How to Test for an Electrical Draw When the Engine Is Off

Car batteries are the modern gremlin's preferred source of nutrition.

Derived from the Old English word gremain meaning "to vex or frustrate," the Gremlin was thought by turn-of-the-century aircraft mechanics to be a tiny, mischievous creature that invaded biplane electrical systems and caused them to malfunction in flight. Later developments in aircraft maintenance nearly drove the gremlin into extinction, forcing it to flee from its natural high-altitude habitat and into more land-borne conveyances. While once an endangered species, the gremlin has come to thrive in the horrendously complex tangle of wires that is the modern automobile. Fortunately, the gremlin's only natural predator -- the digital multimeter -- is available at any local hardware and requires little persuasion to assist in the hunt.

Things You'll Need

• Basic hand tools
• Digital multimeter

Instructions

1. Set your digital multimeter to read in volts and test the battery voltage. It needs to read at a full charge or close to it of about 12.2 volts so that the battery doesn't throw off your resistance test. Once you've ascertained the battery charge, disconnect the positive and negative battery cables as well as the thick, red cable going to your starter. Unplug any accessories you have plugged into the car, close all the doors and trunk and pull out the light bulb that illuminates your engine bay.

2. Set your multimeter to read resistance in ohms and touch the probes to the positive and negative battery cable clamps to test for a major electrical short in the system. The meter should read at least 100 ohms; if it reads 0 ohms then you have a major short. This test isn't technically necessary to test for current draw, but attempting to read amp draw in a system with a major short will probably destroy your multimeter. If you get some kind of ohm reading, then proceed to the next step.

3. Reconnect the negative battery cable and set your positive wire terminal next to the positive battery terminal. Do not allow the two to touch. Plug your multimeter's probe wire into the bigger "unfused" terminal and set the multimeter to read current in the 10-amp range. Double check your wire position and multimeter setting; your multimeter is about to become a load-bearing component of your electrical system, so a mistake here will melt the thing into the next world.

4. Touch the black probe to the battery cable clamp and the red probe to the positive battery terminal. With all of the doors closed and the accessories off, you should get a reading in the 20 to 35 milliamp -- 0.020 to 0.035 amps -- range. This is pretty typical for the amount of current coursing through the relays and electrical contacts in the off position. Anything higher than that indicates a power draw somewhere in the system.

Tips & Warnings

• Here's a fun little formula to figure out exactly how long that current draw will take to kill your battery once you've diagnosed the draw. Look at the top of the battery for its rating in amp-hours, often abbreviated Ah. Divide the amp hours capacity by your amp draw and the result is the number of hours it'll take for that draw to kill your battery. If the battery lists a reserve capacity -- or RC -- divide that number by two and then divide that by the amp draw.

References

• "How to diagnose and Repair Automotive Electrical Systems; Tracy Martin; 2005
• "Foundations of Electrical Engineering"; John Cogdell; 1995