Ignition System Basics

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.