TS2008

Ok. I need to ask this because I didn't find any answer with google search for my question.


My question is why we need spark tester instead of an old spark plug ? For myself, I always check ignition system's health with an old spark plug and I like to know what is the advantage to use a spark tester instead of an old spark plug.


thanks.

ezone

C&P:
The simplest way of measuring high voltages is by their maximum spark length. The maximum spark length is determined by applying the high voltage to a pair of electrodes and bringing the electrodes closer to each other until a spark jumps over.


I'd rewrite the last line a bit for practical use on an engine....if I'm using a movable tester (most of the time it's a simple screwdriver) I start with a small gap and increase to find the maximum distance the spark can jump across.

Yeah I said it. I usually just use screwdrivers and my eyeball ruler to check spark.
I have four ST-125 spark testers (30kV) and an adjustable tester that rarely get used anymore.


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If you only need to know if spark is present, a spark plug laying on the valve cover might be adequate, but that alone cannot stress the ignition coil to find its max output.



For a spark/coil test we (usually) need to know if the coil is capable of generating 20-30kV (depending on the type of ignition).

If an ignition coil can only generate enough voltage to produce a a spark that jumps 1/16" gap, that isn't enough to run an engine and carry a load. (this is the most common way the coils fail in 6th gen and earlier cars)

A spark plug gapped at 0.044" only needs about 2000 volts to ionize the gap at atmospheric pressure.

But if 2000 volts is all the ignition coil can muster, that 2000 volts will not run the engine.

When the pressure inside the cylinder increases (by compression) the voltage required to jump the 0.044" gap increases dramatically.

TS2008

Ok Thanks ezone for the explanation. I was curious because when I read posts about "engine can crank but no start", people always suggest spark tester instead of an old spark plug. I just wondering if I miss something.

ezone

A fixed (non adjustable) spark tester may or may not be the correct tool for the job--- if you don't know what the coil is supposed to be capable of, and how many volts the tester can test for, the test results may lead to wrong conclusions.


The ST-125 is a 30kV tester usually specified for testing GM HEI and DIS coils, some of which could hit far beyond 60kV. That particular tester doesn't always work on other systems.


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I also believe critiquing the color of a spark is pointless. Color does not determine voltage.

I only care about how large a gap the spark can jump.

TS2008

Not sure but a gap too big can harm the coil. isn't it?

ezone

Depends.

Most of the time I personally haven't seen problems resulting from it.
However.....

On the earlier cars with the coil failures I mentioned, the typical fail is a breach of insulation from the coil secondary windings to the nearest ground.


(If I'm testing for spark that means it's already got a problem LOL)

If the coil is already high mileage and aged, a few spikes with the output lead open can cause the coil to go ahead and burn through the already weakened insulation internally. Sometimes you can peel the metal shield off the side of the failed coil and find a burn mark where the spark found its nearest ground.


Many times this happens while someone is doing a compression test, I've seen a few posts about it.
I know I finished off a coil once this way.





On some Chrysler products they say to not open the spark gap more than (1/4" or something) because it can spike their computer. Lack of protection on their part?

ezone

You'll never figure this one out using a service manual or an ohm meter...

...because it's not really an electrical failure. Electrically it's still fine and working.



Dynamic testing is where it's at.