Under the Hood: Tracking a power drain takes a little work

Q: My other half's 76-year-old aunt has a 1995 Buick Century. She has no money to replace the vehicle, which has been costing her ton of money lately.

About a month ago, she came out to a dead battery. They replaced the battery. The next day, the battery was dead again. They checked the battery and found no problems. So they replaced the alternator. The next day, the battery was dead again. So they then replaced the starter. And again, the next day the battery was dead.

She then took her vehicle to another mechanic. He put the car on a scope machine, but told her that he could not find anything wrong because the car was too old. He told her there is a short somewhere and her only option is to install a battery kill switch. Do you have any idea(s) what may be draining the battery?

A: It kills me to hear of parts being thrown at a car instead of someone employing sensible diagnostic steps to fix the problem. It sure sounds as if the Century is suffering from a parasitic drain condition, rather than a short, or a fault that can be diagnosed with a scan tool. All modern vehicles lose a small amount of electricity while parked; it's needed to keep memories alive for the clock, sound system, engine computer and other modules that need to keep one eye partially open. The maximum allowable amount is about 1/5 of what it takes to light a tiny light bulb, such as a side marker light.

Common causes of excessive parasitic drain are: a faulty switch allowing constant operation of a trunk, glove box, or courtesy light; a relay that fails to open after use, possibly enabling the fuel pump or some other component to keep functioning; and a faulty diode within the alternator. In newer, multiplexed vehicles, one may also find a module failing to go to sleep, likely due to an incorrect door switch input.

Checking for parasitic drain is fairly simple for a pro or savvy home mechanic. A multimeter, set to amps, is connected between one of the battery posts and its disconnected terminal. All doors are closed, the under-hood light is disconnected, and a half-hour is allowed for all modules to go to sleep. The meter displays the amount of electrical current flowing from the battery to the car - the amount of drain can be a clue to the size or type of the offending component.

Next, fuses are removed and replaced, one at a time, while observing the meter. The likely suspects are fuses where voltage is present at all times, but it doesn't hurt to check them all. If the meter display falls below 50 milliamps while one fuse is removed, that's the source of the power drain. At this point, you now know what circuit the drain is in, and can follow a list or electrical diagram to narrow the search to a single source.

In most cases, the cause of a parasitic drain is a simple fault that's inexpensive to fix. You could also do a little snooping around before taking the car in for inspection. Whatever is draining the battery is producing light, heat, sound, motion or magnetism. Keeping an eye and ear peeled can sometimes result in a lucky find, such as a trunk light continuously glowing or fuel pump humming.

Brad Bergholdt is an automotive technology instructor at Evergreen Valley College in San Jose, Calif. Readers may send him email at under-the-hood@earthlink.net; he cannot make personal replies.


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