Spark plug wire resistance

[Graham Archer]

I'm chasing a very poor running condition and have a question about the resistance of my 10 year old spark plug wires (83 braided beru). From the cap end of the wire to the connection with the spark plug, they all measure about 3.8 k-ohms. The connectors alone measure 2.9 k-ohms. The wires alone measure 0.9 k-ohms. The spec listed in the Bentley manual is 3 k-ohms from "plug connector to distributor cap end". I assume my measurment of 3.8 compares poorly to the 3.0 spec. Am I comparing measurments correctly? Is this bad enough to cause an idle so bad that the car will only sputter for 10-20 seconds and then die?

Thanks,

[Early_S_Man]

Graham,

If you unscrew the distributor ends from your wires, I believe you will find that the distributor connectors are measuring the 900 Ohms ... they are nominally 1K Ohms. I believe the Bentley writer and editior didn't verify the correct resistance of the SC wire's components when they re-used most of the text and pics from the Carrera 3.2 manual in compiling the SC manual!

I don't believe the sputtering-and-die problem is due to your spark plug wires. I do believe you should replace the 'wire' part of the cables with new, solid-core copper 7 mm bulk cabling ... the braided shields need not be refitted to your wires. New M3 crimp-on connectors need to be purchased and attached to the bulk cable ... Mercedes dealerships have the connectors and will probably crimp them for you, free. The following document has details of the M3 crimp connectors and how the cables are assembled:

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

What condition is the green coax signal cable coming out of the distributor? If it is discolored dark brown or has cracked insulation ... it needs to be replaced!

Here is a page with more information about your ignition and CDI system:

6-pin SC/Turbo CDI unit repair documentation

[Graham Archer]

Warren,

Thanks for the reply. The "green" wire coming out of the distributor only looks green for first couple of inches; the remainder of the wire is brown. The insulation seems intact. Do you think I should replace it anyways? Can you give an idea of how to replace it and where it can be obtained (I assume it is not just a typical wire).

Thanks,

[Graham Archer]

Warren,

Is this green wire the $61.25 one listed as 930-602-907-01-M14? Please say it isn't. That about $10/inch.

[Early_S_Man]

Yes, it is. Sorry.

[RoninLB]

Originally posted by Early_S_Man
. The following document has details of the M3 crimp connectors and how the cables are assembled:

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

very nice

[stlrj]

Checking the ignition wire resistance will not tell you anyting about the dielectiic strength of the insulation that would keep them from shorting...so what's the point?

[Graham Archer]

Is there any other test for the wires?

[TerryH]

Checking resistance can tell you if there's a broken wire.

Have you ruled out a clogged fuel injector(s)?

[stlrj]

http://tpub.com/doeelecscience/electricalscience2172.htm

[911pcars]

"Is there any other test for the wires?"

Along the lines of what stirj suggested:
"Checking the ignition wire resistance will not tell you anyting about the dielectiic strength of the insulation that would keep them from shorting...so what's the point?"

Graham,
Here's one home remedy. Use a test wire with alligator clamps on each end and a well-insulated, long-handled screwdriver. Clip one end of the test lead to ground. Clip the other end to the metal shank of the screwdriver.

Start engine. At various rpm levels, pass the tip of the screwdriver along the length of each spark plug wire. If the insulation is weak, a spark or sparks will jump to ground via the test wire. Avoid using your finger in lieu of a screwdriver or a screwdriver with a metal cap on the end that's probably contacting the end of the screwdriver blade inside the handle; otherwise your future offspring might be "different". :-)

Some ignition systems will exhibit a light show in a darkened garage without the grounding gear.

Sherwood





__________________
Graham

[Graham Archer]

A question about this "green" signal wire. I get 590 ohms across its terminals of the connector to the CDI. When cranking I only get 0.5 volts. From previous posts, it appears that I should get over 1 volt. Is this an indication of bad wire?

BTW, the green wire does not appear to have a connector between the distributor and the CDI. Is this normal? (I have an 83) The wire does not look like the picture shown as 930-602-907-01-M14.

[SCWDP911]

sub'd

[rallyracer]

Originally Posted by Graham Archer

BTW, the green wire does not appear to have a connector between the distributor and the CDI. Is this normal? (I have an 83) The wire does not look like the picture shown as 930-602-907-01-M14.

I would love to know an answer to this. Why does the Pelican PET show a plug on the CDI end? My wire is brown for about 5 inches from the distributor, then green to the wire harness. Sounds like I should replace it, but how to connect?

Graham, FWIW (old thread, same info need), my Beru wires also read 3.71-4.17 kohms, and 1.13 for the coil wire, and the car does not run well. I have no idea if it's related, but process of elimination is starting.

Thanks,
David

[stlrj]

The best test remains the simplest and safest.

Spray some water on the wires while the engine is at idle and you'll quickly find where they short.


Joe

[Kraftwerk]

The oem wires are clad in mesh wire or what reason exactly? To ground the radio or something?
so they fail better?

[RWebb]

RFI supression -back when P AG thought that was causing motor problems...

[Early_S_Man]

Graham,

All of your questions about the the Green coax signal cable should be answered in the following thread:

coil to buy question

As far as the meter readings from the distributor magnetic pickup coil ... they sound fine. 590 Ohms is a good value ... easily within the 600 Ohm +/- 10% spec. at room temperature. The 0.5 Volt reading means it is generating a signal ... different meters can give readings all over the place on that waveform which isn't a sine wave, so average responding meters tend to read low, where my true-RMS meter would give a higher reading.