Abstract: The disclosure describes an ignition system for an internal combustion engine fitted with a plurality of spark plugs. The engine rotates a distributor that includes a primary coil which induces voltages in a secondary coil. The secondary coil voltages are sequentially distributed to the spark plugs in order to operate the engine. The engine is provided with a source of common potential, generally referred to as chassis ground.Improved apparatus for displaying the waveform of the voltage produced by the secondary coil includes an oscilloscope having a first input and a second input. A conductive probe is positioned close to the secondary coil, and a single conductor operatively connects the probe to the first input of the oscilloscope. A second conductor operatively connects the second input of the oscilloscope to chassis ground so that the waveform of the voltage produced by the secondary coil is displayed on the oscilloscope.