Ignition System Basics

There are three basic types of ignition systems used on a car's engine:

Each of these systems includes various components, some of which are common to all the systems. These include one or more ignition coils; and spark plugs. Most of them, with the exception of distributorless ignition systems, will have a distributor, while ignition leads will be common in most ignition systems, except those that use coil on plug (COP) systems.

Let's look at each of these components, other than the spark plugs in more detail. We'll look at the spark plugs in more detail when we discuss the spark plug heat range, and the spark plug gap.

The Ignition Coil

Modern ignition coil

A modern ignition coil

Although there are coils of different shape and sizes on the market, they all share a common basic structure, with three functional components: 100 to 150 primary windings of heavy copper wire, which are directly powered by the battery and are insulated to prevent voltage loss between loops; secondary windings, which are thinner and 100 times more numerous that the primary windings and are located within the primary windings; and a metal core around which both the primary and secondary windings are wrapped.

When electrical current is supplied to the coil, i.e., when the switching mechanism, be it contact breaker points or a pickup coil, is closed, the charge in the primary windings builds up slowly and magnetizes the metal core. This creates a magnetic field around the secondary windings. When the magnetism of the metal core cannot increase any further, the coils is said to be fully saturated. Then, when the electrical circuit formed by the switching mechanism opens and the electrical current to the primary windings is disrupted, the magnetic field collapses instantaneously. This sudden change in magnetism induces a high voltage spark in the secondary windings of the coil. This spark is fed through the coil tower, to the distributor and on to the appropriate spark plug.

HT Ignition Leads

The ignition leads should be able to transmit at least 20,000 to 50,000 volts of electricity without leakage and should have very little resistance of less than 5,000 ohms. Ignition leads with a solid wire core have the least resistance but they cause radio interference and are susceptible to cross fire between adjacent leads, which results in misfiring and lack of power. In addition, solid wires tend to vibrate in the insulating sheath, which leads to deterioration of the leads and a subsequent deterioration in performance. A good alternative to solid ignition wires is an ignition lead with a fine wire spiral wrapped around the core. This type of lead not only suppresses radio interference and cross fire, it is also immune to harmonic vibration, making them more reliable and longer lasting.

Regardless of which type of ignition lead you use, you should ensure that there is at least a 1 inch gap between adjacent wires and never run ignition wires or two consecutive firing cylinders next to each other, especially when using ignition leads with a solid wire core.

The Distributor

The distributor is the controlling element in the ignition system. It consists of a distributor shaft that is driven by either the camshaft or the crank shaft, an ignition timing advance mechanism that can be electronic or mechanical and is responsible for advancing the ignition timing at higher RPM, a switching mechanism that switches the current to the primary windings of the coil, a rotor that distributes the spark, and a distributor cap.

There are two things to watch out for in terms of the distributor shaft: sprocket climb, which occurs when the distributor shaft rides up the sprocket gear that drives it; and distributor rattle, which occurs when the bearings that holds the distributor shaft in place are worn and allows the shaft to move fore and aft. Both would cause less than optimal ignition timing. You can check for distributor rattle by pushing and pulling on the distributor shaft but sprocket climb is harder to detect.

The mechanical timing advance mechanism usually consists of a vacuum advance feed from the intake manifold and two centrifugal weights each of which acts against a spring. Usually, one spring is weaker than the other, or one weight is smaller than the other to allow for a two stage advance. At higher RPM, the centrifugal weights overcome the tension in the spring and turn the distributor's base plate on which the switching mechanism is mounted, effectively advancing the ignition timing. When the springs become worn, or dirt and grime prevent the proper movement of the weights, ignition timing would be affected. Electronic ignition advance is much more accurate.

If it is at all possible, you should opt for a distributorless ignition system (DIS) as it eliminates the ignition timing errors and failures that can occur when using a distributor. We'll discuss distributorless ignition systems, as well as inductive discharge ignition systems and capacitor discharge ignition systems in the next few pages. We'll also discuss spark plugs, spark plug selection, and the spark plug gap a little later.

Next we'll discuss ignition timing.