Under the Hood
---- — 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.
Next, you’ll want to remove each of the Corvette’s fuses one at a time until you find the one that extinguishes the light. This is hard to do with the doors closed; trick the driver’s door switch with a piece of duct tape if needed.
Once a particular fuse has been identified, a wiring diagram or other service information will list the components protected by the fuse. The clock/courtesy/dome light fuse is often the one, and the fault could be an always-on under-hood, glove box or other convenience light.
Aftermarket accessory equipment such as radios, amplifiers and security systems could also be the culprit. If none of the fuses kills the light, unplug the alternator’s two connectors.
The test light trick doesn’t work on newer vehicles because they are wakened by modules, then go back to sleep after the battery is reconnected. They can’t do this properly with the test light inhibiting the battery cable pathway.
These vehicles need a battery memory saver temporarily connected to the scan tool connector beneath the dash, which prevents inconvenient issues occurring due to disconnecting the battery, and a digital multimeter to be used in place of the test light.
The meter, set to measure current (amps) and connected properly, provides a good pathway for normal key-off vehicle functions, and an accurate reading of any offending drainage. Many newer vehicles require additional steps and precautions — check service information before proceeding.
Brad Bergholdt is an automotive technology instructor at Evergreen Valley College in San Jose, Calif. Readers may send him email at firstname.lastname@example.org; he cannot make personal replies.