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Old Mar 25, 2024 | 08:32 AM
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Default Drops voltage

I have a 2011 Grand Sport with auto transmission. I just replaced my alternator and belts. All new GM parts. The alternator is a part no. 25888970. It's charging around 14.3 volts, but when I kick it down my voltage drops to around 12.5 and never recovers. Then I get the service charging system warning. I can stop the car shut it off open the door and restart it and I'm back to 14.3 volts. I never had this problem with the old alternator before it went out. The old one wasn't a GM part. What could be the problem.
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Old Mar 26, 2024 | 02:26 AM
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For any low voltage issue you always want to perform a voltage drop check on the charging system with a DVOM…with a DVOM with the RPM around 2000 RPM and with loads on the electrical system check voltage between alternator B+ and battery positive and then between the alternator case and battery negative…you normally see 0.1 to 0.3 volts on a good charging system.
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Old Mar 28, 2024 | 01:56 AM
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Messure back of alternator output terminal (under rubber boot on the back of it, with alternator body as the ground) to get really reading of what is putting out, and remember that dash dial and DIC reading, is giving you the voltage of the ecm down line.

Hence alternator to battery cable at the starter solenoid terminal, battery to engine fuse block, engine fuse block to BCM, then BCM to ecm.

So if alternator output is still at 14.7, but ecm is reading less than 14.3, problem with less than stellar connections between the alternator to ecm, or problem on the ecm ground connection instead.

Also, oem alternator only puts out about 14 amps at motor idle, while radiator fan will draw around 18 amps (ac turns it one) when sitting at a stop light, so if its just longer stop light tiime with the ac on your seeing the problems, then need to update to a 6 pole alternator, that will put out more like 45 amps while the car is idling, to keep with the battery drain on the system at such times isntead. (billet 170amp alternator is such beast)
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Old Mar 31, 2024 | 02:17 PM
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Also keep in mind there is voltage drop going into the ECM Due to diodes and other circuit designed to protect the ECU , so ECU will always report a little lower than actual provided to the ecu, better to check the ECU with a meter on the wires going in than to read what it tells you via scanner if you are investigating a drop between alt output and ecm input.

Now as to your problem 'when I kick it down my voltage drops to around 12.5 and never recovers.'
I am not sure what you mean by kick it down. Perhaps you mean a voltage kick down (alternator cut-off switch) which many ECU use for various reasons (sometimes at wide open throttle) but I don't remember anything for chevrolet like that, maybe you added it? I also use voltage kickdown because it is safer fueling and more stable ignition at wide open throttle when making big power no more way to blame the alternator for engine failure that way. Maybe you mean when you unplug the excite wire and plug it back in? It sounds like the ECU is throwing a code when it's unplugged from the alternator (hence the CEL) and this is preventing the alt from being turned back on , would be my flat out guess of guesses. But I can't remember any reason why the alt wouldn't be simply turned back on when reconnected. Perhaps you could try manually exciting the alternator (resistor inline or bulb to power for most alternators) - overall the best thing to do is lookup the schematic for the alternator circuit and manually activate to confirm alternator is good if needed. It sounds like it works when it wants to so I don't think you need to do this I'm just providing a diagnostic route for the big picture. If there is some issue with communication then it leaves the wiring (find where the wires go and reconnect them with new wires or fix them) and the controller (ECM usually) there is wire involved with excitation that is computer controlled in modern vehicles which means you must determine whether the computer is giving the command to turn OFF the alternator (When you want it ON) and find out why by reprogramming the service functions or bypassing somehow with your own wiring or circuit which perhaps was done to your old alternator (you might find a resistor in the excite wire or something from previous owner) Because it sounds like you know the old alt isnt OEM but you also arent the one who put it there just guessing again
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