Secondary Ignition – The Art of Spark

What is a coil?
From the early beginnings of the internal combustion engine there have been several different ignition systems to create a high energy spark.  The most popular system and the one that is in use today is a step up coil to generate the high voltage. A coil is nothing more than a transformer which like any transformer, has two separate coils of wire inside it commonly referred to as the windings. These windings wrap around a core of magnetic material, in newer transformers the core is made from a series of plates usually a ferrous type of metal laminated together. The first coil of wire is called the primary; the second coil of wire is called the secondary. The primary winding uses a larger diameter wire with fewer windings which allows the primary to have a very low resistance to current flow. While the secondary uses a much smaller diameter wire with a greater number of windings.

An automotive coil typically is wound approximately 1 to 100 wraps of the conductive wire, in other words, for every 1 winding of the primary the secondary has 100 windings. Most of the standard automotive coils primary windings usually have about 1 to 4 ohms of resistance; whereas, the secondary winding usually has about 8,000 ohms to 16,000 ohms of resistance. The primary and the secondary are electromagnetically coupled so anything that affects either winding is mirrored in the other winding.


The physics are the same no matter the size of the coil. A current flows through a winding which is used to create a magnetic field. This magnetic field build up is referred to as inductance. The magnetic field is proportional to the inductance and the current or in other words, the larger the current the larger the magnetic inductance. This current flowing through the primary coil winding allows the energy to build that magnetic field. As the magnetic field is building, it moves across the primary and secondary windings. This induces voltage in both the primary winding and the secondary winding. However, the effect of this induction is different within the two windings. As the magnetic field builds and moves across the secondary windings this induces an electromotive force which in turn creates the high voltage.


  Between the coil windings there is what is called capacitance. Capacitance is when two conductors are separated by space and current is allowed to flow through the conductor. Electrical potential is built up between the two conductors, while the size of the conductors and the distance between these two conductors determines the capacitance. This allows the capacitance between the coil turns to build electrical energy.


Once the current rises in the primary windings to a point that it basically can’t hold anymore the magnetic field is fully built up and completely surrounds the secondary windings. This is referred to as the saturation point. The larger the current, the larger the magnetic lines of force. Likewise, the smaller the current, the smaller the magnetic lines of force.


A coil takes a finite amount of time to build this charge up. That's the dwell time, normally defined as the degrees of rotation of the camshaft during which the input signal is shunted to ground.  Too little dwell and the coil doesn't have time to charge up fully. Too much dwell and the coil has lost some of its charge, causing a weak spark. Hesitation, low power, misfiring, pinging and a number of other conditions are symptoms of an incorrect dwell setting.

Back in the day of points and condensers checking the dwell was part of an all-around tune up. Since dwell is measured by the camshaft rotation and the camshaft runs at one-half the speed of the crankshaft, for every two degrees that the dwell was off, the actual engine's timing would be off by one degree. Because of the constant electrical arc between the points contact area they would wear down causing the dwell to be changed periodically, which led to readjusting the point gap or replacing them on a regular basis.  With today’s cars and the computer driven electronic ignition, dwell isn’t as big an issue to maintain as it was back then.

Testing the Secondary

Even though you might be able to get the new guy in the shop to hang onto the end of a plug wire while you crank the engine over to check for spark, I doubt he’ll try that twice. Of course you can also see a spark jump across an air gap with something like a spark tester, but the best way to observe the secondary ignition voltage is with an oscilloscope.

Not only is it more accurate than by just yanking a plug wire off, but with a lot of models these days you can’t even get to the plug wires or spark plugs without tearing layers of protective engine covers off or countless hoses and tubes that the manufacturers have so graciously put in the way. So being able to observe what’s going on inside the combustion chamber without tearing things apart is not only going to save you time, but save you from changing parts that may not need replacing. Learning how to interrupt a secondary wave form doesn’t take long at all either.  Once you’ve got the leads hooked up, the display adjusted, and the engine running chances are you’ll see the problem right in front of you without even turning a wrench.

Unlike using a spark tester which only shows you if there is spark or not, the scope can give you the value of that spark as well as the duration, the input voltage and the oscillations associated with the signal.  If you’re unsure what it is supposed to look like that’s not a problem either.  There are several websites that offer known good patterns to verify what you’re seeing as well as helpful tips on how to interpret the scope readings.

Another advantage of using a scope to diagnose the secondary pattern is the amount of information that can be gathered by understanding the wave pattern.  From finding a shorted/grounded plug wire or spark plug (low firing line - Kv) to an open or disconnected plug wire (high fire line-low spark line – spark duration).  Even a faulty injector can be diagnosed with your scope hooked up to the affected cylinder and performing a throttle snap and watching the firing line and the spark line.

I’m no expert when it comes to scope readings by far. So I spend a lot of time listening and observing other technicians and instructors in the use of a scope. I’ve seen some very ingenious ways different techs have found to hook a scope up and read the secondary ignition, too many to list that’s for sure. If you’ve used a scope in the past there’s not much I need to explain to you, however it’s the guy who is thinking about it that I hope to reach with this article.  Once you’ve got the hang of using a scope and reading the secondary you’re probably going to retire that old spark tester.

Practice when you can on a car without any secondary ignition problems so you can get familiar with what a good pattern looks like on your scope.  Then create a failure such as an open plug wire, or grounded wire, or watch what happens when an injector isn’t working.  When you’re familiar with the pattern changes your job of diagnosing secondary ignition problems is going to get a whole lot quicker, and a whole lot easier. 


Return to Knowledge Page