Question on spark voltage

[blown motor]

What should the voltage be at the spark plug? Is there some way to measure that? I'm thing of something that you could put between the plug and the plug wire that would measure the voltage at that point.

[JH27N0B]

I'm sort of an electrical ingnoramus so somebody will correct me if I'm wrong.  But isn't it what the output of the coil is?
According to this article, that would be 30,000 volts.  I'm not sure how you measure the output of a coil to tell if it's weak.
https://www.hagerty.com/media/maintenance-and-tech/ignition-coil-basics/#:~:text=The%20coil%20becomes%20a%20transformer,secondary%20side%20as%2030%2C000%20volts!

[tparker]

I have no idea but I don't think you can measure it with a DMM because  it takes an average reading. You would need something more specialized like an occiliscope.  I'm not sure about the  range though, cause like @JH27N0B mentioned, it could be as high as 30K volts or  what not.

As far as  taking  a reading, you can  just remove  the sparkplug  boot  and read  off of the thing that connects to the plug. Hmmmm.... Just figured out I  don't recall what that is called. LOL. You could  probably just  slip the meter probe in between the wire and plug cover. But that would require a special tip as most oscilloscopes have a special tip to clamp onto wires.

Interested to hear ideas.

[cuda hunter]

Side Electrode is the J shaped piece that throws the spark. 

[jimynick]

Murray, the Bruce County method was to pull a plug wire off and hold it about an eighth of an inch from your nose and have an accomplice crank it over. It won't take too long to ascertain the condition of the coil, so you won't have to worry about that for a while. 

[bdschnei]

These things work pretty well for a quick check.
Poor man's o-scope. 

https://www.amazon.com/OEMTOOLS-25069-Adjustable-Ignition-Tester/dp/B0014WDFLY/ref=asc_df_B0014WDFLY/?tag=hyprod-20&linkCode=df0&hvadid=642203366870&hvpos=&hvnetw=g&hvrand=12470439660660122183&hvpone=&hvptwo=&hvqmt=&hvdev=c&hvdvcmdl=&hvlocint=&hvlocphy=200637&hvtargid=pla-1948269718969&mcid=e29a4db9ee39312db3d9391f6d2e6461&th=1

[blown motor]

Checking for spark isn't the question. I'm wondering if you can actually measure the voltage getting to the spark plug. My car has been getting hard to start with a cold engine. So one of my thoughts is what if I have weak spark because the coil is starting to break down or the wires are starting to break down and not delivering the full charge from the coil. I'm looking for a way to test that.

[tparker]

I  couldn't find  a really good video  but this one  kinda  explains it. Although the tool tip isn't what I figured  it would be. Anyways, he is using a hardware box that plugs into the computer that runs the ocilliscope software. I think  it  runs about $100-$200  and requires a windows machine. I recently purchased a  handheld ocilliscope the size of a digital multimeter, which it also has built in.  That set  me back about $150. I don't have that lead since iit is not a automoitive specific ocilliscope.

https://www.youtube.com/watch?v=rvWKgBwRMfk

[bdschnei]

"Checking for spark isn't the question. I'm wondering if you can actually measure the voltage getting to the spark plug."

The spark tester I linked to has an adjustable air gap. It has a scale that'll give you an idea of the approximate voltage at the plug. Short of that only an automotive oscilloscope is capable of measuring that high of a voltage accurately. A handheld (labscope) with an inductive pickup might give a decent approximation, but the cheap spark tester would be just as accurate if not more so.

You might want to plug the spark tester directly into the coil wire and see how large a gap the spark jumps there and then compare that to the available spark at the plug(s).

[blown motor]

You might want to plug the spark tester directly into the coil wire and see how large a gap the spark jumps there and then compare that to the available spark at the plug(s).

I suppose that would give an indication of any voltage drop through the plug wires.

[blown motor]

Maybe I'm missing something here but...how does it test the spark plug when the wire is plugged into the tool and the tool is not connected to the plug? Or am I getting too technical about the description?

[bdschnei]

Yeah that is a goofy description they give.
The idea behind the tester is pretty simple. The end with the alligator clip is clipped to a  good near by ground. The sparkplug wire is removed from the sparkplug and pushed onto the other end of the tester just like the wire would normally attach to the plug. The sparkplug remains in the engine. Then start the engine and let it run on the other seven cylinders while watching for spark at the tool. You could disable the fuel on a fuel injected engine and just crank the motor and monitor the spark while cranking. It really comes in handy when  diagnosing no start conditions.

So basically it's just a spark plug with an easily adjustable gap. Nothing magical but it's a handy little tool to have for quick troubleshooting

If you're wanting to measure voltage at the sparkplug while the plug is installed in the engine, then you'll need an automotive oscilloscope or an automotive lab-scope with an inductive pickup that can graph the firing / plug voltage. Before the days of OBD2 when you had to diagnose misfires these things were indispensable. Since OBD2 came along, just plug in the scanner and it'll pretty much lead you to which ever cylinder is misfiring. That's really nice but unfortunate that younger techs no longer know how to troubleshoot ignition/fuel problems without that crutch.

[blown motor]

Thanks @bdschnei I ordered one. New toy to play with, woohoo!!