How To Test and Replace Spark Plug Wires

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Microscopic cracks in spark plug wires can cause them to short out when they are wet. Locating them and replacing them is a simple matter with an inexpensive spark plug wire tester.

One of my best friends operates a mobile auto repair business. I do not get to spend much time with him since I bought this home up on Walch's Mountain because most of his clients live in the flatlands. My friend has been in the business of bringing the garage to the customer's car for over thirty years. You could have knocked me over with a feather this past Fourth of July when he told me that he hated working on cars. One of the things he hates more than anything else is working on a car with electrical problems because the cause of the problem can be elusive. I nodded with full understanding. Back in the days when I was studying radio and television repair, we called those kinds of problems "Dogs."

A few days ago I was down in the flatlands getting parts for a dune buggy engine when I spotted his truck parked behind a new car. He looked outraged, so I pulled my Jeep up behind his truck and asked what the problem was. The car had a rough idle and stalled out when it was wet. It was not raining then, and all his expensive, high-tech test gear showed everything to be in proper order. He had a "Dog" on his hands. Some times, low-tech works better than high-tech. I suspected we had a high-tension leakage problem and I was right.

High Resistance Leakage To Ground

You need an analog or digital Multimeter to check conventional spark plug wires. Spark plug wire has a resistance of 10,000 to 15,000 Ohms per foot when new. Any reading significantly higher than 10,000 to 15,000 Ohms per foot indicates a faulty wire. A reading of infinite resistance indicates a wire with an internal break. What an Ohm meter reading will not tell you is if there is any high-resistance leakage to the ground when the wires are wet. Back in the days before electronic ignitions and on board computers, we used a screwdriver and a jumper wire to find high-tension wires that had microscopic cracks in their insulation, and were arcing over to ground. We grounded the screwdriver with the alligator clip equipped jumper lead and then ran the blade slowly along the high-tension wires. If there were a microscopic break in the insulation, the high-voltage would arc over to the grounded screwdriver.

If you are working on an older car with a conventional breaker-point ignition system, you can still use the grounded screwdriver to find a faulty spark plug or coil wire. It is not a smart idea to use this approach with modern High-Energy Ignition Systems and on cars with on board computers. It is possible to cause irreparable harm to sensitive electronics using the screwdriver method. Lisie automotive tools make a spark plug wire tester that does a terrific job, and you can add the Lisle 26900 Spark Plug Wire Tester to your tool box for less than $10.00.

The Lisle 26900 Spark Plug Wire Tester is easy to use. Clip the grounding clip to any solid ground and slide the concave end of the probe slowly along each high-tension wire. When a microscopic break in the wire insulation is encountered, the probe will light up. It is a good idea to dampen the wires with water from a spray bottle before making this test. The water will find its way into microscopic cracks and set up a conductive path for the high-voltage electricity.

How to Replace Defective High-Tension Wires

A word of warning is in order here. Do not buy high performance wire sets although they may cost less than conventional wire sets. All high-performance wires are not resistance wires. They work extremely well on race cars but play havoc with the on-board computers on everyday cars because of the radio frequency energy they emit. With that in mind, let us move on to the task at hand, replacing the wiring.

Most spark plug wires snap on the spark plug and push in the towers on the distributor cap, but not all. Spark plug wires on some of today's cars is secured in the distributor by towers by spring clips inside the distributor cap itself. To remove those wires, you need to remove the distributor cap and then remove the spring clips with needle nose pliers. Some wires screw into the distributor cap's towers. Check the service manual for your car before you start trying to pull the plug wires from the distributor cap's towers.

To remove plug wires without damaging the wires, use a pair of Spark Plug Boot Puller Pliers. Spark plug boots become stuck on spark plugs with age, removing them without using boot pliers can damage the wires. With boot pliers, all you have to do is grasp the boot and give it a twist to break the seal before pulling the boot and wire free of the plug or distributor tower. Replace the wires one at a time so you replace them in the right firing order. The service manual for your car will show you the correct order for placing the spark plug wires in the distributor. Replacing them one at a time is a time saver. Replacing the spark plug wires one at a time makes it easier for you to match up their lengths correctly.

Buy After Market Parts Whenever Possible

There is one particularly compelling reason to buy replacement parts from Auto Zone, Advance Auto Parts, Pep Boys, NAPA, or some other auto parts store rather than a car dealer, the cost of the parts. A set of plug wires for a V-6 engine could cost $200 from a dealer while an after market set could cost $35. There will be times when you will have to face the situation and go to a dealer for a part, but it should not be often.

Coil on Plugs (COPs)

If you have a car that has individual coils, one for each spark plug you are looking at a dealer only parts situation. A set of boots will cost about $25 but if the boot and coil are one piece you are looking at about $100 per unit. Normally I recommend replacing spark plug wires and boots as sets but if you are facing an integral COP unit, just replace the faulty unit.

Coil on Plug ignition systems are rapidly replacing the single coil and plug wiring harness the way the electronic High-Energy Ignition Systems replaced the old breaker point systems. In many ways, they are easier to troubleshoot, and I will be doing an article on how they work.


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