Ignition System Basics

As long as there have been internal combustion engines, there has been a need for an ignition system to ignite the air and fuel mixture in the cylinders. From the earliest days of automotive ignitions (where you advanced or retarded the timing via a lever on the steering wheel of Ford's Model A) to today's distributorless computer-controlled systems, the ignition systems haven't changed much in the way they work to get a spark across your spark plug's electrode gap. Your ignition system is critical to a smooth running engine (or your engine not running at all!), and while virtually maintenance free, there are important things to remember when choosing an ignition system, maintaining one, or even diagnosing one for repairs. Our goal here is to explain how all of the Mustang's various ignition systems through the decades work and what to look for when you have a problem.

Starting with the first generation of Mustangs from the 1960s, Ford used a breaker points based ignition system housed inside a distributor on the engine. The distributor is mounted at the front of the engine and is geared to the camshaft, which is in turn rotated via the timing chain connected to the crankshaft. The camshaft moves at half of the crankshaft's revolutions, thus the distributor does as well. The distributor houses a movable breaker plate that can advance the ignition via a set of centrifugal weights or with a vacuum canister diaphragm system. Positioned on the breaker plate are the ignition points set and a condenser unit (essentially a capacitor). The points are opened and closed via a cam lobe on the distributor's shaft. The points opening and closing are what controls the ignition coil's output.

The ignition coil has two sets of internal wire windings--a primary and secondary winding (not to be confused with primary and secondary ignitions, which we'll cover shortly) that share a common magnetic core. The secondary winding of wires is wound at a much higher capacity (more wire turns of finer wire) than the primary winding and is proportional to the step up voltage of the coil's output. Battery voltage is applied to the coil's primary winding from the ignition switch and the negative side of the primary winding is wired to the ignition points in the distributor. The primary voltage causes a magnetic field (EMF) to build up, storing energy in the coil between the two sets of windings. When the points contact opens due to the cam on the distributor shaft, the magnetic field collapses, inducing a high voltage in the secondary winding which causes a spark to occur in the coil's secondary tower where the coil wire attaches. The magnetic field collapsing can damage the ignition points, thus the condenser is used to absorb any back-flow from the EMF.

While the distributor and coil are some of the most important parts of your ignition system, the spark isn't going anywhere without a distributor cap and rotor, plug wires, and spark plugs. These components make up the secondary side of the ignition system (your battery, distributor, and coil are the primary ignition components).

The secondary ignition system is the high-voltage side of the ignition system; where as the primary side is the low-voltage side of the system. Once the ignition coil's EMF has collapsed and created a spark, the distributor, utilizing the rotation of the engine via its internal geared shaft, distributes the high voltage spark to the proper spark plug via the distributor cap's wire towers and the rotating rotor button inside the cap, which sits on the top of the distributor's shaft. The spark plug wires attached to each tower of the distributor cap hand off the high-voltage spark to the spark plug at the end of its wire run, where the spark jumps the electrode gap at the tip of the spark plug as it finds its way to ground. Hopefully, if everything is timed correctly, the spark will jump the gap just as the piston is compressing its cylinder of air and fuel to top dead center (TDC), igniting the mixture and causing the explosion that will force the piston downward to rotate the crankshaft. Now factor in six or eight cylinders all in time with their individual explosions pushing on the crankshaft's throws to rotate it in your engine block. There's a lot going on to turn that crankshaft and it all has to work together.

Triggering the Coil

While we mentioned ignition points in our hardware descriptions, points are rarely used these days, as electronic ignitions are much more accurate, have no moving parts and thus no wear, and can create more voltage and produce hotter sparks in the combustion chamber. Ford moved from points to their first electronic ignition, dubbed Duraspark, in 1975. Duraspark was used right up to the mid-1980s and is still quite popular today as a conversion for street rod and kit car builds, and even some Mustang owners enjoy the simple conversion. Ford moved away from the Duraspark system when it started fuel injecting its cars, first with the TFI (Thick Film Ignition) system and later with EDIS (electronic distributorless ignition system) as used on the V-6 and modular engines with no distributor and one or two coil packs. Finally, Ford switched to a COP (coil-on-plug) setup in the late '90s that is still in service today. These later systems use a series of sensors, such as cam position, crank position, and throttle position along with a computer, to determine when to fire the coil packs or individual coils.

The Duraspark modules are fairly hearty, but have been known to fail, whereas the EDIS coil pack system and the newer COP setup are sturdy systems that rarely fail. The Duraspark module is easily tested by the end user, but the EDIS and COP systems are a bit more technologically advanced and take some real know-how, test equipment, and manuals to diagnose.

Routing the Spark

Spark plug wires don't last forever. They have to fight off high engine heat and the temperature extremes of weather while carrying 40,000 volts of electricity hundreds of times a minute. We routinely toss new spark plugs into our Mustangs as part of a tune up "regimen," yet how many people actually test their plug wires when screwing in those new plugs? You can easily inspect plug wires visually and by feel, and if you suspect any issues you can measure the resistance of the plug wires with an ohm meter. Many stock-type wires average 1,500 ohms to upwards of 6,000 ohms per foot, where as some performance wires may only be 50 ohms per foot. Check your plug wire manufacturer for their specs and if you can't verify the information, measure all of the wires to determine if any of them stand out with higher resistance than the others. It's sort of like giving your engine a compression test.