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Draw test using voltage drop
Posted to Technical Tips Forum on 3/1/2011 84 Replies

I had a car in today that would have a dead battery after sitting for a couple nights. Checking for a draw I found it had a 220 ma draw and was going to start with the usual pulling fuses and checking my Amp meter. One of the other techs brought over the February 2011 edition of Motor Age and told me to read an article and see if it works. I did and it did. I don't remember the title of the article but I can get it if anyone is interested.

The article went on to say that basically, everything has a resistance value and if there is current flow and you can measure on each side of it, there will be a voltage drop. It said to check for a voltage drop between the 2 exposed tabs of the fuses. So, setting my D.V.O.M. to the mv scale I proceeded to do a voltage drop test on all the fuses under the hood. When I got to the last 2 (of course) they each showed a .4 or .04mv drop, can't remember exactly but all the others were 0mv. They were the fuses for the headlights (low beam right and left).

Now, If I had done the old pull a fuse, see no change in the system draw, put the fuse back in, then neither one of those fuses would have shown a reduction in the system draw due to the way the circuit is wired and the location of the problem.

Had I done the pull a fuse and leave it out then pull the next and leave it out, etc etc etc... I may have been looking in the wrong location thinking it was just the circuit pertaining to whichever headlight fuse was pulled last.

Not all of the fuses will be 0mv as there may be some current flow at the fuses that power clocks and or modules but those fuses should have a very small voltage drop. It (the article) also stated you can find the amount of current draw if you know the resistance of the fuse in question. Doing the voltage drop way you are also not disconnecting power to module which could wake it back up when re-installing the fuse and possibly spiking the current draw, popping the fuse in your meter or making you wait for the module to go back to sleep.

For my first test using this method, I can say it worked much better than my old methods so I thought I would pass it on.

Harvey from California

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