Testing spark plug wires with a multimeter (ohms)
 

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!

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... :)

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?

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

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!

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.

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.

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

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

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

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

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

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. :(

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.

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.

Wayne,
Glad I could help. :)

Just don't call me Shirley (Airplane, 1980).
<iframe width="420" height="315" src="https://www.youtube.com/embed/0A5t5_O8hdA" frameborder="0" allowfullscreen></iframe>

Fixed it. Sorry bout that.

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.

Since I'm old and look it, I prefer not being replaced even though everything sorta works. There's a test for leaking SP wire insulation, so I personally wouldn't replace it solely on the basis of years in service (an admitted variable). My silicone-jacketed SS SP wires were firing on all cylinders for over 50 years up to the day I sold them and the engine they were attached to.

New ***** malfunctions too even though newness can also be deceiving. It can also get expensive depending on one's perception of oldness. And driving a car that's 50+ years old can present a conundrum of sorts. "Hmm, everything's old. Should I replace the front control arms this week?"

Of course, you car and YMMV.

Sherwood

A better test is done with a spray bottle at idle. Unmistakeable miss when you hit the bad part of the wire or cap.

That's a good test to confirm the insulating jacket is doing its job. However, that test doesn't confirm nor identify which wire is causing misfires due to a bad/poor conductor path.

As for resistance values, remember that you probably have resistor spark plugs in each hole which adds about 3000 ohms to each spark path. You only need enough resistance to mitigate any RFI (radio frequency interference) that may disrupt vehicle electronics. More resistance (including an open circuit) does nothing except reduce available/potential voltage to jump the spark gap.

Sherwood

Unfortunately, the reason ignition cables fail is due to failure of the insulation, not a failure of the copper or resistor to conduct. The fact that a miss is detected only confirms the conductor has found the nearest most convenient ground prior to reaching the spark plug.

Spark plug conductors are manufactured from different materials to provide the correct radio suppression requirements. Some are more robust than others, and their subsequent handling and operating conditions are variable factors in their lifespan. To state that all SP wire failure is due to poor insulation is to discount failure of the conductor within.

The symptom you describe can be caused by a failure of the insulation jacket. However, it can also happen when an open circuit develops due to a conductor failure. At that time, high voltage current seeks the least path of resistance to ground. Thus, your observance of current jumping to ground somewhere along the wire path. What happens first, the chicken or the egg; open circuit or insulation failure?

Sherwood

One more variable to add.

If you are using 3K ohm Beru connectors, they can also go bad. They have a series of resistors inside that can lose continuity. When I would test my connectors, I always would shake/tap on the connector body to see if there was a loss of continuity. Sure enough, I found several over the years that would register "infinity" ohms when vibrated. It is one more thing to check before deciding if a wire/connector combination is "good."

Indeed, the innards of the connectors can go bad. This connector is of the machine screw thread kind - the crimped end of the plug wire is screwed into the metal threaded part of the top of the connector. This one pulled apart as I was pulling it off its spark plug - never had one grip that hard. So it has two resistors, held against eachother and the ends by the spring, with the aluminum rod in between. Lots of places for some extra resistance to develop.

The older style connector has what looks like a small wood screw sticking out. You screw the bare end of a plug wire into that screw, which contacts the center conductor of the wire.

I get 2.3K ohms from one of these resistors, and can't get anything from the other, though the plug was firing just fine.
http://forums.pelicanparts.com/uploa...1428281188.jpg

The open circuit is always present in the form of the spark plug, so chicken and egg do not apply.