Engine Diagnostics Part Two

The Ignition System
The ignition system is really responsible for the whole spark part of the fuel/air/spark/compression formula. Its job is to produce and distribute 12 volts to the ignition coil and at least 40,000 volts to the spark plugs. If there's a problem here, the car won't start, and if it does it will misfire, hesitate, stall or fail an emissions test. An ignition system consists of a primary and secondary system. The primary system is made up of the battery, ignition switch, primary windings of the coil(s), pickup coil or crankshaft sensor and ignition module (igniter). The secondary system is made up of the secondary windings of the coil(s), distributor cap and rotor (if applicable), and spark plugs and wires (if applicable). That's a lot to chew on. It's also a lot to contemplate when an ignition problem is suspected. The trick is checking things in the right order, with the right tools, and knowing how to interpret the results so you're not throwing money at the wrong parts.

One of the first things you should do is check for spark if you suspect an ignition system problem. Remove a plug from the cylinder, reconnect it to its respective wire, position the electrode a few millimeters away from any metal part on the engine, crank and look for spark. It's not as accurate as using a spark tester, which forces the spark to jump a wider gap, but it works. But you'll be able to tell if there's no spark or an intermittent one in either case. This could be due to a faulty coil, plug wire or ignition module. Check for voltage at the coil's positive terminal with a multi-meter (by the way, if you have a coil-on-plug ignition, this is where you'll want to begin testing). Look for an open circuit in the ignition wiring if there's no voltage. Check the negative terminal while cranking the engine over. The coil will pulsate voltage when working properly, and voltage readings should vary. If they don't, check the individual coil, ignition module or wiring. It's fairly easy to check to see if a coil's OK, just measure the resistance between its terminals with a multi-meter and compare with your service manual. The results will tell you if the coil's primary windings are fine. To check the secondary windings, set the multi-meter to read kilohms, instead of ohms, and measure the resistance between the primary terminal and the secondary coil tower. Again, check what your service manual has to say about this. While you're checking the coil tower, make sure the screw's tight (if applicable), as this is a common source of ignition problems.

But before diagnosing any ignition problem and breaking out the tools, check and make sure all of your spark plug wires are routed in the right firing order and connected properly (if applicable), that your plugs are clean, and that the distributor cap and rotor (if applicable) are in good condition. A damaged or corroded plug wire or cracked cap will reveal a spark if you check things out at night. Be sure and measure the resistance of the plug wires to make sure they're within specifications too. You can also check to see if intermittent engine missing is due to a spark-related issue with nothing more than a spray bottle full of water. Just lightly mist water over the spark plug wire and distributor area and look for any signs of arcing while the engine is running. Hooking up a test light to the coil's negative and positive sides will also reveal whether or not the ignition is the cause of a rough-running engine. The light should be dim and steady. If it pulses or varies, there's excessive resistance somewhere in the circuit.

Even ignition timing can affect engine starting and operation. Excessive timing, either advanced or retarded, will make for difficult, slow starts and possible overheating, not to mention a loss in power. Be sure and use a quality timing light, preferably one that hasn't been dropped on the shop floor, and adjust the timing to the factory specifications.

The Charging System
We know the battery supplies the current and that it's part of the primary ignition system, but without a charging system, it wouldn't last very long. Alternators, also called generators, use electromagnetic induction to generate electricity in order to keep the battery fully charged. Alternators generate alternating current (AC), which must be transferred to the battery. But car batteries can only use direct current (DC), so diodes within the alternator convert AC to DC to make everything happy. There are a couple of tests that should be performed if you suspect a problem with your vehicle's charging system, or if you just saw your charging light pop up on the dash. But before doing anything, make sure the alternator belt is tensioned properly and that the ribs aren't worn out. Now would be the time to look for any blown fuses. Then check to make sure the alternator's not producing AC voltage. With the multi-meter set to read AC, connect the leads to the battery terminals and run the engine at 2000 rpm with the headlights on. If you see more than .5 volts of AC, the diodes are bad.

You'll also know there's a problem with an alternator if you keep going through batteries. To check charging system voltage, connect both multi-meter leads to the battery terminals and measure voltage at your service manual's recommended rpm point. If voltage reads too high, check the alternator's and voltage regulator's electrical connections. If it's too low, it could still be a wiring problem or the alternator itself, but more tests should be done. Checking the alternator's rear bearing for magnetism will tell you whether or not the voltage regulator and generator brushes are working, and if the rotor's producing a magnetic field. With the engine running, use a metallic object to check for magnetism at the rear of the housing. Charging current can't be produced without this magnetic field present, so make sure it's there.

Like we did for the starter, you can also check voltage drop for the alternator. Operate the engine at a fast idle again with the headlights on, and connect the multi-meter's leads to the output terminal of the alternator and the positive terminal of the battery. Readings over .4 volts mean there's too much resistance, while readings equal to the battery's mean there's an open circuit somewhere. Check that the alternator is properly grounded by connecting the multi-meter's leads to the negative battery terminal and the alternator's housing. The reading should be less than .2 volts under a fast idle with the headlights on.

Keep in mind, we've barely touched on all of the tests that can be performed on a vehicle's cranking, ignition and charging systems, but these will get you started and, most importantly, can be done with less than $50 worth of tools. There are several internal components of both starters and alternators that can each be tested and replaced. Tests like these require specialized, not to mention expensive, equipment that even we don't have. But that's OK. With the tests mentioned above, we can at least pinpoint the problem to, say, the starter, with an educated guess that it just might be its pinion clearance, for example. This narrows down the possibility of buying a bunch of parts that we don't need.