EagleTribune.com, North Andover, MA


July 28, 2013

Patient testing can track down battery drain

I grew up in the ‘50s, when the huge “car craze” started. I still try to do my own maintenance to our cars. I am now working on a relative’s 1979 Corvette that is in need of some TLC. It has what you call a phantom battery drain. The battery goes dead unless I drive the car every two days. I’ve had it in two shops, and both say there is no battery drain.

I rebuilt the alternator, and it has a new battery. I think the problem is in the interior courtesy light system. How can I diagnose this problem to be sure? Will you help me with this annoying and frustrating problem?

If this problem is happening consistently, it’ll be easy to track down. Your phantom battery drain is being caused by something in the car that is converting electricity to light, heat, magnetism or sound. All vehicles will lose a tiny amount of battery energy to such things as computer and radio memories over time when parked, but there is clearly a large drain occurring with the Corvette.

The method I’ll propose is simple, but works only on older, pre-computerized vehicles. I’ll explain why, and mention the more techie method needed for newer cars shortly.

Start by obtaining a $5-$10 unpowered automotive test light, which resembles an ice pick with a bulb inside the clear handle, and a wire with alligator clip at the end leading from it. Disconnect the negative battery terminal. Connect the test light between removed cable and the battery by inserting a bolt into the vacated side-terminal threaded hole so that the test light is serving as a battery cable extender. Now close both doors and the hood, and wait a couple of minutes.

If the test light illuminates brightly, this means the battery drain is presently occurring — a good thing, since you’re trying to find it. If not, try jiggling doors and the glove box door, or check again later while at a differing ambient temperature. A bright light also tells us the drain is fairly large, more so than that of the test light.

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