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How to properly check the plug wires?


NAXJA Forum User
When I first picked up my XJ about 4.5 years back I put an aftermarket iginition kit on it... yeah, yeah, I know it was pretty much a waste of money.

Anyway, it was the Jacob's kit and it came with hi-performance wires. It's been about 73k miles and I've change my plugs, cap and rotor a couple times but not the wires yet.

How can I tell if these are bad?

They all measure about 250 ohms except for the wire going to the coil which measures about 900 ohms (It's about 3 times longer than the others so you could mount the aftermarket coil anywhere in the engine bay)

None of them have any cuts or gouges.

The strange thing is I recently changed the wires on a friends 2.5L YJ and the new wires actually had more resistance than the old but it still seemed to idle and start smoother... then again the plugs were changed at the same time so who knows.

(I'm no expert)

The resistance test is a good start.

The other area of failure would be the insulation jackets on the wires. There is a piece of equipment that can be used to test the integrity of the insulation. It's called a HIPOT(high potential/voltage) tester. Not exactly the kinda of thing found in your standard toolbox. You can do a makeshift test of the insulation.

(Here's where I get a little weird. Do not do this unless you understand the safety implications of doing this test.)

In the dark, with the engine running, take a grounded wire, and pass the other end along the lengths of the plug wires. Maybe even make a little loop that surrounds the spark wire. The end or loop should be bare wire. You will be looking for tiny sparks/arcs which will indicate that the insulation has started to deteriorate.

Most of the performance wires that I've seen have a dielectric breakdown of ~100KV, which means that you should not be seeing are arcing unless the insulation has failed.