Normal/good OHM resistance through spark plug wire

[tdskip]

Hi guys - what should I expect to see by way of measured resistance on a spark plug wire? Will it vary some by length?

Thanks!

 

[bigjones]

My Dodge Polara handbook says to replace if over 8k ohms/ft for suppression wires.

Cheers!

 

[tdskip]

Wires in question - actually from the coil to the distributor;

Reading on the longer of the two;

And the shorter of the two;

And yes, these are um, for the Alfa but I thought it might be useful to post here since I haven't seen this discussed in some time. And that seeing how red these are would make Paul happy.

 

[CJD]

Those numbers look perfect to me. The shorter wire is about 2/3 the length...and about 2/3 the resistence. If you had one twice as long, it should have twice the resistence. In my experience, a bad wire will go infinite resistence, and may come back if you twist it around while checking.

John

 

[TR3driver]

Looks good to me too.

I have seen bad wires show up as high numbers, but they are really high like megohms instead of Kohms.

Whether it varies by length depends on the wire, but most suppression wires do vary by length. The center conductor is also the resistor, so longer means more resistance. But the set on my TR has a discrete resistor in the plug cap, and copper wire.

 

[tdskip]

Thanks guys!

 

[DNK]

I thought not all wires are the same.
You can get info on that from the Magnecore website on theirs.

 

[TFB]

Just a thought ,but most problems I have had with high voltage ignition wires is with the insulation ,not the conductor.A megger can check insulation condition.
Tom

 

[DNK]

Megger?

 

[TR3driver]

He means a megohm meter; which can measure very high resistances. It's a valid point, but I wouldn't bother buying one just to check ignition wires. You can accomplish much the same thing just by watching the engine run in the dark. If it puts on a light show, time for new wires &or cap.

Or just run your hand along the wires and see if it bites you

 

[TFB]

Hey Don,this is a common electrical testing tool that generates a high voltage to a conductor to check insulation.The old ones had a hand crank,while new ones use a small battery.
tom

 

[DNK]

 

[Gliderman8]

Glad to see you finally broke down and bought a new one don

 

[Rhodyspit75]

Mine's a little newer. I just realized the red lead is connected to the "earth" terminal. Must be positive ground. You can't see it in the picture but upside down on the meter face it says registered "Megger"

 

[lbcs_r_fun]

Mine's a little newer. I just realized the red lead is connected to the "earth" terminal. Must be positive ground. You can't see it in the picture but upside down on the meter face it says registered "Megger"

That instrument is useless for this test!
The purpose of a Megger is not to measure resistance, as in resistance to the flow of current. It is used to measure the insulation resistance of the wire. It is more of a qualitative test. When you crank the handle, approx. 500 volts dc is produced across the terminals. For example, you could use the Megger to check the insulation of a starter motor or even a dynamo in your TR. You would connect one of the insulated winding conductors to one terminal and the other to the case. For this level of insulation, anything less than infinity is a sign of trouble.

This instrument would be useless for evaluation of this type of high-tension spark plug wires. Perhaps it could be used to find a puncture in a solid copper ignition lead.

Regards,
T

 

[DNK]

Yo mean I gotta take mine back?

 

[lbcs_r_fun]

Yo mean I gotta take mine back?

No, not at all! A megger is a very useful instrument, but not suitable for testing resistance as the original poster asked. Actually , measurement of "true" resistance of a modern spark plug lead is complicated. The value of measuring DC resistance of a sparking plug wire is dubious, because in operation the current is a complex wave shape. To define the resistance of the wire would require some very specialized instruments. The results would yield a DC component and an AC component (inductive reactance) of resistance at some assumed value of frequency.

Of course all of this electrical engineering talk has made me thirsty. I think I'll go get a coffee.

T

 

[TFB]

yes megger leads dont go from conductor end to end.One lead to conductor end ,the other to ground.I mentioned it as I know of no other test for insulation,rather than the look for sparks at night method.If the wires are in a metal conduit,say a steel loom on a flathead ford,you could ground the loom,and test loom to conductor,or to other coductors.The ones I have used go to 5k volts,so you wouldnt be able to get in the 10-20k range,but it may show breakdown on a bad wire.I have checked vintage cloth covered wires this way .You have to lay wire on metal bench or put in steel tube to test.
Temp and humididity also have an effect,and most car owners will and should just periodically replace wires.
In the vintage bike world there seems to a shift towards conservation where possible,rather than restoration,and people are using original wiring if possible.I havent seen much of this in the car world yet.

Have fun
Tom