Spark plug wires are also called Ignition wires or High-Tension leads or Ignition leads etc. These wires carry the high-voltage power from the coil to the individual cylinders of the engine. With Coil-On Plug (COP) Ignition systems becoming the standard, ignition wires have now being reduced to the repair market. If your petrol engine (if it uses ignition wires) is hard to start or misfires during driving , the first thing to check should be the spark plug wires. After visual inspection for cracks etc in the rubber / silicon insulation, do a continuity test with multimeter. The resistance or continuity test would not indicate anything on the state of the insulation of the wires, which needs to be tested separately. One way of doing it has traditionally been starting the engine, looking at the plug wires in the dark and noting any sparks or arcs from wires to the other parts or between wires themselves. Another way to test the wire is with a variable-gap spark tester at various engine rpms. This variable-gap spark tester replaces the spark plug and is attached to the wire. If there is a spark at a wide-gap (at least double the spark plug gap), then wires are ok.
If found faulty, you would need to repair or replace them. Here is a demo for repair.
The first thing to do is to pull up the insulating rubber boots from the terminals of the wires like shown below. The metallic terminals would be exposed.
Remove the metallic terminal with care and cut an inch or so of the outer layer of the wire to expose the internal conductor. Do not shorten the wire too much otherwise it would not fit.
Fold this newly exposed conductor around the wire and put the terminal on.
Crimp the metallic terminal over the wire with a crimping pliers (that can crimp spark plug wires) or even better with a crimping die. The crimping die would require a vice or at least a C-Clamp to do the crimping.
Push down the rubber boots over the terminals and your Ignition cable is ready to be installed back in the engine.
Easy starting and no more misfires (if there is no additional problem).
If you want to replace your ignition wires, then do some homework before purchasing them. There are various classes of wires like B,C,D,E and F-Class with increasing temperature tolerance and insulation strength. If the wires are more than 10 years old, it is better to replace them with new F-Class wires. An important parameter to check is the resistance (in ohms or kilo ohms) of the wires per meter or per foot. Japanese, French and Italian reasonably-priced cars (BBC Top Gear term) use wires with resistance in the range of 5-9 kilo-ohms per meter.
The distributor cap is usually fitted at the back end of the camshaft and wires then run from the distributor cap to the individual spark plugs. This setup results in the four different lengths of the wires with different values of resistance. Now it can be easily imagined that the resulting spark would also not be the same in all four cylinders. The cylinder with the shortest wire (thus least resistance) would get a fat and long-duration spark. With Coil-On Plug ignition, there are no plug wires and all cylinders get the same spark energy.
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