Ignition Coil Diagnostics
Ignition coils have evolved many times in the past century. Whatever the ignition coil looks like, it always does the same function, creating a spark by converting amperage to voltage. How efficiently the ignition coil operates has changed dramatically as well. The ignition coil is always made up of three parts, the primary circuit, the secondary circuit, and the iron core. A magnetic field is created around the soft-iron core when an electric current flows through the primary circuit or winding. When the current flowing through a few hundreds of turns of primary winding is interrupted, the resulting magnetic field collapses into many thousands of turns in the secondary winding. By “cutting” the magnetic field many thousands of times, the secondary winding multiplies or transforms low battery voltage into the voltages needed to create an ignition spark. The actual output voltage varies.
The Primary circuit includes battery voltage (B+) terminal attached to a 12-volt current source and a ground (B-) terminal attached to a power transistor that controls primary current flow. To create a spark, the transistor is commanded by the PCM to form a magnetic field by grounding the primary coil. The PCM then commands the transistor to interrupt the primary circuit, collapsing the magnetic field, and creating an ignition spark. Some import models attach a transistor to the coil directly, but it is usually on a separate ignition control module. (ICM) Most systems also incorporate the transistor into the PMC. The core temperature is regulated by changing the on-time/duty cycle at high and low engine speeds.
The secondary circuit is made of the secondary ignition coil windings, distributor cap, distributor rotor, spark plug cable, and spark plug. Distributor-less systems have no distributor cap or distributor rotor. The Secondary circuit transfers the spark to the spark plugs.
Ignition Coil Diagnostics
Intermittent ignition coil failures are tough to diagnose because the windings are heat sensitive. This can cause a coil to pass shop tests but fail under loads. Measuring a coil’s resistances can indicate whether it is defective or not. Another test is to see how well the spark jumps through the air, but there will only be a spark if there is 10v or more charged on the battery. Also, the air gap must be constant. Most technicians working on ignition coils use a computer based digital oscilloscope to measure the waveforms produced.
On modern COP ignitions, secondary waveform testing is almost impossible, so most technicians use a lab scope and a low amperage inductive current probe. Depending on how the waveform looks, flat-topped or pointed, determines whether the circuits are limiting or non-limiting, respectively. The primary circuit can be accessed through the ignition fuse in the fuse box. In COP systems with no other access, a pair of jumper leads can be used to attach an inductive current probe. If the coil driver in the PCM or the ICM is ruined, check the ignition coil for shorts. If the coil is shorted, it could ruin the brand new PCM or ICM replaced, which could end up being a costly replacement.