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How to Check Spark Plug Resistance?

blueflyer

Well Known Member
My multimeter reads "1" on all 8 plugs. It never waivers off of "1" When I touch the two multimeter leads together, I see a value change, so I feel like my mulit meter is working. What could I be doing wrong? I am using the picture below (which I downloaded from one of Kahuna's posts I think) to show where I am touching the spark plug with my multimeter leads.

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What meter are you using? Some digital auto-ranging meters, like many Fluke meters won't work.
 
What happens when you touch the two lead together without the spark plug in the circuit?
 
What ohm meter is suggested for using when checking spark plug resistance?

I'm using an el cheapo digital ohm meter from Walmart. When I touch the two leads together the value on the screen jumps to a big number, then when I pull the ohm meter leads apart, the reading goes back to "1"
 
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Select the ohm range you require on your multi meter (bottom left of your meter) 200 ohm, 20K ohm, etc. Touch two leads together you should read zero ohm. To check ohm on spark plug touch leads to each end of the spark plug as shown in you picture. Make sure the multi meter probe reaches down inside the spark plug to make contact or it will read open (1 or zero). Try not to touch probes tips with your hands as this could effect your ohm reading. Hope this helps.
 
I had it set on 20K ohms....i'll go back to the hangar today and try it again after sleeping on it last night. Maybe I was doing something wrong and I didnt realize it. Thanks for the help.
 
You may not necessarily be doing something wrong. Last week I removed 12 Cahmpion fine wire spark plugs that were all showing infinite or extremely high resistance. I checked them with 2 meters, as well as the Tempest spark plug checker.
If your plugs have infinite or greater than 20K resistance it will show as a 1 or "open" on your meter.

Do make sure you are getting a good contact on the center electrode. Sometimes the ignition residue needs to be cleaned off.

Vic
 
Thank Vic. I read some of your old posts about checking spark plugs...very helpful. I want to buy that Tempest spark plug resistance checker, but for $140, im gonna wait just a little longer before I pull the trigger on it.
 
I prefer to use a Simpson meter when doing this check. Digital
meters can give you funny readings if the scale won?t adjust manually. Some just do it automatically. Also, it is important to have a clean surface. Good luck!
 
I have the Harbor Fright version of the Walmart meter used by the OP and it works fine for checking plug resistance. Getting a good connection on the terminals sometimes requires some effort when there is combustion or electrical residue on the terminal or electrode.
 
If you aren?t that versed in using the ohm meter, just get a resistor in the range you want to test for and ohm it out with your meter before you test your plug.
Any resistor in the 1k to 5k ohm range will tell you if your meter is working correctly and then you can confidently test your plug.
 
What meter are you using? Some digital auto-ranging meters, like many Fluke meters won't work.

I use a Fluke with auto-range but it can be disabled. I usually see plugs reading 1.2k to 1.9k Ohms. Above that, to me, is a concern. I know they are supposed to be good up to 5k Ohms.

That said, what are people seeing for resistance on their automotive plugs (e.g. NGK (5044) BR8EIX Iridium IX)? Or do you just replace them every year and not worry about the resistance check? I went with Iridium because they should last longer.
 
I simply replace my autoplugs with new at annual - they are too cheap to mess with.
 
The auto plugs are already around 5K when new. Quite different than the aviation plugs, which usually show 700-1K when new. As someone mentioned, 1.7K-1.9K is OK on the aviation plugs.
I do replace the auto plugs every 100 hours. Cheap insurance.

Vic
 
update

Just to update this thread, my 8 new Tempest (old ones were Champions) plugs came in. They all tested between 950 and 1300 ohms with the same el cheapo ohm meter that was giving me "1" readings on all 8 champion plugs. Those champion plugs had about 400 to 500 hours on them.
 
Things might have changed but in the day, the only way to test spark plugs was with one of these.

Otherwise the plug might check out OK but under combustion pressures they might be breaking down.

We never checked them with just an Ohm meter.

Here's Champion's manual for cleaning, adjusting and testing aviation plugs.
 
HOW to measure plug resistance ?

Can someone explain how the NGK BR8ES is constructed. My plugs measure new>5-5.7K. Used 100 Hr =4.5 to 5K. Here is my dilemma:

Touching the wire boot terminal and the electrode I get nothing (open circuit)

Touching the wire boot terminal and ground strap I get nothing, BUT

Touching the wire boot terminal and putting the other probe in contact with the electrode AND ground strap simultaneously, voila, 5K !

Why is this ?
 
Can someone explain how the NGK BR8ES is constructed. My plugs measure new>5-5.7K. Used 100 Hr =4.5 to 5K. Here is my dilemma:

Touching the wire boot terminal and the electrode I get nothing (open circuit)

Touching the wire boot terminal and ground strap I get nothing, BUT

Touching the wire boot terminal and putting the other probe in contact with the electrode AND ground strap simultaneously, voila, 5K !

Why is this ?

The "R" in the part number tells you that the plug has a built in 5k resistor. If you dont want that, get B8ES spark plugs.
 
Resistance is required for Pmags

Resistance is OK. The question is the illogical, to me, observation that the probe must be touching the electrode AND the ground strap simultaneously to get a reading.
 
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