Testing spark plug wires with a multimeter (ohms)

[EA911]

Ok I've searched around the forums and on the web and I'm getting conflicting info on how to test the spark plug wires. I know there are other more definitive ways to test the wires but I wanted to just do a quick ohm test.

This is what I did but please educate me:

Set mutimeter to ohm or Ω setting at 20K
Connect the red meter cord to one end of the wire and the black meter cord on the other end
Reading is 1.56 (wire is about 1.5ft)

So I'm assuming this is good since it has low resistance but I'm seeing conflicting articles on the web that say it should be reading between 6-8.

So whats right? or is my wire ok base on this quick check?

Thanks!

[timmy2]

My memory tells me they should all read 4k ohms with boot attached for factory Beru wires. 3k on wire 1k for boot.
My memory is failing, reverse theose numbers...

[911Freak]

Sub'd I'd like to know definitively as well. There has to be a variance between plug wire lengths and the acceptable ohm range, no?

[manbridge 74]

The standard has generally been 1000 ohms per foot for non resistor wires. Not sure on Beru as I've never checked.

[EA911]

I have the Beru wires and I'm pretty sure they've been changed out since new. Can't imagine the wires lasting that long.

Ok rookie question: is the beru a non resistor wire?

Thanks!

[john walker's workshop]

the steel core wire won't give much resistance. you need to take the ends off and measure them separately. 3K is normal for the plug ends and depending on which distributor end you have, it's generally no resistance or 1K. later wires have a crimp on end that threads into the connector and it can get stuck and lead to wire damage if you're not careful getting it loose.

[EA911]

Thanks - I have a 1969 911T and I switched out the distributor for a bosch.
I'm trying to understand what you mean by 3K for plug ends - the way I'm measuring is removing #1 wire and testing the continuity. and its coming out to 1.56 with the multimeter set at 20K.

[john walker's workshop]

3000 ohms resistance. (3K). some of the early cars had black connectors that measured 1K. the brown ones are 3k.

[EA911]

ok - the connectors I have are gold in color so they should be at 3K.

[Hugh R]

Why so much resistance? To raise the voltage? IIRC, on Lucas copper plug wires, I'd get like one ohm or something.

[wwest]

Originally Posted by EA911

Ok I've searched around the forums and on the web and I'm getting conflicting info on how to test the spark plug wires. I know there are other more definitive ways to test the wires but I wanted to just do a quick ohm test.

This is what I did but please educate me:

Set mutimeter to ohm or Ω setting at 20K
Connect the red meter cord to one end of the wire and the black meter cord on the other end
Reading is 1.56 (wire is about 1.5ft)

So I'm assuming this is good since it has low resistance but I'm seeing conflicting articles on the web that say it should be reading between 6-8.

So whats right? or is my wire ok base on this quick check?

Thanks!

The difference is the result of availability of nichrome wire resistance vs carbon cranules.

[EA911]

Are the Beru wires nichrome or carbon cranules?

So far I know they are standard non braided steel wires so they are nichrome? which means 1.56ohms is good?

Cheers!

[911pcars]

Originally Posted by EA911

Are the Beru wires nichrome or carbon cranules?

So far I know they are standard non braided steel wires so they are nichrome? which means 1.56ohms is good?

Cheers!

Or just steel, plated steel or stainless steel plug wire conductors. Early 911s don't have electronics (other than a radio) that may be compromised by RFI caused by these wire conductors.

You may also want to check the resistance from each spark plug connector to the appropriate copper terminal under the distributor cap. Corrosion and/or a loose connection can increase the resistance to a threshold that creates borderline misfires.

Sherwood

[EA911]

So here is the latest. I tested all spark plug wires with the following results:

#1: 1.52
#2: 1.40
#3: 1.58
#4: 1.50
#5: Nothing - did not register
#6: Nothing - did not register

Looks like #5 and #6 are dead as the meter did not register anything. Basically no continuity.

[Oh Haha]

Ok, please help me understand. The SC is running like crap.

I am down to checking the plug wires.

Meter set to 20k on the ohm setting.
I tested the wires w both connector ends attached and come up with readings of 4.15 for cyl 2 which is about 1.5 ft long.
Is this good or bad?

Wires are Beru shielded.

[911pcars]

Depends on the knob setting on your meter.
Analog or digital?
I assume different resistance range settings
If "20K" is the measurement range, 4.15 could be 4000 ohms. Plug wire is fine.
If "20K" is a multiplier, 4.15 could be 80K ohms. Plug wire is NG
How about the other plug wires? That would give you some reference point. If the readings are more or less consistent on all, either all are OK or all decided to die in unison (not likely unless subject to a common fire/barbecue).

What is your definition of "running like crap"? While emotionally understandable, it's not too descriptive for potential hints other than "replace everything". Describing symptoms would be easier to diagnose (E.g. at idle, under load, cold vs hot, backfires, misfires, no power at high rpm, etc.).

Sherwood
(not that I could assist even with all that info)

[Oh Haha]

Originally Posted by 911pcars

Depends on the knob setting on your meter.
Analog or digital?
I assume different resistance range settings
If "20K" is the measurement range, 4.15 could be 4000 ohms. Plug wire is fine.
If "20K" is a multiplier, 4.15 could be 80K ohms. Plug wire is NG
How about the other plug wires? That would give you some reference point. If the readings are more or less consistent on all, either all are OK or all decided to die in unison (not likely unless subject to a common fire/barbecue).

What is your definition of "running like crap"? While emotionally understandable, it's not too descriptive for potential hints other than "replace everything". Describing symptoms would be easier to diagnose (E.g. at idle, under load, cold vs hot, backfires, misfires, no power at high rpm, etc.).

Sherwood
(not that I could assist even with all that info)

Thanks Sherwood. I've asked the same of posters(running like crap comment) so I should know better.

I was trying to diagnose my SC which was down on power and rough running, like not banging on all cylinders. I was to the point of checking the ignition system and I had never checked my wires in the years I owned the car. It turns out the issue was a clogged fuel injector and old fuel. She's running great again.

[911pcars]

Wayne,
Glad I could help.

Just don't call me Shirley (Airplane, 1980).

[Oh Haha]

Fixed it. Sorry bout that.

[DanielDudley]

Testing spark plug wires can tell you if they are bad, but it won't absolutely prove that they are good, as the insulation can break down, and arcing can occur on an otherwise ''good'' wire.

If your wires look old, an you have never replaced them, do so. Likewise with the cap and rotor.