Spark Plug Wires - Easy Test

Good day Chuck,

Checking the plug wire resistance is indeed an excellent test for checking for bad or dead wires. Sadly one cannot use this test alone to ensure that your plug wires are 100% fine. Sometimes plug wires exhibit dielectric breakdown which causes ignition missing under a variety of conditions (heat, engine load, etc). Dielectric (insulation) breakdown is a fancy term where the spark plug voltage is no longer insulated from engine ground and "jumps" (arcs) to the closest ground as opposed to going through the spark plug... hence a miss. Resistive measurements cannot check for this, as the voltage used with resistive measurements is far lower than what would cause the insulation breakdown problem.

Cheers,

Sam

 

I had the same problem a number of years ago. On the track at higher RPM the car was back firing and missing. After a lot of digging I found that 3 of the spark plug wires were "open" . The spark was "jumping" across the break/gaps. Installed a new set of wires and had no more problems.

 

I once saw it written here on FChat that you should get your ignition system 100% up to snuff before even *touching* a carb. You can tune, and tune, and tune, and tune, but if the ignition's screwed up, it won't make a lick of difference. Sounds like your experience is a case in point.

Thanks for sharing it with us...

 

Good day Chuck,

Indeed your simple test of dimming the lights and looking for plug arcing can be used. Sadly, this is not overly reliable, as the arcing may occur in areas where you cannot see too well or where the plug wire touches the engine or some other area that is grounded. One visible indicator of breakdown are blackened areas on the plug leads. Other visible clues or possibility of problems are hardened or cracked plug leads or cracked/damaged spark plug extenders. Cracks,etc create paths for the spark plug voltage to arc. Anyway, my original point was that resistive measurements alone cannot guarantee that the plug wire is 100% perfect.

In terms of terminology, "no resistance" means infinite resitance or an open circuit (and obviously zero resistance is a short circuit). "Continuity" is also a relative term, but in automotive circles usually denotes low resistance (i.e. the wires are connected/shorted together). Anyway, yes, spark plug wire does have some resistance and will vary depending upon the materials, wire diameter, and length. In your case, the wire had infinite resistance when measured which is indeed a problem. Usually this is caused by a damaged (kinked, broken, etc) wire or by the spark plug terminal loosening from the wire (at the connector and/or distributer connection). Remember that although the resistance may have been infinite when you were measuring it (low voltage measuring device), it may have had some resistance when the car was running do to the high spark plug voltage possibly arcing "within" the spark plug wire. Obviously this is not optimum and would cause intermittant missing etc. The bottom line is that some resistance should always be measured.

As for the term "dielectric", one cannot use this term interchangably with the term "insulation". By definition, a dielectric is a material that can hold an electrostatic charge, but prevents current flow (insulating qualities). I used the term here, as the spark plug wire's insulation can be thought of (and is) as a dielectric between the spark plug wire and the surrounding grounded engine/parts (a large capacitor if you will). The measurement of a dielectric is called a material's "(relative) dielectric constant" and is defined as the ratio of the dielectric of the material to that of free space (vacuum). There is a more formal definition of this parameter and dielectrics in general, but in the terms of our discussion these definitions will surfice.... But I digress... Dielectric grease is a material that is used between /around electrical connections primarily to prevent moisture and/or oxidation of the electrical connections. By its vary nature it does not allow current to flow through it and so can be used in areas where large voltages occur without concern of it providing an "arc" path (spark plug terminals, etc). Remember that the dielectric grease does not prevent a physical electric connection and so when two connections are mated, the grease is squeezed or pushed out allowing the connection occur. The resulting grease surrounding the connection prevents oxygen (oxidation)/moisture from getting into the connection and thereby potentially increasing the connections resistance... which over time can prevent the circuit from operating correctly. The grease can be used wherever electrical connections are made and are usually in high moisture areas (tail-lights, etc).

I hope I have not went on too much about this, but I wanted to answer your questions thoroughly.

Cheers,

Sam

 

I realize that this may not be the definitve test for whether or not a wire is functioning at it's optimum; However....

I bought this little tool when I saw it a few weeks ago, because it just look like "a good thing to have". As it tured out, I was able to use it yesterday, and I was very glad I had it in my toolbox.

While winding around the seemingly endless spiral in the parking garage at work, my Alfa Spider died. It happened so quickly, that I thought, "It must be an electrical problem", knowing of course that the problem was likely either electrical or fuel related.

Using this little tool, I was able to determine in a a few seconds that my problem was fuel related. Which saved me looking at what would have originally been my first group of things to look at.

So, instead of it taking me a half-hour to determine that my problem was a fautly fuel-pump, I was able to short-cut my search and find the problem in about 5 minutes.

Great little tool, for making sure you have spark. And when I saw this thread, I figured I might as well share...
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A company I worked for was doing some developement work on static engines and the guy from champion gave us all one of those. Great bit of kit, saved me loads of time and effort