my alternator only charges when it feels like it, so I often end up with dead battery

That was my first idea so I got the alternator tested and came out GOOD. On the alternator, apart from the BTTY terminal, there is 2 other wire hooked up to the regulator. To narrow down my search, I've compared what reading I was getting out of the female adapter on my GM Truck and when the switch is on, the L terminal gives me the same voltage as the car btty. But, on my cadillac, I get only half the voltage +/- 5.5 VDC. Since I've got the charging schematic, I can see that the L terminal goes to the voltmeter (light) then ignition. So, why only half the voltage I should get? And also, can someone explain how the regulator gets excited in order for the alternator to start charging the btty? This is an intermittent problem so which part in the system could be on or off depending on the alternator ghost wishes?

Change the regulator if it is separate. Otherwise swap out the alternator for a new one.

taver91, Thanks for your reply but the regulator is not separate and, as I wrote, I had the alternator tested and turned out good. I'm 98% sure the problem lies with a component that tell the regulator to start charging because, as I wrote, with the switch ON, on the L terminal, I'm getting only half the voltage of the battery compare to the reading I get from the L terminal on my GM truck with the switch ON. I know there not the same vehicles and same alternator but there must be something consistent with the way "regulator gets excited and drives the alternator to charge" . That is the information I'd like to know and maybe I could bypass the faulty component and confirm my theory. So, can someone explain to me how the regulator gets excited and drives the alternator to start / stop charging?

Try the plug that goes to the alternator. I had the same problem on an 08 and after 2 shops 3 battery and alternator returns & including the dealership and 1,800 hundred later. I spliced and changed the plug and never had a problem again