Resistance Of Plug Wires

[Bay13]

Well this morning I measured a brand new set of OEM plug wires.

1 is 25" long and reads 4.68K ohms

2 is 22" long and reads 4.17K ohms

3 is 19" long and reads 3.428K ohms

4 is 15" long and reads 3.05K ohms

5 is 12" long and reads 2.4K ohms

The coil wire is about 12" and read 2.454K ohms

The math guys can figure out the Ohms per inch or foot :P

[gdizzle]

Well this morning I measured a set of OEM plug wires.

1 is 25" long and reads  4.68K ohms

2 is 22" long and reads 4.17K ohms

3 is 19" long and reads 3.428K ohms

4 is 15" long and reads 3.05K ohms

5 is 12" long and reads 2.4K ohms

The coil wire is about 12" and read 2.454K ohms

The math guys can figure out the Ohms per inch or foot

←

Nice rich, I wonder what the resistance is on a set of magnacores etc.. When I upgrade the T5 I'll take some measurements.

[kenhoeve]

varies from .18 to .20 ohms/inch in your wires.

never thought about that, but it means the #5 gets double the voltage of the #1 cylinder. that really can't be ideal one would think.

[lext]

never thought about that, but it means the #5 gets double the voltage of the #1 cylinder.  that really can't be ideal one would think.

←

Not necessarily.

If it's a closed circuit then that would mean the current going through wire#5 will be double the current through wire#1. But in terms of voltage loss, I think the difference will be small between voltage at spark#5 and spark#1, since the source voltage is so high and the wire resistance (i.e. voltage loss) is, relatively, so small.

[kenhoeve]

but we have a distributor. that means each wire gets the same current. and if V still equals IR, last i checked it did, that means half the voltage. granted it's a ton of volts, but still half in #1.

[James A Sousa]

I think that a circuit with a surge of voltage and current and a gap in it will behave differently than a conventional closed circuit with several resistances.

For example, voltage drops across several resistances in a closed circuit will add up to the total voltage applied. In that case, the different wire resistances would cause different voltages to remnain to ignite the fuel.

However, a spark plug gap represents a very great resistance when compared to several Kohms, and the resistance in the wires may be relatively insignificant when compared to this gap.

The secondary ignition circuit is quite different than a closed circuit, as the current only flows when the spark plug gap is overcome by the voltage applied.

Measuring wires intended to carry large currents with an ohmmeter is not a complete test of suitability. I can subject a small resistor of the same resistance as your plug wires to the output of your ignition system and blow it to smitherines. Losing current and voltage to ground is an issue also.

Plug wires with the proper resistance may not be faulty, but this test is insufficient to prove that the wires are good.

If this opinion is faulty, it is because it is only meant to be food for thought. I may pipe in later if my thinking makes more sense to me!

[Graham McMicken]

Measuring wires intended to carry large currents with an ohmmeter is not a complete test of suitability. I can subject a small resistor of the same resistance as your plug wires to the output of your ignition system and blow it to smitherines. Losing current and voltage to ground is an issue also.

Plug wires with the proper resistance may not be faulty, but this test is insufficient to prove that the wires are good.

If this opinion is faulty, it is because it is only meant to be food for thought. I may pipe in later if my thinking makes more sense to me!

←

I think your right, and the ohm test is conducted for 2 reasons.

1) The resistance is sufficient enough to suppress radio interference.

2) To check for abnormaly high resitance, which will most likely indicate an aged/defective wire.

I think that if the wire is out of spec. it is 90%+ likely to be bad. If the wire is in spec though, I dont think this necessarily means the wire is still good, as you mentioned before. Using an Ohm meter isnt meant to be a perfect determining factor, but it does help.

We would need something like... an ignition coil to test it thoroughly

- G

[Kashif]

Can this Data be added to the repairs section of the Website for future reference? It is always nice to know these kinds of things when one contemplates replacing their Spark plug wires.

[Kashif]

Well this morning I measured a brand new set of OEM plug wires.

1 is 25" long and reads  4.68K ohms

2 is 22" long and reads 4.17K ohms

3 is 19" long and reads 3.428K ohms

4 is 15" long and reads 3.05K ohms

5 is 12" long and reads 2.4K ohms

The coil wire is about 12" and read 2.454K ohms

The math guys can figure out the Ohms per inch or foot

←

I am sort of a math guy ,

so here it is in Ohms per foot

1 is 2.25k Ohms/foot

2 is 2.27k Ohms/foot

3 is 2.17k Ohms/foot

4 is 2.44k Ohms/foot

5 is 2.40k Ohms/foot

and Ohms per inch

1 is 0.1872k Ohms/inch

2 is 0.1900k Ohms/inch

3 is 0.1804k Ohms/inch

4 is 0.2033k Ohms/inch

5 is 0.2000k Ohms/inch

[Tom]

The primary failure mode of ignition wires is not related to resistance from one end to the other. It is related to resistance between the core of the wire and the outside surface, which ideally should be infinite all along the wire - that's the insulation. If the insulation fails at any point, then when it rains the current travels to the outside of the wire and grounds through the moisture on the outside surface, bypassing the spark plug. Testing the wire resistance completely misses that problem - it only finds a problem in the relatively rare event that one wire becomes an open or near-open circuit.

Tom