Ask Anything is the portion of What's Your Problem where readers send questions for industry bigwigs, and we get the answers. So pick a hero, come up with good questions, and send them to CarCraft@primedia.com.
Steve Rambone, Fairfield, CT: I have a '70 Chevy Impala that I've been working on for about a year and a half. It's a coupe with a small-block 400, TH350 trans, and a 10-bolt rear, and it's mostly stock, save for the four-barrel carb and intake I added and the dual exhaust I had a shop fabricate. I got the car from a relative, who had owned it since the early '70s-when the relative got older, the car sat in a garage most of the time. It has about 95,000 original miles, and the body is in pretty good shape, and now that I'm finishing the mechanical restoration of the brakes and suspension, I want to drive it more than just around the neighborhood. Trouble is, if I leave the battery connected, it goes dead in about a day. I didn't really notice the problem before because the car would sit in my garage for several days or even weeks between starts, and I would always disconnect the battery when I parked it. This car has factory power windows, rear defog, A/C, and some other options, so I'm a little overwhelmed when I think about trying to diagnose the source of the drain. How can I narrow it down?
Joe Armstrong, Technical Expert, Painless Performance: This is a common malady found on older and late-model cars alike. Often it is the result of an aftermarket alteration to the factory wiring harness, but sometimes it can be caused by a faulty switch or relay. The following information will help diagnose this issue.
For this test you will need the following: a 12-volt test light and whatever tools are required to remove the positive battery cable, the wires from the charging system, and the wires from the ignition switch. You should also have a sheet of paper and a pencil. Make sure the battery is fully charged before you begin.
1) Remove the positive battery cable and put the test light in line between the positive battery-cable terminal and the positive post on the battery. This will measure the current flow from the battery to the electrical system. Look at the test light and take note of the brightness before you change anything.
2) Disconnect the test light from the positive post on the battery. Make sure the positive battery cable does not touch the positive post on the battery. Remove the wiring to the charging system. Be sure to insulate the ends of the wires, as these are battery hot-circuits, and you do not want them to short to ground. Reconnect the test light to the positive post on the battery. Take note of the brightness. If you see a dramatic change, the charging system is causing a drain. Your alternator or generator will need to be tested, rebuilt or replaced. If there is little or no change, go to step three.
3) Remove the test light, and replace the positive battery cable on the positive post. Close the door to your car, and make sure the dome light goes off when the door is shut. If the dome light works properly, remove the dome light bulb (be sure to remove any courtesy light bulbs as well). Using the test light, make a list of each fuse to determine which fuses have power with the ignition switch in the off position. Disconnect the positive battery cable from the positive post on the battery. Put the test light back in line as described in step one. Remove each fuse on your list one at a time. Check the test light after you remove each fuse. If removing a fuse causes a dramatic drop in the brightness of the test light, that circuit is causing the drain. You will need to identify what is connected to that circuit (we can help identify what could be connected to each circuit if you determine which fuse has the drain). If no fuse causes the drain, continue to step four.