Possible check spark without removing or disconnecting plugs?

On a couple of my cars, manifold covers plugs, so it’s impossible to disconnect electric connectors to each plug without rigamarole of removing manifold. Is there an electric induction tool that could tell me the sparking status by holding it close to each wire going to each plug while running? Will a good OBDii device tell me sparking status of each?

If there’s no Check Engine light with misfire codes, then the sparking status is good.



About 50 years ago I bought one of those testers from jc Whitney. Just hook it on the wire and look to see if the light in the window lights up. Of limited use though for identifying which wire is the problem since it picks up pulses from adjacent wires. Gotta believe they are still out there somewhere. I’m not sure where it is but bought a better testera few years ago.

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Hi Tester & Bing–
Tester, I figured a misfire code, e.g. p0302, could indicate either a valve prob, a lack of spark, or poss some other probs too. Is that the case? If so, I’d like to be able to narrow down the diagnosis to plugs.

Bing-- I had an induction electrical tool like that for home electrical work, but I was just wondering if there was sth like that for 12v car use. I have searched sites like OReilly w no success. Maybe I don’t know the proper name of the tool.

Voltage to the spark plugs can be as high as 100,000 volts.



Hi Tester-- yes that tool would likely do the job if I could reach its “antenna” behind the manifold. (I’d have to experiment reaching other stuff back there first).

Re voltage in wires going to coils, I think that’s still 12v, no? I think the coil then coverts it to the higher voltage at the spark gap.

The voltage to the coils is 12 volts while cranking the engine over, but the computer may drop the voltage below that once the engine starts.


The spark gap conducts considerable current when the plug fires. The current creates a strong magnetic field. I’ve wrapped a couple loops of thin wire around the spark plug wire and connected my o-scope to view the induced voltage on the two ends of the thin wire. Very easy to see on the o-scope trace. This is the basis for how the old Sun ignition diagnostic machine worked I think.

Most diy’er don’t own an o-scope, but used ones can usually be found, not overly expensive. I found mine at a flea market, $50. I’ve seen them on Craiglist in that price range. O-scopes for auto use don’t need to boast fancy specifications. There’s definitely a market for small, portable, inexpensive, easy to use o-scopes for automobile diagnostics , but for some reason vendors aren’t stepping up to the plate. I presume most would-be vendors believe it would take too much training time. Plus I’m guessing a Tech 2 (pro level) scan tool already has similar functionality.

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Just google automotive spark tester. Under $10. I’ve only used mine on cars and the mower, not house wiring. I have a separate one for that.

Current to the spark plugs is extremely low.

That’s why you can grab a plug wire and get jolted so it knocks you on your ass.

But it’s not going to fry you because there’s very little current.

It’s the high voltage that creates the spark.


Depends on the def’n of “large” I guess.

From the Denso website:

“At the start of the discharge, the spark is generated by the electrical energy stored in the secondary circuit. The current is large but the duration is short.”

Electric current refers to the flow of electricity in an electronic circuit, and to the amount of electricity flowing through a circuit. It is measured in amperes (A). The larger the value in amperes, the more electricity is flowing in the circuit.


Hi George–that’s a great macgyver, looping small test wires around the wires to the coil/plugs and reading with the o-scope. If a particular spark/coil combo weren’t doing their job, then there would there be just no induction possible in that test wire, correct?

Yes, I’ve seen o-scopes in Goodwill at least a few times–never thought to buy one till now.


It depends on the ignition system configuration. Coil on plug is different than coil/distributor. The testing I described applies to both my cars, which are coil/distributor. Definitely a brief very healthy current flow there, very easy to capture using just a couple of loops of wires. (The Sun system didn’t require wrapping a pickup wire around the spark plug wire, it used special magnetic material inductance probes that spring-clamped over the wire.)

With coil on plugs, if the signal between the ignition module and the coil is 12 volt range, the average current at that part of the circuit in the coil on plug configuration would be considerably higher than the coil/distributor configuration, but not sure about the peak current. Inductance method only works if current is changing. Wrappping wire technique wouldn’t capture a signal for a constant current.

Have I made anyone mad yet today? On my go cart I managed to grab the uninsulated part of the plug wire a couple times. Yeah it won’t kill you but it is a very unpleasant experience as your whole arm gets zapped up to your shoulder as you try to let go. I’ve zapped myself a couple time trimming wallpaper around outlets with a box cutter too, but that was nothing like a spark plug wire. I always screw the screws down on outlets but don’t tape them. Of course most were done by real electricians that don’t do either.

Always fun times when you get hit by your MSD (multiple spark discharge) ignition system with a street/strip coil that produces higher spark energy… It will light you up… lol