Ask Away! with Jeff Smith: Tracking Down a Phantom Battery Voltage Drain in a 1967 Chevy

I have a 1967 Nova that drains its battery if I leave the car sitting for more than a couple of days. I’d like to try to fix the problem myself rather than take it to a shop because I’ve heard that wiring problems can require a ton of time to trace. Do you have any suggestions on how to find the problem? The dome light is not the problem and I make sure the headlights are off—it’s more mysterious than that. Can you help?

D.P.

Finding electrical drains can be an elusive search, but there are a couple of techniques you can try to narrow the search down to a specific circuit or circuits. I learned this trick from a friend Mark Hamilton who has specialized in electrical work with a company called MAD Electrical. His suggestion was to find a small 12-volt buzzer like the old seat buzzers used in 1970s cars.

To begin the test, remove the positive battery cable from the battery and connect the buzzer in series or in between the battery cable and the battery. Any current flow or electrical drain from the battery will make the buzzer sound. Now you can begin to isolate each individual circuit by removing a fuse in the fuse box one at a time.

Remember to do this test with the dome light disabled, otherwise you will be chasing that circuit and may miss another one this is the real culprit. We usually just tape the door switch closed to make this easier. You can remove the dome light, but we had a situation one time where the wire feeding the dome light fixture was bad and had melted to create a direct short. This is what was draining the battery. We mention this because you need to work carefully and slowly in order to detect the problem.

If you don’t have access to a buzzer, you can use a test light hooked up in the same manner. The light will illuminate only when there is current flow. The problem with this technique is that you have to look up from the fuse box to see if the light is on or off. That’s why the buzzer is a better idea because you can instantly hear the buzzer.

This issue is compounded and complicated when dealing with an intermittent problem that only surfaces rarely or with unknown causes. Also be careful of other, inadvertent causes. We had a problem with a poorly adjusted brake light switch that would leave the brake lights on, which is a non-switched circuit which causes the lights to drain the battery if you do not notice the brake lights are on.

We also had an issue with a intermittent horn switch in the steering column that would trigger the horn relay. We disconnected the horns but forgot about the horn relay, which was the old, original mechanical style that pulled amperage to maintain the relay connection in the “on” position. We only noticed it when the horn relay box was hot to the touch when it should have been cold. Lesson learned.

You may discover that you may not be able to find the problem even after much searching. We had a similar issue where an intermittent short would kill the alternator. After burning up three alternators on our 1965 Chevelle we decided the only way to repair the problem (we never did find the problem) was to replace the original wiring harness with a new one. This solved the problem but was a bit of an expensive repair when possibly all it really needed was to replace or repair a section of wiring. However, the electrical system is now far better than in the past with greater reliability so it was worth the effort and expense.

This is a common problem with older cars that are over 30 years old. The factory never envisioned that gearheads like us would still be messing with these cars five or six decades later, so frankly nearly all of these cars could benefit from a new wiring harness, especially those cars with the older glass fuse boxes. Those fuse holders don’t take long to begin to rust and lose their electrical connection.