Electrical gremlins??

[Erik Beckett]

I just recently purchased a 68 Camaro from a fellow member here. Its a great car but I am trying to sort a few electrical gremlins on the car. The previous owner just recently installed a GM headlight switch, due to the other one being faulty. I took ownership of the car last weekend and am having a weird humming noise coming from the battery when both cables are connected. As soon as you disconnect the cables the humming goes away. Also, the electric Autometer speedometer shows the mileage even though the car is off and the key is out. As soon as you disconnect the battery the mileage is no longer lit up. He says everything is hooked up to the keyed ignition not the battery.

I am probably the worst when it comes to electrical but its something I guess I have to learn. Any ideas what could be drawing the power from the battery when the key is of and out? He says he never had this issue but who knows, its not his problem anymore. The only thing he did was install a new headlight switch.

Any ideas?

Thanks, Erik

[First Bird]

Could be alot of things. But I'd start with two actions. 1) Check grounds, especially the headlight switch. 2) Use a meter to read how many amps of current are flowing when the battery is connected but everything is off.

1) Remove headlight switch. Sand the sheetmetal behind the dash where the headlight switch mounts. It grounds here. 40 years of corrosion is bad. 220 grit to a shiny surface. Apply a little motor oil. Wipe it off. Re-install switch tight to dash. Then go around and verify all ground wires, esp. battery are tight and corrosion free.

2) With key off and battery grounded, take off the postive cable at the battery terminal. Put the multimeter in amp reading configuration. You need a meter capable of reading at least 10 amps. Connect one lead to positive battery cable and other to positive battery post. If you have a major short somewhere, you just might blow a fuse in the meter. Hopefully not. With the setup I just described my electrical system draws 23mA - tiny amount of "quiescent draw". So here's the reason we're doing all this - go to the fuse panel and pull each fuse out one at a time and see what effect it has on current flowing from your battery. This might help you narrow your focus on where the problem is.

Others can weigh in here but it sounds the the PO might have wired a component to constant Batt +, when it was supposed to have switched power.
Electrical is tricky stuff, but get a meter and a book and think logically. Good luck.

[TnBlkC230WZ]

To add to First Bird's comments. While your parking lights are fused, the headlights have a circuit breaker in the headlight switch so you have to unplug your light switch to kill the power to it. Don't assume the speedo is wired correctly. Double check it to be sure. and trace the power circuit as First Bird described to see where it is getting its power from.

The humming battery sounds ominous. Are you sure it is good.