ORIGINAL: tkg

If you have an CH or CHEXL ignition with a metal sparkplug cap then it is self grounding when a spark plug is correctly inserted. If you have a CH unit with a rubber plug cap then you must connect the ground braid to the plug to test it.
If you fire a CH unit with out having a grounded plug in the cap then you have a 50% chance of knocking out the sensor. Lets see 50% then 50% of that and then 50% of that , 3 sparks and you have a 12.5% chance of still having a good sensor.
When you fire an ungrounded ignition the spark has to go some where, the sensor is only 1/8'' wide the spark will seek the ground path through the sensor. Most transistors/IC chips do not take kindly to have 25,000v ran through them. If the spark can find another way to ground because of the internal resistance of the chip is high it will, but if it can't then its through the sensor and POOF.
Also IF your CH or CHEXL is constantly buzzing a spark when battery voltage is applied, then you have blown the SCR and your ignition is toast. Although is some rare cases to much voltage will cause the SCR to oscillate, but that usually just before it dies.