Testing spark plug wires with a multimeter (ohms)

[911pcars]

Originally Posted by 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.

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

[stlrj]

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!

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

[911pcars]

Originally Posted by stlrj

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

[stlrj]

Originally Posted by 911pcars

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.
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.

[911pcars]

Originally Posted by stlrj

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

[ossiblue]

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."

[Walt Fricke]

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.

[stlrj]

Originally Posted by 911pcars

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

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