I need more info on this unit, what effect the class 2 serial data to and from this unit has on the rest of the systems.

Long story short, Went to the dealership today to get some connector pins and some info, wire diagrams on my "GENERATOR" namely the blue wire in pin a of the plug. I know it goes to the pump motor but then where and why, ENGINE ON SIGNAL. ok what else does that do?????
my problem is that the volt meter does not register and the dash sometimes flashes BATTERY NOT CHARGING even though it is. I figured that no sense in figuring that out unless all the wires are connected, right? the blu one has been hanging for ever.
The service dude stopped a tech and asked him to help me for a couple mins, I told him about the problem and the swap well there went my 2 mins answeing his questions LOL. Any way he explained that ALL OF THE MODUALS communicate thru the class 2 serial data. Things that you never would think are related or connected are., He said that most certainly my guage not reading was because on no bcm in the loop. IDK about that?

The blue wire on the alt gives about 6.5 volts to the LOGIC in the pump motor, for the ENGINE ON SIGNAL,

Hear is the MILLION DOLLAR QUESTIONs , WHY DOES THE LOGIC NEED THAT SIGNAL, WHY IS THAT THE ONLY PLACE IT ENTERS, AND WHAT ELSE DOES IT HAVE AN EFFECT ON?