Auto Repair Q&A



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How To Find a Parasitic Draw

Car Battery (generic)

Finding a parasitic draw in your car is fairly easy, but it's a tedious process that can take a lot of time. If you have a dead battery, be sure to first check that the battery itself is OK - it is not out of date, low on water, etc. - by checking it out and having it tested. Most auto parts stores will test your vehicle's charging system (battery, alternator, etc) for free. Many garages will as well.

Once a defective battery is ruled out, check every light and switch in your car to be sure that one of them is not stuck, on, or shorted. Quite often, the parasitic drain on your battery is actually a light that gets chronically left on or a switch that isn't being shut off somewhere.

At this point, you can begin tracking down your parasitic draw. It may still end up being a shorted connection or defective switch, but you may not find it until you've gone through these steps.

Remove the negative battery cable. Disconnect it from the battery and hang or place it somewhere where it will not complete a circuit (ground). A twist tie to the hood or wrapping the end of the cable in an old inner tube and setting it down are good options.

Place your multimeter's ground to the negative battery terminal. Make sure your multimeter is configured to read at the 2-3 amp level (or lower), otherwise you are likely to miss your parasite. Place the positive side to the negative (ground) cable. Alternatively you could choose a ground point on the car instead, but this will not help rule out a defective ground with your battery's negative cable.

Once you've made the connection between the battery's negative and the ground, wait for your car to go through its cycle. Once the computer reconnects and then "sleeps," you should be reading zero Amps on your meter. If you are reading anything above a few milliamps (which may be present from atmospheric static), you have a draw somewhere that is pulling from the car. Remove the connection from the negative battery cable and place it on the car at a ground point (bare metal). Watch again for the draw. If it's still there, you've ruled out an issue with your ground at the battery. Reconnect and wait for it to stabilize after the computer "sleeps" and leave the multimeter connected.

Go to the engine's fuse box. Starting with the minor fuses (smaller), pull each of them one at a time and see if the meter changes, replacing each one before pulling the next. If it drops after pulling a fuse, that is the circuit with your problem. If none of them change anything, try the larger (main) fuses.

Go to the in-car accessory fuse box. This is usually one (sometimes two) fuse boxes inside the passenger cabin. Go through each fuse again, starting with the smallest and working up. Again, if the drain stops when you pull a fuse, that's your defective circuit.

Check devices on the circuit, one at a time, from easiest to hardest. Once you've found your defective circuit, begin testing each device on that circuit until you find the one with the drain. You'll need to disconnect each lamp, turn ever knob, disconnect and reconnect each switch and control on that circuit until you find the culprit.

Repair and retest. Once you've found your draw, repair it and retest it. If the draw is gone, you're done. If not, double check your repair and then continue through the circuit or check the fuses again to find the next circuit. It's not unusual to have more than one defective link in a circuit.

Once finished, reconnect everything and test the battery itself again, as per our second step.

If after all of this you still cannot find the drain, have your starting circuit tested (if you didn't already). Often, a bad diode on the alternator can be the cause of the drain and this should be noted when the vehicle's charging system is tested.