Saw this in the October RockAuto newsletter. Sounds like a great idea for tracking an abnormal battery drain, so thought I'd post across the fora (sorry for the x-post). Reproduced in full...
Tracking down the circuit that is draining the battery gets harder if the problem is intermittent. Current drains can often be found by unplugging connectors or removing fuses and then using an ammeter or test light to bridge the disconnected electrical contacts. There is current draining away if the test light turns on or the ammeter gives an amp reading. However, disconnecting connectors or pulling fuses might also sometimes only hinder diagnosis by inadvertently making the problem temporarily go away.
This is especially true for newer, computerized cars (“newer” includes my son’s 1990 Lincoln!). Disconnecting power might reset the computer controlling a problem circuit so that the problem temporarily disappears. For example, maybe a body computer is staying on forever because it is receiving a signal that a door is still ajar. Disconnecting power to the computer or the door sensor might reset everything back to normal until the next time the offending door is opened or some other trigger reactivates the problem. Disconnecting things can delay tracking down intermittent problems on cars from almost any era. Diodes, capacitors, electromagnetic relays and other basic electronic components with problems might be temporarily “reset” if their host part is unhooked from the vehicle’s positive and/or ground connections.
Here is another method for troubleshooting battery drains. Instead of unplugging stuff and using the ammeter function on your multi-meter, leave everything hooked up and use the multi-meter’s voltmeter function set to millivolts. Voltage (V) equals current (I) multiplied by resistance (R) or V=I x R. Everything, even the best automotive wire or fuse from RockAuto, has at least some tiny resistance. R is always greater than zero. Therefore, any current (I) flowing through a fuse (with R resistance) will create a measurable voltage (V) drop across the fuse.
Remember to set the multi-meter to millivolts. The fuse’s resistance is very small so the voltage across the fuse will be small. Put the meter’s probes on the exposed ends of old style glass fuses. Put the probes on the two exposed contact points on the backs of blade fuses (see photo). If the measured millivolts are anything above zero, then current is passing through that fuse and the circuit protected by that fuse might contain the malfunctioning part that is draining the battery.
Picture: Put the probes on the two exposed contact points on the backs of blade fuses
It might be normal for current to be traveling through some fuses even when the engine is off and the key is out of the ignition. New cars often have systems that are designed to stay powered up for minutes or even hours after the car is turned off. Old cars may have clocks, alarms and other systems that always draw current but not enough to quickly drain the car battery.
Comparing millivolt readings from known and unfamiliar fuses can help track down something abnormal. Let’s say the fuse for the dome light circuit has four millivolts across it when the dome light is on. That means another circuit with a seven millivolt reading across its fuse would definitely drain the battery if allowed to draw current indefinitely after the car is shut off. Continue to track down the problem by using the owner’s manual or service manual (found under “Literature” in the RockAuto catalog) to find out what parts and systems are on a suspect fuse’s circuit.
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