how to test plug wires w/ ohm meter?

[b. hilliard]

Hi-
I have a cylinder #3 spark plug that seems foul over time and I suspect the plug wires. I'm not sure how many miles the wires have on them, but i've put about 40k on..
I bought an ohm meter, but I don't know what to do from here!
Can anyone help?
Thanks, Brian

[Howard M]

I assume you have resistance wires of some sort (carbon core or wire spiral). Resistance should be around 10,000 ohms + or - 2000. Put the meter on the X1000 scale.

[coloradoporsche]

An ohmmeter will only tell you the resistance.

Unlike most wires, plug wires are normally designed to have a lot of resistance.


EDIT: Howard is probably right. Ignore my advice regarding 1ohm (deleted).

http://www.inct.net/~autotips/plugwire.htm


The meter will not tell you if you have a problem with the insulation that could be resulting in shorting/grounding to another part of the car or from wire to wire.

good luck.

[b. hilliard]

The wires are the braided and grounded type, but I'm not sure of the internal makeup.
Pardon my ignorance, but do I pull the wires off to test the resistance from one end to the other? Electrical issues are NOT my strongpoint...

[911mot]

pull the lead from the sparkplug and the corresponding end from the disributor cap. Now take the ohmeter probes and put one in each end of the lead, making sure they are touching the metal contacts. Make a note of the resistance. Now do the same to the other leads and see how they compare

Edit: replace wires back in cap between tests - it's easy to mess up the firing order otherwise

[Bill Verburg]

The oem s/s sheathed wire will have a solid core w/ almost no resistance. The ends however have a resistance built into them.

I always tested by holdin an end ~3mm from a ground and looking for a fat blue spark.

Spec end to end including the resistance in the connectors is 3kW

[ZOA NOM]

An ohmmeter won't tell you much about plug wires. You need to use a Meg-Ohmmeter (Megger), which cranks up a high amount of current to break down any weaknesses in the wire. The current used with an ohmmeter is provided by a 9v battery. It won't tell you if there is a weakness in the wire that will cause problems when 20kv is crammed down it from the coil.

[Early_S_Man]

Brian,

There are two issues with the OEM spark plug cables ... leakage from old cracked wire insulation, and open resistors in the connectors.

You can tell if the wire between the ends needs replacing just by looking for a 'light show' on a running engine on a dark night ... you can see arcing and corona effects [blue glow] at the defective spots in the insulation.

The end-to-end resistance test tels you if the Beru ends are open and causing a miss. They can be intermittent, which is why I recommend a 'shake test' while holding the ends securely to the test leads while shaking the connector sideways.

The ends unscrew from the cables ... and can be reused. New bulk cable can be purchased -- name brand only -- Belden, Packard, Autolite, AC-Delco, Accell, Magnecor ... and can have M3 screw terminals installed ... Porsche pn 911.609.310.00 or Mercedes pn 000.159.1438 can be installed with special crimping tool at Porsche or Mercedes dealership!

Here is a very nice Beru PDF document that illustrates the connector system and how the ends are installed:

http://www.beru.com/download/produkte/TI05_e.pdf

[stlrj]

You could also eliminate all the problems associated with the OEM Beru stranded copper ignition cables and install conventional carbon core igniton wires.

Joe

[RarlyL8]

Great info Warren!!

Question: How long do/did the OEM wires last in the SCs and Carreras? There are stock wires still on cars running today, can they possibly be in good shape?

[RoninLB]

My stock wires lasted over 90k mi and 20yrs in lala land when I replaced them as preventive maintenance.

[TerryH]

An ohm meter can absolutely tell if you have a broken wire/terminal. I bought a used stock braided Beru set in the Pelican classifieds and I ohm'd them out and found one to be broken. Now if the wire is leaking due to poor insulation, then an ohm meter won't help.

Warren's described "shake test" found this culprit and to the sellers credit he offered to take them back or pay to repair. Jeff at Aase Motors in Fullerton CA. fixed it for free.

[stlrj]

Another reason carbon core wires are superior, no corrosion.

[TerryH]

I would imagine the corrosion is a by-product of arcing and not materials involved. The wires were on a car that supposedly ran well and were only changed out when the motor was switched out.

[stlrj]

I would imagine the corrosion is a by-product of arcing and not materials involved.

Corrosion is the result of the use of copper conductors and moisture resulting in the powder greenish copper oxide.

[Early_S_Man]

I have disassembbled 30+ year-old VW and Porsche wires with reusable ends that were as stiff as jerky [old Neoprene/Hypalon gets rigid with heat and age], yet the copper wire and brass screw ends were pristine and shiny ... no corrosion at all! Oil seems to be a preservative against cracking [that causes arcing and corona effect] under the grime on those black Hypalon wires! Of course, this was in West Texas, where moisture in the air is seldom a problem!

[real550A]

While on the subject, are there major advantages to either carbon, copper, or steel core wires on a MFI early car?

[RoninLB]

more entertainment

Plug heat range- Educate me

Originally posted by RoninLB
most ignitions need resistance somewhere for operation. If the system doesn't need this resistance in the plug wire it means the resistance is made up in the coil afaik.
As far as radio noise suppression it can be either the resistance wire or spiral wound. Spiral suppresses by induction and allows max available current flow. There is also an argument around about resistance wire or resistor plugs lowering the frequency of the current thus increasing plug life and conversly no resistance means more current available to prevent plug fouling.