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...the only way to truely check the ignition switch that I know of is by substitution. If you check the switch with a meter or light, you are testing its static operation. This will show a high resistance fault between the switch poles, which is a common enough failure mode, allright. However, many times the switch fails because of dynamic current flow. The sparking which occurs in the Kettering system can induce high sympathetic voltages across the switch. Now, the spark has more or less a square or saw-tooth wave form, which makes it behave more or less like alternating current. This means the insulation in the switch is no longer functioning only as insulation from ground, but also as a dialectric in an ad-hoc capacitor, with a switch pole as one of the plates, and the grounded switch body as the other. When the insulation is new and clean, that's all well and good; the switch is, after all, designed to function in this environment. But if the insulation developes even hairline cracks or accumulates too much dirt, the high voltage induced across it can exceed the breakdown voltage of the ad-hoc dialectric, and it can arc to ground. Once an arc starts, it sets up a low resistance path. So, the 6 or 12 volts, which couldn't cross the insulation before, can cross the now broken down dialectric. The result; much if not all of the energy intended for the ignition is dissipated across the path created by the arc. You can find this kind of fault with an oscilloscope if you know or can extrapolate the waveform you are looking for; but finding it with a volt-ohm-meter or light is nigh unto impossible. This is in part because the volt meter and the light present a higher resistance path to ground than the arc; so the current goes to ground through the arc rather than the measuring device. In addition, the duration of the arc can be very brief, adding to the difficulty of seeing it with a volt-ohm-meter or light.
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