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Diagnosing Parasitic Drain
Sent to me by Andrew - ixixAndrewxixi

Battery keeps being drawn down, and you don’t know why.

Something in your car is not shutting off or going into “sleep” mode.

Tools needed:
Amp meter, Most commonly included in a multimeter.

Turn car off, Let cool down. Disconnect the Negative (-) battery cable. Set your meter to milliamps, and put one lead on the Negative (-) post of the battery, and place the other lead on the Negative (-) battery cable. In case, you will be remaking the circuit in series with your meter. Your meter should now jump up and possibly bounce. Be patient, and wait (up to ten minutes) until the meter drops to a lower level, and observe the reading. A rule of thumb for a car is 300 milliamps. Now some Cadillac automobiles are allowed up to 700 milliamps, so keep this in mind. But with our cars, you should see well below 300 milliamps. If your meter shows below 300 miliamps, your problem is not a parasitic drain. If you have a high number, or close to 300, and would like to investigate anyways see below.

IMPORTANT: DO NOT, turn on the key of the car, or anything that will draw decent power while meter is hooked up. Most meters are capable of 10 amps. Turning on your key will power up all devices and fuel pump, and there is a good chance it will fry the meter.

Now that you know you have a drain, make sure your meter is attached well to the battery and cable with rubber bands or such, and make it visible from the drivers seat. Remember that opening the door will turn on the dome lights, which is a small draw, so your meter will rise. This is normal. Just remember to Close the door to get your reading back. Now one by one, pull and put back a fuse out of the fuse block watching for the meter to drop drastically, when it does, you’ve found which circuit the draw is on. Now with either a wiring diagram or just looking what’s on the circuit, go to each component on that fuse, and disconnect it watching for the meter to drop drastically. When you find the component that is causing the draw, replace it. You may have more than one draw. So this process may need to be repeated. Reconnect battery.
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