Coil operation on the vehicle
Even though the coil may be capable of an output of 40,000 volts, this almost never happens in real life. What will happen is that the voltage will rise as high as it takes in order for the spark plug gap to ionize. Based on the variables discussed earlier, the voltage required could be as low as 3,000 volts or as high as 30,000 volts.
The lower voltage can be enough to ionize the gap under high speed, light load conditions. The maximum required voltage happens under heavy acceleration where the load is high and the fuel mixture is rich.
Typical ignition firing voltages are between 8 kv and 22 kv. Having a higher voltage available from the coil may not be an advantage. Spark plug wires, boots, caps and rotors can be damaged by excess voltage. If the plug has not fired by 30 kv, the connection may have been lost either internally or externally.
As the voltage increases further, and has no place to go, it may jump out of the side of the boot, the spark plug wire or even the coil package itself.
Allowing the coil voltage to rise too high often results in system damage.
Typical ignition coil waveforms
By far the best tool for diagnosing ignition coil problems is the oscilloscope. On some coil designs it is possible to measure the primary and secondary coil resistances from outside of the coil. The primary resistance is a low value (typically 0.5 ohms) and is difficult to measure accurately even with a digital ohmmeter. While it is possible to measure the secondary resistance, the answer you get might really not tell you very much. The coil’s resistance is quite sensitive to the actual temperature inside the coil. The temperature related change might effectively “hide” the resistance change caused by shorted turns in the coil secondary. Even a small percentage of shorted turns can dramatically reduce the available coil output voltage.
There are three distinctive coil waveforms that define the performance of the coil. The first is the coil primary current waveform (see Figure 1). A properly operating coil will have a smooth, linear change from 0 amps at the start of the dwell period to whatever value the car maker determined was required for a full energy spark. Please remember that some coils will show a flat area at the top of the ramp that represents coil current limiting.