Technical Information for Gunsmiths and for Classic Car Enthusiasts

MG Restoration

Spark Plug Wires

Les Bengtson

One area of interest to most owners is ignition tune up. Most people understand about replacing points, condensor, rotor, distributor cap and spark plugs, but very few understand how to check the spark plug wires to find out if they also need to be replaced.

There are two basic types of spark plug wires—copper and silicone.   The copper wires are great for conducting the high voltage current from the coil to the distributor cap and from the cap to the spark plugs. They have a long life and seldom need replacement. When they do, it is normally due to the insulation of the wire breaking down and causing some of the high voltage to leak. In most cases, they will still conduct electricity, but at a reduced voltage. There is only one real problem with copper wires—they create a minor radio transmitter and produce electrical interference with TVs and radios.

To correct this problem, silicone wires were introduced. These wires have some degree of internal resistance which surpasses the radio/TV interference. The silicone wires became more popular back in the late 60s and early 70s as the car producers began to offer more sophisticated radios. FM was becoming popular with the masses as the stations expanded and cassette and eight track tape players became popular. Prior to this time, people with the very expensive (back then) radio systems had to fit resistors to each individual copper wire to suppress radio interference. With the silicone wires, none of this extra suppression was required.  The only drawback to the silicone wires was that they wore out. In the early versions, rather quickly.  Today, silicone wires, much changed from the earlier versions are the standard. Unfortunately, they still do not last as long as a good set of copper wires and need to be inspected to see if they are functioning properly.

The first step in inspecting the wires (of both types) is to check to see that they are clean. Dirty build up on the exterior of the insulation may allow some of the current to be lost. It can also speed the breakdown of the insulation, leading to current leakage. Examine each wire and, if dirty, clean with either waterless hand cleaner or dish washing detergent.  Dry and wipe clean before reinstalling. It is best to remove one wire at a time to prevent mixing them up. Most old hands will be able to install the wires on a bare cap and get them in order with no problems. But, we all make the rare mistake and doing one wire at a time will help to keep the mistakes rare.

The next thing to check is that the ends of the wires are firmly attached to the spark plugs, the distributor cap and the coil. Four cylinder, in line engines are not the smoothest running of beasts and, sometimes, a wire will work its way loose. This is especially a problem at the cap, but Bob and Gil found two wires loose at the spark plugs on two different cars when they were helping me a couple of weeks ago.  Always check to see that all connections are properly seated.

The next test requires darkness.  You need to start the car with the hood open and run it while looking for blue sparks off the wires or a blue glow surrounding them. This indicates the current is leaking through the insulation and the full current is not being carried to the distributor cap and then to the spark plugs. In really bad cases, this can actually light up the right side of the engine compartment.  WARNING: It is dangerous to work around the engine compartment in the dark with the motor running. Put your hands in your pockets when performing this inspection and do not take them out until you are ready to turn off the engine. Running the car in the garage will help to cut down the ambient light, but make sure the door is open to prevent the build up of carbon monoxide.

If you see blue sparks, you need to replace the wires with a good quality set of replacement wires. The ones by Robert Bosch seem to fit the B very well and last well. They are available from BAP and other sources.  One problem with the silicone wires is that they do not work well with the screw in, side terminal caps on the Mark I cars. This is not a significant problem. If it is a show car, get the copper wires, which were originally correct for this model.  If it is not a show car, the 68-74 distributor cap will fit the distributor and allow you to use silicone wires that push into it. You will also need to install a different coil, one with the push in style terminal, but this would be a good time to install a Lucas Sports Coil anyway, right?

If the car seems to be running well, this is all the testing you need to do. If, however, you seem to have a miss, there is one further test you can run. This requires an ohm meter.  An ohm meter measures resistance and is normally a feature found on volt meters. In fact, most volt test meters are actually Volt-Ohm Meters (VOMs).  Good quality analog meters may be had for under $20 at Radio Shack and other sources. Some dwell/tach meters also have a volt and ohm feature. I prefer to have a separate VOM as it allows me to do tuning using both the dwell/tach and the VOM when necessary.

The first thing to do is turn on the meter and set it to ohms or resistance function.  Then, touch the two probes together and watch to see the meter’s needle swings to zero. This shows that there is zero resistance as it should be. Some of the more expensive meters have a zero function where the probes must be held to zero and the scale adjusted to zero.  The less expensive models do not have this feature and it is not needed for this type of work.  Having confirmed that the meter is working properly, remove the distributor cap from the car, having disconnected the spark plug wires from the plugs and the coil wire from the coil. A small piece of masking tape on each wire with the number of the cylinder the spark plug wire came off of  makes reattaching easy.

Then, take one probe and stick it into the spark plug end of the wire. You can probably insert it between the metal terminal and the boot to hold it in place. Then, you touch the probe to the terminal inside the distributor cap.  This tests both the cap and the wire. Make a note of the resistance reading, then check the other plug wires in the same manner. Finally, check the coil wire from the end that goes into the coil to the carbon brush in the top, center of the cap.  All of the spark plug wires should have about the same resistance. If one is very much lower or higher than the others, the set may need replacing. If one shows infinite resistance, the set may need replacing. How to determine whether it is a wire or a cap problem?

Simple. Remove the wire showing the infinite or high resistance from the cap and measure it again. If it now shows resistance similar to others, it is a problem with the distributor cap. Firmly seat the wire again into the cap, making sure it is fully engaged and check again.  If it still shows a problem, the cap is at fault.  If, however, when you test the wire by itself, it shows high or infinite resistance, the wire is bad and the set should be replaced. This is where the “lifetime warranty” pays for itself. Take the wires back and exchange them for another set.  I go one step further and keep a spare set of wires on hand and, when I need to exchange them, install the spare set and return the old set in the box.

The final question is how long will the silicone wires last. The best examples may do as long as three to four years. Often, however, the Arizona heat and high under hood temperatures will have them breaking down in two years or so. Testing the wires while doing a tune up only takes a short time. Good wires will give better fuel economy, reduce pollution and not leave you stranded when the car does not start. Time well spent.

This monograph may be reproduced only for non-commercial use without other permission of the author. Reproduction for commercial use only by written permission.

Copyright © 2001 by Les Bengtson