*Troubleshooting Parasitic Electrical Drain In Your Vehicle*
By: Morpheus
30 May 2011

Have you ever gone out in the morning to start your car only to find that the battery was dead? Then you jump-started the car and drove it for a day or two and after letting it sit over a weekend, the battery was dead again? I recently had friends who went through a problem like this with a late-model Camaro, and the repair bill for troubleshooting was $600. The end conclusion was that the vanity mirror over the passenger seat had a light that was staying on, draining the battery. Were I the mechanic, I would have been too embarrassed by my own incompetence to charge exorbitantly to isolate and fix such a simple problem. With a little bit of knowledge, a light tool kit and a multi-meter, you can likely save yourself troubleshooting costs and potentially a significant portion of the repair costs.


  1. Don't open the hood and start working on your vehicle until you remove rings, watches, necklaces and other jewelry. Necklaces and watches can get caught in rotating belts and fans. Rings can short electrical connections and melt into your appendages due to current flowing through them.

  2. Before starting work on electrical/electronic components, disconnect the negative terminal of the battery. If you leave it connected and use tools on positive circuits, your tools can short out on frame and/or chassis, resulting in rapid battery discharge, tool meltdown and potential battery explosion.

**If you own a BMW, I recommend taking it to the dealer. When BMW batteries are disconnected, the computer dumps its memory and the car has to be towed to a dealer for re-programming.**

Before starting, you may want to charge your battery with a household/110V charger, start the vehicle and drive to an auto parts supply store. Most auto supply stores have equipment to check your battery and charging system and will help with initial diagnostics free of charge . If old or an Ever Start (Never Start?) from Wal-Mart, the battery may be defective itself, and shorting itself internally. The alternator may also have problems and not be charging adequately to maintain the battery.

There are two tools you can use to check for current draw, a 12 volt test light or a multi-meter. Multi-meters come in two varieties, analog and digital, either will work for vehicle electrical system troubleshooting.

Before you get started, make sure the radio and any other audio and video accessories are off. Close the vehicle's doors and/or turn the dome light and any other interior lights off. Disconnect the negative cable from its terminal on the battery. Adjust your multi-meter so it will read 12V direct current (DC) and connect the meter's positive probe to the negative battery cable and the meter's negative probe to the negative terminal on the battery. If you're using a test light, connect the clip/lead to the cable and touch the probe to the battery's negative terminal. If the test light comes on or the meter reads 12V, some circuit is drawing voltage from your battery. At this point you can check the amperage of the drain using your multi-meter, but if the drain is higher amperage than your meter is rated for it will blow the meter's fuse. If the test light is bright or the amperage reads more than an amp, the draw is significant and needs further investigation.

To begin isolating the offensive circuit, I usually double check to ensure that all interior lights, radios, and accessories are off. If they are, a common cause of drain is a failed diode in the alternator's regulator causing voltage drain. To check, disconnect all of the wiring from the alternator. If the alternator was the culprit, once it's disconnected the test light should be dimmer or not come on at all when connected between the cable and the battery's negative terminal, or the multi-meter should show less amperage draw or an absence of voltage when connected. If you have doubts about your alternator's serviceability you can remove it and take it to the auto parts store as most have alternator test racks and will be able to tell you if it's serviceable or not.

If the alternator isn't the culprit, starter solenoids are also a likely culprit. To check the starter solenoid, remove the large positive wire from the battery to the starter from its post on the starter solenoid, then check for voltage and amperage as described above. Once I've disconnected a component, I prefer to leave it disconnected until I have zero draw from the battery as evidenced by the test light not coming on or zero voltage on the mult-meter.

If the neither the starter nor the alternator are causing the drain, the next place to check is fuse panels and relay boxes. Most modern vehicles have fuse panels under the hood as well as those in the passenger compartment. Use the owner's manual if available or search on-line to find the location of your fuse panels and relay boxes. One by one, remove fuses and relays until there is no voltage showing on the multi-meter or the test light doesn't come on when attached as described above. Once you find the fuse or relay that's supporting the current flow, you'll have to troubleshoot that circuit and replace defective components it powers or disconnect it and live without it if possible. If you find several circuits active/drawing that shouldn't be with the ignition switch off, it's possible that the ignition switch itself is bad and allowing current to flow when it shouldn't. Simply disconnecting the ignition switch (at the bottom of the steering column for most vehicles) should stop the voltage drain if the switch is faulty.

Some circuits like interior clocks, security systems, and capacitors in the airbag and PCM computers will draw minimal amperage full-time, but it shouldn't even be an amp.

For one example, one evening I was driving to the local airport and noticed that my voltage was reading a little high on the dash gauge. Within 3 days, my Suburban wouldn't turn over or start. I bought a new battery and went through this process. The ECM was pulling minimal amperage, but the alternator was defective and drawing heavy amperage. Unfortunately the low voltage to the starter from the drained battery destroyed the starter as well so I had to replace the starter, alternator, and battery all at once. I haven't had any problems with it since.

A friend of mine had a similar experience with his Ford truck and found that his alternator was bad. Even when the alternator was disconnected, his dash, power-train control module and clock pulled .17 milliamps, which confounded his troubleshooting efforts a little. He replaced his alternator and his truck has run reliably and well since.

Most importantly, remember the safety notes above and don't compromise your health and welfare. If you feel like you're in over your head, try to find a competent mechanic who can advise, support, and take over if needed. Your demonstrated knowledge of the vehicle's systems and problem should stop the mechanic from fabricating charges and lessen the time a good mechanic will need to spend troubleshooting your vehicle's problems as you've eliminated several of the potential problems.

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