DeTomaso Mailing List: June 2001, Message #46

Doug Braun wrote:

"Your 20K meter setting means it's reading 20,000 Ohms full scale.  The .26
reading means 260 Ohms, the .74 means 740 Ohms and the 2.4 means 2400 Ohms.
Ohms are a measure of resistance to electrical current flow.  In the case of
a spark plug wire, lower readings are better, but how low is good enough?
In the "olden days", spark plug wires were made with a fiberglass or other
high temperature type filament core material impregnated with carbon
particles.  The "rule of thumb" for these old carbon core wires was that
they're bad if they measure greater than 1000 Ohms per inch.  This means
that on a typical 20" plug wire, a value of less than 20,000 Ohms was
perfectly acceptable and worked just fine.  Then along came helically wound
core plug wires with a much lower DC resistance, which means a more reliable
spark but wait, the lower resistance sometimes results in radio
interference.  Since the automakers liked the more reliable spark provided
by helical core wires (for low emissions) but also wanted to sell radios in
their cars, the spark plug manufacturers began putting resistors in their
spark plugs (It's now difficult to buy a non-resistor plug!).  The typical
resistor in a spark plug is well controlled; I've measured them anywhere
from 1,500 to 14,000 Ohms and always throw out any new ones I find that are
over 10,000 Ohms.
For testing helical wires, I would be suspicious of any reading above 200
Ohms per inch but you can also  compare the Ohms/inch readings amongst the
wires.  If you find one with a significantly higher Ohms per inch reading
than the rest, consider it to be worthy of replacement.  Remember to always
divide the meter reading by the length of the wire for the comparison.  Hope
this clears it up for you.Doug Braun73l #5505"

Doug -

Super discussion on plug wires.  I had no clue (so much for 4 years and an
EE degree. . .).

- Curt  (PA  74L 351c)