Learning from the ignition system

Let's play with some ignition systems and see how the spark teaches us.

electrical ignition systems vehicle ignitions no starts starting complaints repair shop training technician training A/C training automotive aftermarket There aren't too many issues you can have with an ignition system. It either has no spark, a weak spark or a spark happening at the wrong time. The function of the ignition system is also pretty basic. At the heart of the system is the ignition coil, made up of two windings: the primary and the secondary. Power is fed to the primary coil, a magnetic field is created in the process and then the power is shut off at a specific moment. This causes the magnetic field to collapse and that collapse energizes the secondary, stepping up the voltage to a level needed to jump the plug's gap and begin the combustion process.

Ignition systems can be one of three basic designs: the distributor ignition system, the distributorless (DIS) ignition system or the coil-on-plug (COP) system. They all perform the same function, but even in the individual systems, design differences make offering you a "one size fits all" troubleshooting guide a little difficult. So instead, let's play and take a look at how to approach examples of all three.

A 1989 Jeep Comanche

The distributor ignition system has been around for as long as I can remember. Originally, a mechanical set of electrical contacts called points was used on the ground side of the coil primary to control current flow. They were opened and closed by small ramps built into the distributor shaft, which the camshaft drove.

Another contact was mounted on top of the shaft, called the rotor. The rotor turned with the shaft and acted as the path for the secondary side of the coil, connecting each cylinder in the firing order by passing near matching contacts in the distributor cap. With the introduction of electronic ignitions even this oldest of the designs we encounter has nuances of its own. As you've heard me say many times, be sure to read up on the theory and operation of the specific system you are working on.

In the Jeep's case, the coil is part of an ignition control module, but it is serviceable separately. The Electronic Control Unit (ECU, an early ECM) handles coil control via the ignition module. Coil output is routed via a conventional ignition wire to the distributor, where it is fed to the six individual plugs on this early 4.0 liter in-line engine. What's kind of cool is under the distributor cap. It is a synch pulse generator — an early camshaft position sensor if you will — that the ECU uses to synchronize the injectors.

If you can access the spark plug lead, a quick test of any ignition system's health can be performed with a spark tester. This handy tool allows you to adjust the air gap between the test points and is installed in the ignition lead in place of the spark plug. The other side of the tool is firmly attached to a good engine ground. Disable the fuel system to avoid flooding the cylinders with gas and crank the engine over.

Older single coil systems didn't put out the kind of energy more modern ones do, so I would set my tool to simulate 10,000 to 20,000 volts and look for a clean, consistent blue spark. If the spark is weak or drops out, move your tester to the end of the coil wire where it connects to the distributor. Now, if the spark is good, you know the problem lies in the distribution: distributor cap, rotor or wires. If not, focus on troubleshooting a weak coil output, a bad coil wire or a fault in the ignition module/ECU circuits.

A 2006 Kia Sportage

This four banger uses a distributorless, or DIS, ignition system. Instead of one ignition coil for use by all the engine's cylinders, a DIS system uses one coil for each pair of cylinders. Also called an EI, or electronic ignition, system by some manufacturers, this design puts out a significant increase in spark energy of 30,000-plus volts, which is needed to ignite the leaner mixture in later model engines.

The Kia coils share a common power wire, with individual grounds leading to the Engine Control Module (ECM). As in the earliest distributor systems, coil primary current flow is controlled on the ground side. Some DIS systems utilize an ignition control module in addition to the ECM, but not this one. Timing control is strictly via the ECM, based primarily on inputs from the Crankshaft Position Sensor (CKP) and the Camshaft Position Sensor (CMP). In addition, the engine uses a Knock Sensor to retard timing should the sensor detect any sign of detonation, or spark knock.

Here's a question for you to consider, that is, if you've been keeping up with our electrical discussions. Every circuit has a source of EMF and a load. I've stated that in the majority of cases, the battery is the source. But what is the source in the DIS secondary? And what is the load?

While you think that over, back to our Kia. A DIS coil fires both plugs at the same time, every time. If the cylinder is on its compression stroke, that plug firing is called the power event. If the cylinder is on its exhaust stroke, the plug is firing on the waste event. Back to my question.

The secondary coil winding is the source, and the two spark plugs are the loads in the circuit. You know the battery has a positive and negative side as does the secondary. Current flows from the negative side of the coil, through the first spark plug and the cylinder head to the second plug and back to the coil. The compression plug is going to need the most energy, but if a problem occurs on the first plug's side of the coil (excessive gap, weak coil output, etc.), there may not be enough left for the second plug. That is why you can have a single misfiring cylinder even when the coil is shared.

Testing the spark energy can be done with the spark tester, just like with the Jeep. A scope and amp clamp can be used to test these systems, and often spot weaknesses you may miss with a mechanical tester.

A 2007 Toyota Corolla

The Toyota uses a version of the newest ignition system design, the coil on plug or COP ignition. In these systems, one coil is assigned to each cylinder. Some of these systems can produce as much as 50,000 volts, so I wouldn't hold one in my hand while it was working if I were you!

Many COP coils are controlled like all the rest — on the ground side by the ECM. One of the challenges of modern engines is accessing the plugs in order to scope the secondary ignition system. On any two-wire DIS or COP coils, you can attach your scope lead to the primary control side (ground side) of the coil to trace the primary event. This will produce a few hundred volts, so make sure you use an attenuator to prevent damaging your scope. The pattern, with the exception of the firing line, will mimic the secondary on most designs and allow you easier access by connecting at an exposed harness connector or at the ECM itself.

The Toyota, however, uses an ignitor internal to the coil that actually controls the primary side ground. This is a transistor that is turned off by the ECM on what Toyota references as the IGT line. If no IGT signal is given, spark will not occur at all. As a feedback, a return signal is sent to the ECM on the IGF lines, letting the ECM know that the coil is working. If no IGF signal is seen, the ECM will enter a model specific fail-safe mode and store a code, all to protect the catalytic converter.

And the igniter COP coil is not unique to Toyota. If you see more than two wires headed into a COP coil, spend some time studying exactly what makes that system tick before beginning any fault diagnosis. Troubleshooting ignition systems is still based on some old principles, but it's how they are applied on a modern car that can create the challenge.