Spark plug wire resistance testing
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Cargo Master
Joined: Aug 2002
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From: Cajun Country
Spark plug wire resistance testing
First of all,I know very little about electricity or electrical systems.And,what I do know will most likely get me in trouble. 
Just recently I purchased a digital meter. Woohoo,i'm going places now! Anyway,I was checking resistance on some old plug wires. Some numbers I was getting are like 6.60-7.90 on the shorter ones(18-20"). Are these good readings? On the longer ones (24-30") I was getting like 9.50-10.62. Are these good readings? I then checked my factory wires I changed out at 64k miles and those readings were like 15.20-16.50. Do higher numbers mean a bad reading? 
Just recently I purchased a digital meter. Woohoo,i'm going places now! Anyway,I was checking resistance on some old plug wires. Some numbers I was getting are like 6.60-7.90 on the shorter ones(18-20"). Are these good readings? On the longer ones (24-30") I was getting like 9.50-10.62. Are these good readings? I then checked my factory wires I changed out at 64k miles and those readings were like 15.20-16.50. Do higher numbers mean a bad reading?
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From: Virginia
First off, you probably need to move the decimal three places to the right for the readings above(multiply by 1000). If you look at your meter, you might see a "k" on the screen. This means "kilo-ohms, and means times a thousand.
A good plug wire will have less than 10,000 ohms per foot(or 10k ohms/foot) but more than 1000 ohms per foot(or 1k/foot).
A good plug wire will have less than 10,000 ohms per foot(or 10k ohms/foot) but more than 1000 ohms per foot(or 1k/foot).
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From: Virginia
2M is 2 megohm. Multiply your readings by 1,000,000 or move the decimal over six places. So .007=7000 ohms
Your other scales have the "k" so move the decimal 3 places over.
06.4K= 6400 ohms.
6.39K= 6390 ohms
What is happening is the meter is rounding the results off. The 2M scale is for reading very high resistances. So if your resistance is not very high, it will be on the very low end of that scale, so the reading is not very accurate.
You can also see it rounding off on the 200k scale, though not as bad. The 20k scale is giving you the most accurate reading.
The guy in the book said not less than 1000 ohms/foot and not more than 10,000 ohms per foot. So if your wire is 18 inches, that is a foot and a half.
So 1000 X 1.5 ft = 1500 ohms is the lowest this 18 inch wire can be.
10,000 X 1.5 ft = 15,000 ohms is the highest this wire can be.
You have 6390 ohms so you are right in the middle. Sounds like it is a good wire, if the insulation is in good shape.
Your other scales have the "k" so move the decimal 3 places over.
06.4K= 6400 ohms.
6.39K= 6390 ohms
What is happening is the meter is rounding the results off. The 2M scale is for reading very high resistances. So if your resistance is not very high, it will be on the very low end of that scale, so the reading is not very accurate.
You can also see it rounding off on the 200k scale, though not as bad. The 20k scale is giving you the most accurate reading.
The guy in the book said not less than 1000 ohms/foot and not more than 10,000 ohms per foot. So if your wire is 18 inches, that is a foot and a half.
So 1000 X 1.5 ft = 1500 ohms is the lowest this 18 inch wire can be.
10,000 X 1.5 ft = 15,000 ohms is the highest this wire can be.
You have 6390 ohms so you are right in the middle. Sounds like it is a good wire, if the insulation is in good shape.
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I have a digital voltmetre, and I know where to find the spark plug wires--how do I check them? What do I attach the red and black wires on the voltmetre to, to get the readings you got? I am assuming to attach them to the outside insulation of each end of the plug wire, while the motor is running--yes? (or will I blow myself up doing that?)
Thanks,
Frank.
Thanks,
Frank.
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Cargo Master
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From: Cajun Country
Frank, the wires must be removed.Take the black lead and touch the metal part inside the boot which attaches to the plug.Touch the red lead to the metal end that goes into the coil. Your meter should be set on ohms to get the reading. 
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Hotshot
Joined: Oct 2003
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From: Ashland City, TN
whenyour echecking resistance, you can place either lead on either end of the wire.
When you get into voltage and ampers its different.
Remember though. The longer the wire, the more resistance there will be, because wire has a natural resistance also.
Pluss to make current you need resistance.
When you get into voltage and ampers its different.
Remember though. The longer the wire, the more resistance there will be, because wire has a natural resistance also.
Pluss to make current you need resistance.
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From: Durham, NC
Originally posted by Talyn
Pluss to make current you need resistance.
Pluss to make current you need resistance.
Ohms law:
voltage= current * resistance
current= voltage / resistance
resistance= voltage / current
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I have an additional question in regards to the plug wire resistance. On my 97 F150 the check engine light came on. Testing shows that I have a #8 cylinder misfire. In talking to a mechanic he suggested that I try several options prior to paying up the nose purchasing parts. Items to check were the plug, plug wires and ignition coil. The plug looked good and the ignition coil ohmed out correctly on the primary and secondary side but the plug wires seemed a little hiigh. I checked the wires for the number 2 & 8 cylinders as their both off the same ignition coil and next to each other on the left side of the engine. #2 ohmed out at 10K ohms while #8 ohmed out at 16K. I was told by my friendly Autozone teller that the wires can vary quite a bit depending on Manufacturer. Does the 16K ohms on #8 seem a little high causing me my miss fire? Not sure I want to spend $50 on a set of wires and then find the problem to be an injector...
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From: Virginia
If you are not certain about the wire being the problem, see if one of the other cylinder's wires are close to the same length. Swap the two wires and see if the miss-fire trouble code moves to the different cylinder. I think I would try to move the wire to a different coil pack for the test.
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From: Hamilton Ontario
Take a spray bottle , like a plant mister , fill with water , adjust the nozzle to stream. Start the engine and wet those wires. If it starts to stammer and shake or miss , change the wires.
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