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Electrical Testing!

Subject: Electrical Testing!
Date: Tue, 15 Oct 96 10:17:45 -0400
I keep reading about people doing continuity testing and voltage testing
with meters, and wanted to put in my two cents.  Sometimes using meters
can be more misleading han using a light bulb type tester.  I thought I
would add a litlle primer for those of you that would like one.

Lets say you have a dirty connection that is 2 ohms (ohms are the units
of electrical resistance):  (BTW 2 Ohms isn't all that much)

If you measure the voltage from the load side of the connection to
ground without the load connected (the light bulb is not in the socket),
then the voltage will be the battery voltage, about 12 volts.

If you plug in the light bulb, you have connected the load.  Since the
light bulb may draw 2 amps, you will get a voltage drop of E=I*R or 4
volts.  If you now measure the voltage at the same point you will only
read 8 volts.

If you try to read the continuity using an ohmmeter, you will probably
not be able to read the 2 ohms.  Most meters have the lowest range of
0-200 ohms, and lets face it 2 isn't very high on that scale.  Some good
digital ohmmeters have scales in the 0-20 range or 0-10 range, but these
are more rare and more expensive.  Also the meters that measure down to
low ohms may put several amps through the circuit while testing.

If you have a 0.1 ohm dirty connection in your starter circuit: 
Starters draw lots of current.  I don't know what the current draw of an
MG starter is, but wouldn't be surprised if it were in the 100 amp
range.  So the voltage drop across the 0.1 ohm connection would be 0.1 X
100, or 10 volts!

So if you want to measure continuity, or even voltages for that matter
make sure you do it in a method that would provide reasonable results.

1.  Only measure voltages with loads connected.

2. If you want to measure continuity either buy a real good ohmmeter or:
measure the voltage dropped across each connection when the full load is

For example:

   -----      A        B    C     D      
   | B |------o--------o---o_o----o------Bulb--
   -----                                       |
     |                                         |
     V                                         V

In the above diagram, several connections are shown(A, B, and D) and a
switch (C) and a light bulb.  The V's represent the chassis ground which
is the return path.  

1. Turn switch C on. 

2. You can measure the voltage across connection A, B, or C. to see how
much voltage drop you have.  Put one probe on the wire that goes from
the battery to A, and the other probe on the wire that goes from A to B.
As a rule of thumb, if you have 0.1 volts or more. I would clean the

3. If you can't get to the connection, for example lets say that B in
inaccessable, measure from the A-B wire at A to ground, then measure
from the B - C wire to ground at C.  The difference is the voltage drop
across connection B and the drop in the wire A-B and B-C.  in this case
I would worry about 0.2 volts being too much.

4. Don't forget to measure the drop from the ground side of the light
bulb to a GOOD ground.  Take your probe and scratch through the dirt,
grime, paint, rust, and oxide until it connects to bright clean metal. 
Also measure from one ground to the other to see where the voltage is
being dropped.

Have I left anything out?

Good luck

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