<VV> Electrical question, now measuring drain current

jvhroberts at aol.com jvhroberts at aol.com
Sat May 2 13:05:47 EDT 2009

 Even easier is to use something like a marker light bulb in a socket with a pair of alligator clips in each fuse socket. If the light lights up when it's not supposed to, bingo. 



-----Original Message-----

From: craig nicol <nicolcs at aol.com>

To: virtualvairs at corvair.org

Sent: Sat, 2 May 2009 9:20 am

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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