MSD Tech: Coil and Spark Plug Wire Troubleshooting

By: Ray Frescas | 11/14/2013 < Back to Motor Life Home

Coils and Spark Plug Wires Trouble Shooting

There are not a lot of things that you can check to confirm that the coil is operating. Also, due to the number of different coil variations available, it makes it difficult for specific specifications.

If you have an ohm meter, you can check the primary and secondary resistance of the coil. First, be sure to have the ignition turned off and the coil disconnected from the ignition. The primary resistance is measured with the probes on the positive and the negative terminals of the coil. This value will vary between coils, but a rule of thumb on MSD’s coils would be between .03 - .7 ohms.

Secondary resistance is measured between the coil + terminal and the secondary tower (where the spark plug connects). This reading also will vary between the make of the coil, but it should be somewhere between 200 – 14,000 ohms! Quite a range. The important thing to look for is that there is not an open or a short in the windings.

Spark plug wires can easily be visually inspected for tears, burns or black spots that may pinpoint a voltage leak. Close inspection of the boots and terminals should also be checked. You can also measure the resistance of each spark plug wire, but like coils, values may vary between wire brands. MSD’s 8.5mm Super Conductor Wire will measure about 50-ohms per foot of wire while the Street Fire brand wires have about 500 ohms per foot. As an example, if you have a 4-feet long Super Conductor wire, the resistance should be about 200 ohms. The important thing to check is that the resistance values of your wires should be consistent throughout the entire set. Lengths will vary the values, but they should all be within a similar value.