OutdoorKing Repair Forum Forums General Discussion Forum Off Topics Testing for spark

I have always used my timing light so I can test for spark. It only has an LED in it so it doesn't take much for it to light up It gives me an indication if the coil is dead or not but what I want is some tester that will give me an indication if a spark plug is working under compression. I haven't seen anything like this but I am wondering if anything out there that might do the job?

Hi Norm,

Yes you can get a spark plug tester but it's expensive compared with a new spark plug.


I was more looking into getting a Tachometer that also checks coil output on a motor.

the GTC GTC505 Engine Ignition Analyzer tests coil spark output on both cranking (non-running) and running engines, and it functions as a tachometer by displaying real-time RPM

Cheers
Max.

Thanks Max,
That is a plug tester, I am looking to see if the spark from the coil is breaking down when the plug is under compression if there is such an item

No problem Norm ,it was just the bit I read about you wanted to know if the spark plug had spark under compression but
you want to know if the coil has spark under load.

As you know there are coil testers under load but again expensive unless second hand ,I saw a mower shop selling a second hand tester for $300. One thing with this type you remove the coil to test.

The second half of my first answer is the only type I know of that's not too expensive and will test coil's output.


Below was a quick search so I don't know how accurate prices are , most likely us dollars.

GTC TA100 purchase options
General Technologies Corp Smartach+ Digital Tachometer And Ignition Analyzer - GTC-TA100
$174.22
·
www.garageandfab.com favicon
Garage And Fab

Official product page with specs and links to purchase the TA100 Smartach+ wireless ignition analyzer and tachometer.
Smartach+ Digital Tachometer and Engine Analyzer
$172.99
·
·
FCP Euro

Retail listing for the TA100 Smartach+ digital tachometer and engine analyzer, typically around a bit over 200 USD.
General Technologies Corp Smartach+ Digital Tachometer And Ignition Analyzer - GTC-TA100
$174.22
·
www.garageandfab.com favicon
Garage And Fab

Another retailer carrying the TA100 Smartach+ ignition analyzer and tachometer, useful if you want to compare price and shipping.
More advanced analyzers
General Technologies Corp Smartach+Copen Multisystem Ignition Analyzer - GTC-TA500
$209.91
·
www.garageandfab.com favicon
Garage And Fab



A TA100-style ignition secondary voltage tester will show a weaker or failing coil as a lower kV reading under the same engine speed and test conditions, but it can’t perfectly simulate every “under load” situation.
How the TA100 works

The tester senses the ignition system’s secondary voltage while the engine is running, then displays approximate peak kV the coil is producing on each lead.

If one coil or cylinder consistently shows noticeably lower kV than the others at the same RPM, that can indicate a weak coil, internal breakdown, or another issue in that cylinder’s secondary path (lead, plug, or plug gap) .or just compare reading to a good known coil

​

“Breaking down under load” question

A coil that is breaking down only when cylinder pressure is high (heavy acceleration, heavy load, high RPM) will usually produce less usable secondary voltage and may misfire, which often shows up as lower kV on a running ignition tester compared with a healthy coil.

​

However, if you only check at idle or low load, the coil might still look “okay,” so it is important to compare readings at higher RPM or under as much load as is safely practical for the test.

​
As we know a weak or breaking-down coil will generally show lower output

An oscilloscope can show the coil output on a lawn mower, but you must connect and set it up correctly for high‑voltage ignition signals.
What the scope can show

On a lawn mower with a magneto/ignition coil, the scope can display the primary voltage waveform (low‑voltage side) and, with the right pickup, the secondary spark waveform that reflects coil output and firing behavior.

​
From the waveform you can see if the coil is firing consistently, if the spark duration is normal, and sometimes spot issues like shorted turns or insulation breakdown inside the coil.
​

How it is typically connected

For primary side: connect the scope across the coil’s primary (kill/trigger lead to ground) to view the low‑voltage pulse that drives the coil; this is within a safe voltage range for most scopes.

​For secondary side: you do not connect the probe directly to the spark plug lead; instead you use an inductive or capacitive pickup (few turns of wire wrapped around the plug lead or a dedicated clip‑on HV probe) so the scope sees only a small, safe sample of the kV pulse.
​

​Practical tips and cautions

Always use a ×10 probe and a non‑contact pickup for the spark lead to protect the scope from tens of kilovolts.

​If you are mainly diagnosing “does it spark and is the coil good,” an inline spark tester or basic coil tester is simpler; the oscilloscope becomes really useful when you want to study the exact ignition pattern or compare a suspect coil to a known‑good one.


Cheers
Max.

My method of testing a questionable spark plug is to put it into a motor that runs well and see how it goes. Of course don't put a long reach plug into a motor that only takes a short reach plug :-(

Simplistic, you betcha, but works for me Norm.

Hey NormK is another way find out if the coil is working. Have your wife or girl friend hold the plug wire while you crank the engine. If hers eyes light and you get smack you would know the coil is working.

Just kidding though....as that is a mean idea.

Hi AVB,
As we have been married 57 years she is now a wake up to me. Might have got away with it some years ago but not now

Hi MM, I'm not wanting to test the plug, I am wanting to test the coil under load

Thanks Max, I will read through it slowly and see if I can make sense of it

You might find this a good tool for testing spark under load and it is only $33.40.

So she done got wise to your tricks.

Usually the young trusting ones that fall for this trick.

It sorta like holding onto an electric fence wire and getting someone to touch you. They only fall for it once. That was a fun thing when I was a kid. Of course I had on rubber boots.
.

Or the old charged up condenser trick that is pulled on new mechanics.

Bit like my old man when we lived on the farm and a calf kept getting through the fence, old man picked it up in his arms and shoved it nose onto the wire. Problem was the calf didn't have its feet on the ground but the old man did. Mum couldn't stop laughing

Yes your Dad took hit instead of the calf. That is the strange thing about electricity. It is whatever farthermost away seems to always takes the hit first on power surges. In your Dad's case whatever closest to ground potential.

I once had a laptop modem to take a hit even it had to go thru my computer first when there was a lightning five away at the phone company's hub building. I was just checking after hours for the next day service calls and to upload daily reports. Good thing I had copy of the company's software on the home computer as it took a week to get it thru to my area's manager that I needed a new modem.

Now that neon tester shown it good to indicate a spark is present just won't show the strength of the spark. This is where a spark gap type is needed. I several Kawasaki that showed a spark but the overall strength so weak that it couldn't fire the plugs under the compression load.

You could get one of these pictured and adjust spark gap while engine is running.