Sub amp power big 3

Sorry, but I am not sure I understand what you are trying to do. What do you mean the big three? Three large subwoofers? If so, you need to run a feed directly from the battery for them with its own fuse. I would not tap into the charging circuit. You are correct that the PCM controls the alternator but you have a fusible link between the battery and alternator that will fail if you put more loads on this circuit.

Again, you need to run a separate power wire from the battery with its own fuse so that if there is an issue it will pop that fuse and not effect other components.

Let me know if I am misunderstanding. Thanks

I have just never heard it called a big 3. Must be a common thing in the sound system realm. However, if you are pulling that many amps on just the sound system then I think you need to consider a capacitor or even a second battery to run just the audio system.

Your concern is valid with the ECM. You want to steer clear of the regulator circuit. You are going to want to stay on the battery side of the alternator because this vehicle will duty cycle the alternator meaning when more power is needed, it will increase the output of the alternator. This guide talks more about that.

https://www.2carpros.com/articles/how-to-check-a-car-alternator

So if just your sound system is using up to 120 amps then that doesn't leave enough for the remaining systems so I don't think the headlamps are going to stop dimming until you separate these systems or add a capacitor. The capacitor will store enough energy to handle the high demands and will not take it from the headlamps. Otherwise, I don't see a solution using just the vehicles systems. I guess the only thing would be to run a large gauge wire directly from the battery rather then just to tap into another wire for the regulator.

Hello, I'm Danny.

Just to add my 2 cents. I would just install a stereo capacitor. I've attached pictures below. You can use any brand you like because I just found this brand because it came with a picture and description. Hope this helps and thanks again for using 2CarPros.