Well i found out last night the hard way that for some reason that little plug on the top of the alternator means the alternator charges the batteries or not. My truck died cause my gf was watching a movie in my truck while i was cleaning it and it died so i had to jump it. I finally get it started and let it run for like 5 mins so i figured that was enough to charge it for another start so i test it and it is dead again so i jump it again and go take it for a drive and im driving and my lights and everything electrical was slowly dying and i realized i was running on battery power. so i park and jump out and while my truck was slowly rolling back because my p-brake doesnt work and its a 6-speed i had to open the hood and plug it back in and close the hood and jump back in while my truck is still gaining speed in reverse. i finally get back in and rev the engine and everything comes back to life and my air bag light was on. but that went off then next day and everything is fine...soooo yea i learned dont unplug the plug on the alternator. Just thought you might wanna know.