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Ignition System Basics
Learn all the basics to your mustang's ignition system right here
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.
The End of the Road
At the end of each plug wire is the spark plug. Many of us don't give much thought to heat range, plug reach, or electrode type. For the most part, we screw in the plug recommended by the instructions (for aftermarket heads) or that our friends run in a similar application (or even listed in the owner's manual or emissions label for you late-model owners). While all of these factors are important and could warrant a few pages all on their own, needless to say, the spark plugs need to be in good working order with the proper electrode gap in order for them to properly ignite the combustion chamber's air and fuel mixture. A fouled plug due to oil or carbon contamination, an excessively worn electrode preventing the spark from jumping to the ground strap properly, or even a ground strap missing due to pre-ignition (spark knock) are all conditions that will prevent a good burn in the combustion chamber. So as you can see, having all of your ignition's components in top order and working together is how you can provide a strong spark to your engine's cylinders.
When Something Goes Wrong
It's been a fun day at the car show and you're ready to head home. You fire up your Mustang and about ten minutes down the road the exhaust starts backfiring and popping. The next thing you know, your engine quits and you're coasting to a stop on the side of the road. What's going on? Your engine needs fuel, air, and spark to run (and proper timing, but we'll dismiss this since the car was running). For this exercise, we'll skip right to the assumption that you have fuel and air and no mechanical engine issues. So how do you check for spark? The easiest road-side check is to simply pull off a plug wire (or the coil wire from the distributor cap) and place it close to a ground path (fender bolt, valve cover bolt, etc.) but not too close to any fuel source like the carburetor. Have someone reach in and crank the starter. You should see a series of sparks jumping from the plug wire's end to the ground source along with an audio confirmation of the spark in a nice "snap, snap, snap" sound. Again, for the sake of our story, we'll say that you don't have any spark. Where to next?
The spark originates from your ignition coil, so for starters ensure you have power to the coil. You'll see a little "+" and "-" next to the coil's wire terminals. The positive side should show voltage with the key on (you'll need a volt meter or test light in your roadside tool kit). Older points-based systems used a resistance wire from the ignition switch to the coil, so you should only see about 7- to 9-volts, except for when cranking, which will be full battery voltage. Aftermarket ignitions and Duraspark/TFI ignitions will show full battery voltage, as they do not use a resistance wire. If you do not have power to the coil, it cannot build its magnetic field to create the higher secondary voltage.
In a pinch, you can run a wire from the battery's positive terminal to the coil to get you home (note this often will burn your points, but is an option if you just have to get the car moving). If your coil shows voltage on its positive terminal, the next simple test is to move the test light or volt meter to the negative terminal and crank the car again. You should see a pulsing of the test light or the volt meter reading rising and falling. This shows the points are opening and closing (or the electronic ignition pickup and reluctor are working) and that the rotor/shaft is spinning inside the distributor. This pulse is what also runs most tachometers, so if you're cranking your engine and the tach isn't moving, it's an instant sign you have an ignition module or points issue (or your tach just quit!). You can also check the coil's resistance with an ohm meter if you have the coil's correct resistance values with you.
Let's state for our scenario that the coil checked out OK. We have good voltage to the coil and the negative side is "pulsing." Remove the distributor cap to inspect the cap, rotor, points or ignition module, and any moving parts in the distributor. We've often seen the carbon button in the center of the distributor cap fail with age and/or mileage, so be sure it is in place. Look for arcing/tracking on the cap and rotor. We've also seen where the coil wire's voltage goes through the rotor and grounds to the distributor shaft via a crack in the rotor. Excessive moisture inside the cap can be an issue as well. Be sure to have someone crank the engine with the cap off so you can inspect all moving parts and to see that nothing is jammed or broken. A broken rotor can sometimes not rotate evenly or at the same speed as the shaft, or move enough to cause spark scatter and excessive timing retarding. Check the points to ensure they're not burned and at the proper gap. We've seen points set screws loosen or the coil wire come off the points mounting. Don't forget that the breaker plate ground strap should be securely fastened too. For electronic ignitions like the PerTronix that use a plastic cap reluctor over the distributor shaft cam, ensure it is secure and the gap is properly set to the manufacturer's instructions as well. Lastly, make sure the distributor itself is tight and hasn't moved (it wouldn't be the first time someone set their timing and forgot to retighten the hold-down clamp), and that there's a solid ground strap from the engine to the frame and/or body.
Checking the primary side of the ignition system is key, as that is where the majority of "crank/no start" issues we've seen have come from. Rarely will a bad plug wire or spark plug on the high-voltage secondary side cause a no-start condition. It'll run like garbage, yes, but we've driven home on six or seven cylinders before and so can you. Generally, we like to keep a spare coil, ignition module or points set, and some basic ignition tools (feeler gauge, test light, volt/ohm meter, etc.) in our trunk's tool kit as spare parts for road side emergencies. Even if it's not your own ride you get running on the side of the road, it might be a friend or club member you help get home. We've seen it time and time again; it's better to have those spare parts and never use them than to not have them and get towed home. We've got some additional diagnostic tips in the photo captions, as well as showing some ignition setups to consider over points and breaking down the basic ignition tools you should own. Also, be sure to check out www.mustangmonthly.com for our ignition tips video where we show you how to perform these tests.