General question about spark testers - PeachParts Mercedes-Benz Forum

benhogan · #1 11-23-2010, 02:39 PM

I have an in-line spart tester that I am using to track down a misfire in a cylinder. I do not know if it is caused by a bad spark plug or a bad ignition wire.

If I put the spark tester between the plug and the ignition wire and I see a spark. Does this mean that the spark plug is firing or does this just mean that electricity is just getting to the plug.

This is the tool.

I guess I could check the resistance of the ignition wires. Does anyone know what the ballpark acceptable resistance should be for ignition wires?

MattBelliveau · #2 11-23-2010, 02:41 PM

That just means there is power to the plug. I generally test a plug visually. Pull the plug, reattach the wire, and ground the threads of the plug to something (I usually use jumper cables) and have someone crank the engine.

Pooka · #3 11-23-2010, 02:46 PM

Perhaps this could help....

A problem I have run into twice during the last 50 years of messing with cars has been a cracked plug. It looks fine when cold and runs fine with cold but after it warms up and expands a bit it starts to misfire.

You can spot this with an oscilloscope, but just replacing the plug is easier and a lot cheaper.

benhogan · #4 11-23-2010, 02:47 PM

Quote:

Originally Posted by MattBelliveau

That just means there is power to the plug. I generally test a plug visually. Pull the plug, reattach the wire, and ground the threads of the plug to something (I usually use jumper cables) and have someone crank the engine.

That's what I thought. I was hoping to avoid pulling the plug. The plug is on the back bank of a transversely mounted V6.

Oh well. Thanks for confirming.

aklim · #5 11-23-2010, 02:52 PM

Question. My timing light has RPM on it. So, I can hook it up to a functioning plug wire and get the RPM. Then go to the #6 wire and see what the RPM is, correct? If the RPM is off, there is an issue. Hot or cold, it should work.

Chas H · #6 11-23-2010, 04:57 PM

If your spark tester shows a spark, there is a spark at the spark plug. The spark plug is in a circuit, current flows through the entire circuit or not at all.

benhogan · #7 11-23-2010, 06:37 PM

Quote:

Originally Posted by Chas H

If your spark tester shows a spark, there is a spark at the spark plug. The spark plug is in a circuit, current flows through the entire circuit or not at all.

ahhh...makes sense. thanks!!