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RE: Spark plugs -and spark

To: <land-speed@autox.team.net>
Subject: RE: Spark plugs -and spark
From: "Waldron, James" <James.Waldron@CWUSA.COM>
Date: Thu, 3 Jul 2003 15:50:10 -0400
Be careful using a voltmeter.  As I recall, that spark is about 
20,000+ volts. And it's going to be way too fast for a voltmeter 
to see.  And if it can jump that spark gap, it may also jump the 
gap between your finger and the test probe (ouch - and dangerous 
with an HEI coil).  My guess is that it will toast your meter.

You 'can' measure the resistance of the wire with a voltmeter - 
for copper 0 ohms would be good, infinite ohms means the wire is 
broken.  For resistance leads, the length of the spark plug 
cable will determine (and I forget the ohms per foot value - 
I'm sure someone on the list knows).  Infinite ohms means an 
open in the wire.

If you are going to use an oscilloscope, be sure to work out a 
resistor ladder and select a low voltage tap.  (A string of 
resistors in series, plug wire to one end, ground to the other 
end, and the scope probe connected somewhere near the ground end. 
WARNING, I have not run the ohms law calcs on this so I don't 
know how many resistors or where to tap. BEFORE you try this, 
break out the pencil and paper and satisfy yourself that you 
are safe!)

Of course, you might try just putting the scope probe near 
the plug wire and see what sort of induced signal you pick up.

Good luck,
Jim W.

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