plug wire resistance
10-01-2015, 10:13 AM
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plug wire resistance
Anybody know what an acceptable amount of resistance per foot would be on a plug wire? I accidently popped one of my wires out of the crimped on connector the other day. After crimping the end back on, I checked with an ohm meter and was surprised to see 14K. Checked another wire and found 15K. Seems kind of high, but I'm not sure. The car seems to run fine. I saw in the archives that someone said GM allowed up to 8K per foot back in the day. Can anyone confirm/deny this? Wires in question are BB braided wires from Lectri Limited about 28 years old.
10-01-2015, 10:27 AM
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I have read this in GM manuals, and it's been a few years, but I think it's more like 12k per foot allowed.....and most wires I check are usually a lot less, about 7-20k TOTAL, for the full length of the wire. 14k resistance is just fine. No worries at all.
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EMD645F3B (10-01-2015)
10-01-2015, 01:34 PM
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I just looked in '94 Chevy shop manual....(HEI ign) specs are for wires up to 15" long: 3-10k, wires from 15-25" long, 4-15k, and wires 25-35" long, 6-20k. So, about 10k every 15 inches of wire. Specs for standard, old school points-ignition wires are looser, about 12k per foot. If your wires are old, you need to check the resistance while bending the wire around while looking at the ohmmeter. That will help find intermittent breaks in the conductive core.
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EMD645F3B (10-01-2015)
10-01-2015, 01:43 PM
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Thanks for the replies. I thought I had seen the specs in the manuals as well, but I looked through my 66 service manual and can't find anything. I did flex the wire while taking readings and it was a rock steady 14K.
10-01-2015, 02:36 PM
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Depends on the wire. Stock, solid core, Spiral core, etc.
MSD says 50 ohms per foot, Davis 300-350 ohms per foot.
But I think that you really want to see if the resistance is significantly different from wire to wire to determine if there is a problem. You are testing resistance with a battery and a constant low voltage signal, not a pulse of 10,000 to 50,000 volts, so resistance numbers do not really measure acceptability for use. If the normal spec wires are 14K Ohms, and all the wires are within range of each other, great. If normal is a few hundred Ohms and one wire is 14K Ohms, you might want to check that one wire out.
Just remember that longer wires will have more resistance that shorter wires.
MSD says 50 ohms per foot, Davis 300-350 ohms per foot.
But I think that you really want to see if the resistance is significantly different from wire to wire to determine if there is a problem. You are testing resistance with a battery and a constant low voltage signal, not a pulse of 10,000 to 50,000 volts, so resistance numbers do not really measure acceptability for use. If the normal spec wires are 14K Ohms, and all the wires are within range of each other, great. If normal is a few hundred Ohms and one wire is 14K Ohms, you might want to check that one wire out.
Just remember that longer wires will have more resistance that shorter wires.
10-01-2015, 11:39 PM
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Years ago my dad bought me a Radio Shack digital multi meter and I thought I was good to go until I was checking things for resistance. A few years ago I sprung for a Fluke multi meter ($$$) and found out the Radio Shack unit was way off. Let your conscience be your guide.
bcwaller we found when measuring MSD wires 50 ohms per foot was correct, but there is a small fudge factor for plug and cap ends. Typically it was the quality of the connection at each end that effected the 50 ohms per foot numbers.
.
bcwaller we found when measuring MSD wires 50 ohms per foot was correct, but there is a small fudge factor for plug and cap ends. Typically it was the quality of the connection at each end that effected the 50 ohms per foot numbers.
.
Last edited by Randy G.; 10-01-2015 at 11:42 PM.
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bcwaller (10-02-2015)
10-02-2015, 07:14 AM
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Years ago my dad bought me a Radio Shack digital multi meter and I thought I was good to go until I was checking things for resistance. A few years ago I sprung for a Fluke multi meter ($$$) and found out the Radio Shack unit was way off. Let your conscience be your guide.
bcwaller we found when measuring MSD wires 50 ohms per foot was correct, but there is a small fudge factor for plug and cap ends. Typically it was the quality of the connection at each end that effected the 50 ohms per foot numbers.
.
bcwaller we found when measuring MSD wires 50 ohms per foot was correct, but there is a small fudge factor for plug and cap ends. Typically it was the quality of the connection at each end that effected the 50 ohms per foot numbers.
.
