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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