Use Scope to Measure Spark Plug Wire

[bostonman]

Long story short, I have an older car with a cylinder not firing (according to the code). Besides using a tester, I'm wondering if I can use a scope to pick up the magnetic field on the external part of the spark plug wire to see if it's firing, when it's firing, and compare the amplitude to a good one.

Due to the high voltage, I'm fully aware I can't jab the scope probe into the insulation to touch the bare wire (ignoring the fact I'll damage the spark plug wire).

I think years ago I tried wrapping a small wire around as a way to pick up magnetic fields, but didn't have much luck.

[PartialDischarge]

Do the following:

Ground of the scope should be wired to the ground of the engine.
Wrap around the spark plug wire some insulation to make the diameter of the cable thicker (maybe 1/2 inch)
Around that insulation wrap some copper tape (1 turn) and connect that copper to the x10 oscilloscope probe. This creates a capacitive sensor so as to pick the electric field not the magnetic. If you don't have copper tape use a cable instead, but again 1 turn.

On the scope you will measure a voltage proportional to the plug voltage. You can compare among different plugs this way.

[bostonman]

Wrap around the spark plug wire some insulation to make the diameter of the cable thicker

What type of insulation? I have copper tape, so I'm halfway there.

[PartialDischarge]

What type of insulation? I have copper tape, so I'm halfway there.

I've done this in the past and the problem "may" be that if you put copper tape directly on the outside of the cable the coupling capacitance is going to be too high so it will saturate the x10 probe (easily +300V generated). So either you use a x100 probe of decrease the capacitance by making the diameter higher. Any plastic insulation that you like

[CaptDon]

Wrapping about five turns of wire around the plug wire does work and be sure one end of the wire is grounded at the scopes BNC connector or through the probe ground lead. A floating wire connection around the plug wire will likely damage the scope or probe especially if the wire arcs into the probe from poor insulation. You will only see a waveform based on current flow using the magnetic loop coupling. Many years back there were special adapters made to go in series with the plug wire and they were something like 1000:1 voltage probes with compensation. The spark waveform can be very complex and fast so the adapter required good high frequency compensation. One of the stupidest 'mis-fire' codes that kept coming up was on a Dodge van that had a weak 'in-tank' fuel pump. Below 1/2 tank of fuel the same two cylinders kept showing mis-fire faults!!! Weird huh??? I could get a lot of good information from the old 'kettering' ignitions with a scope. The spark voltage peak before arc told many stories about fuel mixture and vacuum leaks. If you had time you could verify distributor cap cracks and arcing and also find weak condensors that would test o.k. for 'capacitance' but back then they didn't test for ESR and high condensor ESR would really screw up the sparks during idle and low load conditions!!! The car would idle like crap but still run fine at W.O.T.!! Cheers!!

[bostonman]

Years back (and the same car) I wanted to see the fuel injector PWM waveform, but didn't have an easy way to access the wire without cutting or poking.

I'm not trying to change the topic to a car repair discussion, but just for a bit more information, it's a 1997 with three coil packs. I believe it works off the principle of "wasted spark" where one plug fires anode to cathode, the other at the same time but opposite direction.

A bullet list of possibilities exist and I'm aware of them. If I don't have to, I probably won't resort to measuring the spark plug wire to avoid any possible damage to a scope, but wanted to know a decent process just in case I need to (although I believe at one point I may have asked a similar question but don't remember).

[NiHaoMike]

Use an electric field probe for that, can be DIYed out of a piece of coax.

[Benta]

Far too elaborate.
Just wrap a few turns of stiff wire around the ignition cable and connect the 'scope probe point/clip to it. Set the probe to x10. Ground the probe as well.
Done.

[jonpaul]

An HV engineering Safe and Simple solution..A . small neon lamp on end of long plastic rod.
Risk zero, cost  a few €£$

Scope is at risk in all these other suggestions!!! Beware  as the fast rise nS range, of a 20..35 kV ignition pulse can couple across even 1 pF to the scope input.

A 1000x probe like Tektronix P6015/A is one safe way.

Finally old ignition Analyzers can be used from 1960s..1970s like HeathKits



Bon courage

Jon

[tautech]

Long story short, I have an older car with a cylinder not firing (according to the code). Besides using a tester, I'm wondering if I can use a scope to pick up the magnetic field on the external part of the spark plug wire to see if it's firing, when it's firing, and compare the amplitude to a good one.

Due to the high voltage, I'm fully aware I can't jab the scope probe into the insulation to touch the bare wire (ignoring the fact I'll damage the spark plug wire).

I think years ago I tried wrapping a small wire around as a way to pick up magnetic fields, but didn't have much luck.

I routinely check 10k+ max revs in chainsaws with a scope by just clipping the reference lead into the probe grabber and holding it near the ignition lead but a 4 stroke engine at idle is entirely another beast with an ignition cycle so slow you need a DSO to see it !
600 RPM = 5Hz/ignition cycle in a 4 stroke.

[CaptDon]

The DSO mentioned for idle speed on a 4-cycle is nice. I believe the old sunscopes had both the P7 'radar' phosphor C.R.T.'s with the yellow tinted plexiglass in front of the tube and later the P2 radar phosphor with the green screen. Both are very slow phosphors and were excellent for detecting abnormalities as you could see the misfire trace integrated with the normal trace in the persistence afterglow of the phosphor. Those old Sun distributor machines and SunScopes are still highly sought after by street-rod motorheads and folks building budget racers or fully 'period correct' vintage vehicles.
 

[pardo-bsso]

Dumb question (and maybe I misunderstood your problem)

can you swap cables with other cylinder/coil to rule out the cable being bad?
After that, if you let the failing cylinder cable dangling near the chasis (or with another plug) do you get spark?

[bostonman]

As for trying to repair the car, yes, that's actually a game plan, and good advice.

About why I started the thread: I wanted to find out if a safe and easy way exists to measure using an oscilloscope. From what I gather, it can be done, but I'm also a bit reluctant to try since a possibility exists I can damage the scope. Although I think damaging is small from what I read, the methods to measure can be done, but I may pick up too much noise to determine with assurance that the plug isn't getting spark

[tautech]

As for trying to repair the car, yes, that's actually a game plan, and good advice.

About why I started the thread: I wanted to find out if a safe and easy way exists to measure using an oscilloscope. From what I gather, it can be done, but I'm also a bit reluctant to try since a possibility exists I can damage the scope. Although I think damaging is small from what I read, the methods to measure can be done, but I may pick up too much noise to determine with assurance that the plug isn't getting spark

Depends of the scope you have and how low its frequency counter will give a valid reading.
Using any ordinary probe configured as a near field probe as an non-contact RF loop will just work detecting ignition pulses.