Mechanics

TROUBLE STARTING/LOW VOLTAGE

2002 Chevrolet Malibu • 186,000 miles

Car looses voltage over a 24-48 hour period. Wont start after 2 days. I Have tested for voltage draw and the test light barely illuminates when connected between the neg batt term and ground cable. I had the battery tested and Autozone said its good. But it will loose voltage out of the car. From 12.5 to 11v in 24 hours just like it does when in the car. While doing the voltage draw test I did notice the test light would illuminate brightly when I removed the fuse marked R/H BEC BATT or L/H BEC/BATT2. But would go off if both fuses removed at the same time. I think the battery is bad but Autozone is saying its my starter or mechanical problem. I was going to buy a new battery but don't want to just throw money at the car. Thanks for any help you can offer. BTW new alternator and battery cables.
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Low-key
April 20, 2013.



You can't use that test light trick to test for a current drain. Some computers can take up to 20 minutes to go to "sleep mode" and will draw up to three amps until then. The test light presents too much resistance. It limits current way too much to allow the computers to shut down. You need to insert an amp meter instead but the procedure involves using a jumper wire to bypass it whenever you switch scales to get more accuracy. The meter's switch breaks the circuit just long enough to wake the computers up again and that can draw enough current to blow the meter's internal fuse which is usually a 2 amp.

If the battery is self-discharging out of the car, which sounds like you've already identified the problem, I would charge it up, have it tested again, then leave it at Auto Zone for two or three days and have them test it again after it has been sitting. Be sure the top is clean and there's no condensed acid on it. That will cause self-discharge too.

Caradiodoc
Apr 20, 2013.
Forgot to mention that since GM redesigned their generators for the '87 model year they have had a huge problem with them developing voltage spikes that destroy the internal diodes and voltage regulator as well as interfere with computer sensor signals. The battery dampens and absorbs those spikes but it loses its ability to do that as it ages. If the battery is more than about two years old it should be replaced to reduce the number of repeat generator failures.

Caradiodoc
Apr 20, 2013.