Let's check your battery and charging system first:
With a voltmeter check the voltage at the battery (should be 12.6V).
Start your car and check the voltage again (you should read anywhere from 13.5 to 15V).
If you have a lower reading, your alternator is not charging your battery enough.
A more accurate test would be to take the battery and the alternator to your favorite auto store and have them bench tested.
Okay, now let's say your battery and alternator test fine, you probably have a parasitic current drain.
Turn off your car and open the hood.
Set your voltmeter on ammeter and hook it between a battery cable and the battery post (make sure you have a battery back-up plugged into your cigarette lighter before you do that). Once you have your voltmeter connected between the battery cable and the battery post, unplug the back up battery, and check the current reading you get.
It's possible that you read up to 800 milliamps for the first 20 minutes or so after you turn off the car.
Normally, you shouldn't read more than 50 milliamps of current drain after an hour. If you read more than that, you have a parasitic current drain.
At this point you have to find out which circuit is pulling all those amps.
With your ammeter still connected to your battery, pull one fuse/relay at a time until your reading on the ammeter drops. When it does so, you know which circuit is the problem.
Once you find which circuit is pulling too much amps, you'll have to check all the relays, switches, module on that circuit to find the bad component.
Hope this help
Tuesday, October 21st, 2008 AT 10:39 AM