<VV> Electrical question, now measuring drain current

[The Robbins]

Can you pull one fuse at a time over a period of time and find which circuit
is draining?

AH of BBRT

-----Original Message-----
From: virtualvairs-bounces at corvair.org
[mailto:virtualvairs-bounces at corvair.org] On Behalf Of craig nicol
Sent: Saturday, May 02, 2009 8:21 AM
To: virtualvairs at corvair.org
Subject: Re: <VV> Electrical question, now measuring drain current

To help Bob figure out his car's electrical drain, someone suggested
removing the battery cable and putting a DMM in series to connect the
battery post to the cable. While this seems like an obvious solution, in
practice this is problematic. Most often this will result in a blown fuse in
the DMM and a zero reading. Common DMMs don't autorange for current, they
just have a fuse or two.  The high-current range, usually 10 amps, is OK for
a gross drain but since the typical drain is say, 200 ma against a 30 ma max
spec, it takes a much lower scale, typically 1A or 500ma. 

The slightest mistake, say opening the door, will blow the meter fuse on the
lower setting. You can't just pull the fuse on the dome lamp circuit because
90% on the drains (IME) are in the courtesy-clock circuit. You also have to
consider inrush current as the wiring is connected - you know, the sparks
that occur when the battery cable is first connected. That requires a little
dance of having the DMM already connected to the *connected* battery cable
and then separating it.  You see the problem?

Here's the solution: Purchase a 10W or 25W 1-ohm wire-wound resistor and
install medium-sized alligator clips on both leads.  Insert the resistor in
series with the battery terminal and cable. Use your handy-dandy DMM to read
*volts* or *millivolts* across the 1-ohm resistor. Using the 1-ohm resistor,
measured voltage drop IS the current flowing in the circuit. 1V = 1A, 1mv =
1ma. With this setup, you can accidentally turn something ON, change scales
at will, and manage inrush without frying your DMM. 

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