If you search the internet, you will find several videos showing you that you can test a spark plug with a multimeter. In this video, we will investigate if this is really the case.
► THE MULTIMETER TEST NR.1
To complete the first test, you set the multimeter at 20 kilo Ohm and measure the resistance between the top connector and the central insulated electrode protruding at the bottom. If the multimeter shows a resistance value of almost zero for non-resistor spark plugs, and between 4000 to 6000 ohms for resistor spark plugs, the test is supposed to be successful, and the spark plugs gets a pass. If the multimeter shows a different value, it is a fail.
The idea behind this first test is to confirm the internal electrode including, its resistor when present, have not been damaged. As we have seen in a previous video, testing between the top connector and the central insulated electrode protruding at the bottom means we effectively try to confirm there is a viable electric path inside the central electrode.
And this test tells you exactly that, nothing more. It answers the question: "Do I have, Yes or No, a viable electric path within the central electrode" which is not the same as answering the question "Do I have a working spark plug?".
You see, there are different ways a spark plug can fail, and this test only cover 1 failure mode as I will quickly demonstrate with this spark plug.
According to this first test, this spark plug is a pass. Looking at the eroded ground electrode, it should however be obvious this spark plug should be replaced: it did however pass the continuity test.
► THE MULTIMETER TEST NR.2
The second test with a multimeter around spark plugs you can find on the internet is about measuring the resistance between the central insulated electrode protruding at the bottom and the spark plug shell. If the multimeter shows an Open Loop, the test is successful, and the spark plugs is deemed Ok. If the multimeter shows a resistance value, the spark plug is a fail.
The idea behind this test if to confirm the central and ground electrodes are electrically isolated from each other’s. If they were not, the plug would no longer be creating any sparks. And this test provides you exactly the answer to that question: Is the central electrode electrically isolated from the ground electrode... at room temperature... at 1 atmosphere pressure... when submitted to the few volts generated by my multimeter. This is there again an answer to a completely different question than the one we really want to answer, ""Is my spark plug working as required in my engine, that is, when submitted to several atmospheres pressure and several thousand volts?"". When voltage increases to several thousand volts, electricity can jump through dielectric material. In other words, while we have tested electric isolation and expect the spark to occur where it should, when the voltage builds in, there is no guarantee it won't spark somewhere else. Maybe cracks in the porcelain insulator or carbon tracks provide a more favorable path for the spark to follow and there is nothing this second test can tell us about that. Our spark plug could still be a fail because it is not tested in real usage conditions.
Lastly, I have quickly mentioned it before, but there are other aspects that make a spark plug work correctly that are not mentioned or touched by those multimeter tests: the aspect, foiling and wear of the electrodes that could cause misfire, the aspect and wear of the gasket that could cause pressure loss, the size of the gap between the electrodes that is essential for proper operations...
► CONCLUSION
So, can you test a spark plug with a multimeter? You definitely can! You may even catch a failed one, but a spark plug successfully making it through those 2 tests still can’t be trusted
Only a Spark Plug Tester + Engine Analyser can tell you that
► REFERENCES
Spark plugs with air gaps:
firetrucksandequipment.tpub.c...
Champion product range:
www.championparts.ca/Technica...
► DISCLAIMER
Always make sure you operate in a safe and secure manner.
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