been going out of my mind here. i have a 66 chevy 1/2 ton with a 250 L6 and I've been battling a charging problem for about 3 months now. the truck will run fine during the day but anytime i drive at night and unfortunately have to turn on the lights it completely drains my battery. I've replaced the alternator and regulator (external type) 4 times now, even changing to a different style. replaced the battery twice. each time i tested everything and all tests were sat. I've traced out my wiring and nothing looks out of the ordinary. I've already contemplated doing a complete overhaul of the trucks wiring through painless, but it simply isn't affordable at this very moment. any insight on something i could be missing would be greatly appreciated. I've had this truck for 7 years now so I'm very intimate with it. I've replaced batteries and alternators before with out having this problem. I'm also a mechanic in the coast guard so I'm fairly competent in what i do. looking forward to any help possible. got a little one on the way and unfortunately it is our only vehicle at this moment so I've got to keep it running.

I had that problem once with an old Nash. I replaced everything, but found the problem to be a bare tail light wire grounding out...also check the light switch for grounding...Just a thought.
Best wishes,
Chuck

Since you only seem to have this problem when your lights are on I would disconnect one light at a time and see which one is giving you the problem. Take a voltage test before you turn the lights on and after at the battery. It should be 13.5 to 13.75 volts average. If it is only around the 12v when the lights are off then you have a problem somewhere else other than the lights of course. let us know what you find and we can go on from there. Good Luck

John

Does your wife know you have been intimate with your truck?
I used to have an ammeter that used the magnetic field around a wire to indicate current. I don't know if they are still in the stores. With one of those, you can move around the harness to see where excess current is going.
The suggestion for voltage measurements is right on. Soometimes a merginal charging system will keep up in the day but not at night.
If you can't get the meter described, a digital volt meter will find a high drain. Get the battery voltage, then move out to the lights and measure there. A leg with a partial short will measure somewhat lower than those operating normally.