There are three general ignition systems:
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.
Although there are coils of different shape and sizes on the market, they all share a common basic structure, with three functional components: primary windings, which are directly powered by the battery; secondary windings, which are thinner and 100 times more numerous that the primary windings; and a metal core around which the primary and secondary windings are wrapped.
When 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 and creates a magnetic field around the secondary windings. A point is reached when magnetism of the metal core cannot increase any further. When this point is reached, the coils is said to be saturated. When the switching mechanism opens and interrupts the current to the primary windings, 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.
The ignition leads should have less than 5,000 ohms resistance. Ignition leads with a solid wire core have the least resistance but these wires cause radio interference and are susceptible to cross fire between adjacent leads. Also, solid wires tend to vibrate in the insulating sheath, which leads to deterioration of the leads. A good alternative 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.
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.
The distributor 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-storage type 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.