Although most of us seldom think about an ignition module until a tow truck arrives with a cranking, no-spark problem, the ignition module is still an important part of ignition system diagnostics and repair. Looking back into history, “solid-state” or “transistor ignition” systems made their first appearance on production vehicles during the 1960s. As emissions standards tightened during the 1970s, electronic ignition systems proved their worth because, unlike contact point distributors, they didn’t need frequent adjustments and service. In addition, their solid-state electronics provided much higher firing voltages with much greater spark timing accuracy than did conventional point ignition systems.
IGNITION SYSTEM BASICS
Although thousands of different design variations in ignition modules now populate today’s import ignition market, it’s important to understand that all electronic ignition systems share the same basic functions. To produce a high-voltage spark at the spark plug, any ignition module must a) time the spark by sensing crankshaft position, b) saturate the primary windings of an ignition coil by turning the ignition system’s primary circuit on, and c) collapse the resulting magnetic field in the ignition coil by turning off the primary circuit.
To better visualize ignition system operation, a soft iron core located internally or externally on the ignition coil is used to create an electromagnet. A process called coil saturation occurs when a magnetic field is created inside the coil’s primary windings by grounding the primary circuit. When the primary circuit is opened, the magnetic field created within the ignition coil’s primary circuit collapses into a set of secondary windings, which multiply the voltage contained in the collapsing primary magnetic field into as many as 60,000 volts at the spark plug.
This on time of the primary circuit is called dwell time, which is measured in degrees of engine crankshaft rotation. Dwell time can also be measured as a duty cycle on a multimeter or digital volt-ohm meter. In the latter case, the operator’s manual usually includes a handy chart to convert duty cycle into dwell time.
CRANKSHAFT POSITION SENSORS
The job of the CKP is to indicate to the ignition module when a piston is approaching top dead center (TDC). As the piston reaches a predetermined point on the compression stroke, the power diode located inside the ignition module or at the ignition coil switches off the primary current, which collapses the coil’s magnetic field and creates a spark that ignites the air/fuel mixture contained in the cylinder.
The crankshaft position sensor (CKP) can be contained within a distributor assembly in early electronic ignition systems or be mounted at the front harmonic balancer assembly or the flywheel on more modern electronic systems. A crankshaft position sensor can be a two-wire magnetic pulse sensor or a three-wire Hall Effect sensor. In brief, the two-wire sensor displays an alternating current analog waveform on a lab scope, whereas the three-wire sensor produces a positive current square wave signal to trigger the ignition module. A third type of sensor, called an optical or photodiode sensor, is usually found in distributor-type ignitions and works on the principle of a shutter wheel breaking a beam of light to trigger the ignition module.
MODERN IGNITION COILS
The basic operating principles of the ignition coil haven’t changed since it was invented more than 100 years ago. What has changed, however, is the way the coil is built and the way it’s designed to perform. Modern coils have evolved from the old oil-filled canisters to molded epoxy to external magnetic-core to the modern, pencil-thin versions found on current coil-on-plug (COP) designs.
Due to the advances in coil design, modern ignition modules are designed to saturate the primary windings at much higher amperages than in earlier ignitions. Amperages have increased from about three amps on the old oil-filled coils to more than seven amps on some of the modern ignition systems.
Obviously, increasing primary amperage flow increases internal heating, which can shorten coil life. With that in mind, modern ignition coils have been redesigned to tolerate higher input amperages and higher output voltages. In addition, more coils are being used to reduce dwell time and thus reduce internally overheating the coil windings.
Electronic ignition systems have consequently evolved from a single coil firing all cylinders to distributorless, waste-spark systems using multiple coils to fire two cylinders each. The most recent innovation in ignition systems is the coil-on-plug or COP system, in which each cylinder is equipped with its own ignition coil.
IGNITION MODULE VARIATIONS
Needless to say, the ignition module has undergone thousands of design variations during its three decades of existence. Early modules were simple on-off switches triggered by a magnetic pulse pickup coil in the distributor. When computerized engine controls made their first appearance, the ignition module’s design was altered to send a square-wave signal to the engine control module (ECM). In this design variation, the ECM calculates the amount of spark advance required for the engine operating conditions and sends a return square-wave signal to the ignition module in order to trigger the spark event.
As operations become more complex, the module is often moved from the inside of the distributor to cooler locations like an engine compartment fender well. In other cases, the module becomes part of the ignition system’s “coil pack” assembly. In more late-model instances, the ignition module is integrated into the powertrain control module (PCM) itself to simplify system electronics.
Ignition modules have also changed the way they control primary current. Early modules saturated the ignition coil by grounding the primary circuit in the same manner as the old mechanical contact-point ignitions. Because many of those systems were made to run at a fixed dwell, a loss of resistance in the coil’s primary windings would increase primary current flow through the module. The module would then overheat and develop intermittent or hard failures.
To prevent the ignition coil from overheating, later ignition modules began to incorporate variable dwell times. Typically, these dwell times will run from about 10 degrees at idle to 30 degrees at higher engine speeds. To prevent module and coil damage, current-limiting modules were also introduced that limit primary current to a predetermined level, which typically ranges from five to seven amperes.
Because there are literally hundreds of different ignition module configurations on today’s imports, it’s important to always consult an applicable shop manual, database or wiring schematic to diagnose specific applications. Nevertheless, a few general guidelines can be used to diagnose ignition modules.
First, always check the B-terminal of the ignition coil for the presence of voltage and the occurrence of a dwell signal. Although some manuals recommend using a test light to test for dwell, the dwell signal itself may not be long enough to fully illuminate the test light. Consequently, testing with a lab scope or a voltmeter with a duty cycle or dwell test mode is more reliable. If the module is switching on/off, both the module and crankshaft position sensor (CKP) or distributor pickup are working correctly.
If the B-coil terminal doesn’t display a dwell signal, use a lab scope or a professional DVOM to test the CKP or distributor pickup for a signal. At this point, also be aware that some electronic ignition systems may also use a signal from the camshaft position sensor (CMP) to help trigger the ignition module.
A two-wire magnetic pulse pickup can be tested with a professional-grade voltmeter, preferably a real mean averaging (RMA) meter that provides an accurate measure of an alternating current signal. Three-wire Hall Effect sensors can be tested with equipment using an LED to indicate switching activity or a digital storage oscilloscope (DSO) to display its characteristic square-wave pattern. If the module is switching voltage at the coil, the module is working correctly in the cranking mode.
Also keep in mind that the PCM may play a role in triggering the ignition module. In these cases, the module may signal the crankshaft position to the PCM. The PCM may then calculate spark advance and signal the ignition module to create a spark by breaking the primary circuit.
In any case, always use a wiring schematic to help you understand how the ignition module is integrated into the engine control system. Diagnosing and servicing an ignition module isn’t difficult if you remember that each utilizes the same basic set of operating principles to create voltage at the spark plug electrode.