug on the alternator itself. I thought it was supposed to be located at the battery. Also my van is a 3500 and may have a different setup?