This isn't really vehicle specific, but is inspired by my Toyota Pickup. I recently suspected some bad spark plug wires when my truck began running rough, so I checked the resistance on each wire. None of them seemed to have any breaks in conductivity, but while the four plug wires each read less than 1,000 ohms, the wire from the cap to the coil read 6,000 ohms plus. Mind you, these are CarQuest brand. So I looked at a new set of wires from the dealer ($83.00/set) and each of those read over 10,000 ohms resistance.

So what gives? I've read that lower resistance generally equates to higher quality, but that did not play out with this test. What on earth should I be looking for? Not withstanding an outright break in conductivity, what reading indicates a bad spark plug wire?

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J.R.
Tucson, AZ
'68 Dart 270 with a '76-'80 engine

That IS odd to have so much in new wires....they must be a different resistance type. My rule-of-thimb has been to pitch them anytime they get near to 10kohms. 3-4k has been average for new ones. Are you confient on the measurement of the new dealer wires?

I am confident, yes. I tested a new low-grade set at O'Reilly the other day they were in the 3-4,000 range. Very strange.

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J.R.
Tucson, AZ
'68 Dart 270 with a '76-'80 engine

Well, you got my curiosity up and I wnet out and measured the ones on my recently acquired /6. These are marked '7mm Suppression" with no other markings so are very likley low $$ parts put on the by prior owner's shop. They have a lot of years but very few miles on them; terminal, boot, and outer jacket condition is good to excellent, and they seem to fire well.

Cyl # Res
1 9.0k
2 7.0k
3 5.8k
4 5.9k
5 7.8k
6 8.3k
Coil 11.7k

The resistance is going up with the wire length but not not in proportion to the length; for example, wire #4 is almost exactly 1/2 the length of wire #6 between the crimp points, but resistance does not drop in half. It is like there is a fixed resistance somewhere in each wire of about 2.5k-3k.

BTW, distributor cap terminals are brass and plug terminal are some sort of steel-looking material.

I am tempted to go get a new set and see how those measure.

And here is another data point: My '62 FSM sstaes "Replace the cable [ignition wire] if the resistance is more than 30,000 ohms." So there ya go....

Checking online, thebnumbers and specs are all oer the map to as high as 1000 per inch.

Look at question 4 in the FAQ's section and the links they provide there.
http://www.magnecor.com/magnecor1/main.htm

Dave

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86 Miser 170,000+
2 1/4" exhaust
Holley 1920 #55
HEI MSD BLaster 2
17.8 mpg