My car keeps ruining coils

Question:

We are restoring a 1965 Mustang convertible and it has blown up three coils. I now have a new coil and a new engine wiring harness. With the engine running at idle speed we measure 14.34 volts at the battery and 10.8 volts at the coil. When the engine is revved up the voltage to the coil rises to 12.08 volts. The wiring diagram shows a resistor from the key switch in the run position to the coil. We are unable to find a value or a voltage for the resistor.

Answer:

Since you have gone through three ignition coils it is logical to assume that there is a malfunction elsewhere that caused or at least accelerated the coil failure. Perhaps the most common cause of coil failure is excessive heat. It is essential for the internal components of the coil to be insulated from one another and this is accomplished by means of insulating cylinders that separate the secondary winding from the iron core, the primary winding from the secondary winding and the primary winding from the case. In addition, the wires of the windings are each encased in insulation. Excessive heat will break down the insulation, causing internal shorts and ultimately leading to complete coil failure.

Heat is an inevitable byproduct of current flowing through a conductor so it is normal for a coil to get hot when it is operating. As a result, coils are designed to withstand a certain level of heat without sustaining damage.

A problem arises when the voltage feeding into the coil increases because increased voltage always leads to increased heat. And since the heat developed is proportional to the square of the current even a small increase in voltage will translate to a large increase in heat. For example, if the current increases from 8 to 12 volts (an increase of 50%), the heat generated will double (an increase of 200%.)

About 6 volts should reach your coil when the engine is idling and no more than about 8 volts should reach it when the engine is at highway cruising speed.

Your Mustang’s ignition circuit originally included a resistor wire. This was a pink-color wire coming off the ignition switch and it reduced the voltage to the coil by nearly 50% when the ignition key was in the run position. If this resistor wire has been bypassed in your car you can replace it with a new length of resistor wire or with an ignition ballast resistor, which is a very common part that can be bought at just about any auto parts store.