Spark plug resistance (not resistor spark plugs)

[hzbloke]

I've been spending a lot of time with a multimeter in my hand lately while I wait for my new coil to arrive (Brown - thanks for asking Steve). There's a thread about that here: http://boxerworks.com/forum2/viewtopic.php?f=1&t=4342

I've checked and rechecked every wire and connection I can find and everything seems fine. Having checked the resistance of everything in the shed (gumboots, shovels etc) I finally got around to checking the spark plugs. The resistance on the centre electrode of new ones is next to nil (.5ohm), the resistance of my new 5k caps is about 5,500ohm. The resistance on the old plugs is 650k and over 1000k (yep, a million ohms). What's going on here?

The plugs are NGK BP7ES and have done about 10,000km. They look fine. As they are not resistor plugs I assume the centre electrode is just a lump of metal. Maybe it has a copper core but basically it's just like a 2" nail in an insulator isn't it? So how can it have such massive resistance? I checked some other old plugs and found some were zero and others were also showing very high resistance. I ask again: What's going on here?

[Duane Ausherman]

They can develope open spots in the conductor, which is infinite, however they will still work just fine. I have never pulled one apart to see what happens inside.

To measure with a multimeter is using only a very low voltage. The ignition voltage is many thousands of volts. Your multimeter can show you a false result due to using the wrong voltage.

[ME 109]

I ask again: What's going on here?

I don't know.

But apparently, resistance is futile.

[barryh]

I've had the similar results measuring a whole bunch of non resistor plugs and got anything from zero ohms up to infinity (open circuit). I assumed the same as Duane that in the case of the open circuit plugs there is a break in the central conductor. The spark will just jump the gap no problem. At one time I'm sure some plugs were purposely designed with an internal gap. The idea being that if you have a fouled plug this secondary gap prevents the plug from firing until a high enough voltage is reached to clear the plug gap proper.

You could try this (very carefully) which amounts to the same thing.

Remove the plug cap and put a spare non resistor plug in the cap. Hold the earth electrode of the spare plug against the top of the plug still in the head and fire up the engine. It will run just fine with two plugs and spark gaps in series. Obviously there is scope here to damage your ignition if you allow an open circuit so I've also done it by connecting the two plugs with a length of wire terminated by strong crocodile clips. Why would anyone do that ? It's supposed to be a diagnostic method of observing the colour and strength of the spark while the engine is actually running.

Perversely I still like to have plugs that measure zero ohms.

[dougie]

But apparently, resistance is futile.

Ageed.
Resistance would be futile.

seven 5.jpg

[hzbloke]

Thanks for your opinions but I'm just as confused as ever. I can see that the coil is going to throw out vastly more volts than a multimeter and hence any gap in the electrode will be easily bridged. I'm not at all clear on how that gap came to be there in the first place. Even if the centre electrode melted the metal has nowhere to go. Nonetheless I'm willing to believe that a perfectly good plug can have what appears to be no conductivity.

I thought the 'resistance is futile' line was from some TV show about a Doctor. Who it was I can't remember.

[Duane Ausherman]

It was standard procedure in my shop to vary the distance between the spark plug cap and the plug. We could temporairly fix a dead plug that way. However, the length of spark that one could make jump and color is a great indication of ignition health.

I still have my Snap-On spark plug tool that inserts into the cap and then onto the plug. It has an adjustable gap inside of it. Back then one had to actually know something to do repairs. Of course, now, after these 100 years I have forgotten it all. Now we trust a computer to do the job and assume that the program is written correctly. The computer won't forget. So, which is better. I vote "computer" every time over my brain.

[Major Softie]

It was standard procedure in my shop to vary the distance between the spark plug cap and the plug. We could temporairly fix a dead plug that way. However, the length of spark that one could make jump and color is a great indication of ignition health.

I still have my Snap-On spark plug tool that inserts into the cap and then onto the plug. It has an adjustable gap inside of it. Back then one had to actually know something to do repairs. Of course, now, after these 100 years I have forgotten it all. Now we trust a computer to do the job and assume that the program is written correctly. The computer won't forget. So, which is better. I vote "computer" every time over my brain.

The computer almost always does a better job at what it is designed to do. When one runs up against something it was not designed to do, it then becomes completely useless. Works perfectly, or not at all.

Kinda digital, that.

[barryh]

I thought the 'resistance is futile' line was from some TV show about a Doctor. Who it was I can't remember.

Not sure which came first, The Borg in Startrek or the Vogons in Hitch Hikers Guide to the Galaxy. I much prefer the Vogons.

[Steve in Golden]

But apparently, resistance is futile.

Ageed.
Resistance would be futile.

In her case, there would be no resistance. Except probably, on her part.

So attachments are working again?