Interesting but Probably Trivia
 

I came across this yesterday and decided to post as general info, but as the the title says it's probably of minimal value....

My car has the halogen projector light setup, several years ago I installed a 35w Morimoto HID lighting kit. Love it but that's besides the point. I was troubleshooting an intermittent slow turn- on at driver side. It was just a bit of corrosion at the negative terminal of the oem plug at the light, cleaned , all is good now. What I discovered though , as I was checking voltage at that plug, was that the plug supplies about 4.5v even when the car is off, headlights off. When lights are activated the voltage then changes to 12v as expected.

I expected to see 0v at the plug when everything is off, so I figured the 4.5v was some sort of reference voltage from the lighting control module. But then if that's the case, it seems that the oem bulb (now the HID components) is seeing that 4.5v unless the LCM doesn't ground the negative side of the plug until lights are activated. Wish I had thought to check that at the time. My tester was just connected to body bolt when I checked the plug. Remember that the halogen don't function as low voltage daytime lights, the car has the hockey puck led's for that. Maybe the 4.5 v is related to resistance verification of a functioning bulb. My HID has the CANBUS connector for this purpose to avoid Bulb Out warning.

Anyway, I thought I'd post this tidbit of info in case anyone was poking around their halogen light setup.

I remember reading on the forum that the car is constantly sending voltage to different parts of the car even when off , I think it had something to do with the back up battery and the car checking that everything is working properly.