I want to check the integrity of my MSD 8.5 Spark plug wires. They claim to be a low 50 ohms per foot. With an ohmmeter, what would be considered "out of tolerance"?

 

What do they measure?

 

Measure in ohms? I haven't checked yet. Measure in length? Standard SBC HEI set.

 

Yeah resistance.

 

I'll check 'em today and report back.

 

If they're supposed to be 50 ohms per foot, I'd neasure their length in feet, and multiply that by 50 ohms. That's what they should read.

I'd guess they should be within 10% or so of the calculated value. Odds are, either they're all good, or some one or 2 of them will be WAY off from the rest. So, if they all check about the same ohms per foot, they're probably good; if all except a couple are close together and those couple are considerably different ohms per foot from the rest, then those are bad.

This isn't rocket science, it's just a car. Don't outsmart yourself by thinking too much, just be logical, and the right answer will usually be obvious.

 

Yeah, they are all checking out ok....about 50 to 75 ohms per foot. My problem was in the advertising. It reads that the wires are ABOUT 50 ohms per foot. What's "about"? Would a weak wire read 80 ohms per foot or would the weak wire show a big difference in ohms? Apparently I'm looking for a big difference.

 

Plug wires with wire in them, rarely fail by their resistance increasing.... more likely, they'd go bad by the insulation failing (typical result of laying against headers and such), which they can be totally fried to ashes, and the ohms per foot would still be fine.

Basically, you're not really accomplishing much by worrying about that particular measurement. Youd do alot better to immerse them in water with from one end to the other, and apply high voltage to them, and see if they leak. That's more likely to catch real-world failures, the kind that would cause a dead cylinder or whatever.