getting to the bottom of an old Wives tale

I continually see dire warnings of Doom and Gloom and threats of the sky falling down on your head if you dare to test your ignition spark without earthing (grounding ) your HT leads during the testing but I have never really thought about this until last night when responding to a post about non starting tests .

What is this all about ? can someone please tell me how you test for an HT spark without grounding the HT lead or spark plug ???? The very process of testing an ignition spark at the HT lead requires the HT lead to be held close enough to ground in order to obtain an arc (spark)

If you test for an HT spark by removing the spark plugs and holding them against the frame or an earthing point (ground) then again the HT lead is grounded by virtue of any spark being able to go to earth /ground via the spark plug body

The only situation where there may be a risk to an electronic ignition system is if the engine was cranked over with either of the HT leads loose so that the HT spark was unable to find an earth to dissipate to . Even in that event I suspect that it would take a lot longer than a brief couple of seconds to damage a TCI equipped bike which is all the time that it would take to test for a spark.

I may be wrong but think it is extremely unlikely that anyone is going to test for an HT spark on any bike TCI or other electronic ignition system without grounding either the HT lead with the cap removed or a spark plug that has been removed and held against an earth point except in the event of the caps of both HT leads being removed and only one HT lead tested at a time .

I think there is far too much confusing or misleading advice spread throughout the net when we don't take the trouble to give accurate information and warnings. The risk to a TCI or unprotected electronic ignition system is when the engine is cranked with the ignition on for other purposes other than testing for a HT spark and I cannot think of a reason off hand why you would need do that ...?

I welcome discussion on this, particularly from anyone who has taken a perfectly good TCI and tested it to destruction by cranking the engine with both plug caps off the spark plugs for extended periods

i just don't buy it . if TCIs were that fragile and flaky they wouldn't last 5 minutes in a high tension environment let alone 38+ years

Good point, peanut, but the warnings aren't urban myth. There's a difference between things that aren't good for the ignition and things that will guarantee catastrophic failure every time. Right, to check for spark the HT lead has to be grounded. That isn't where the ignition is at risk. Some tuning operations involve running on one cylinder. With OE breaker point ignition a person can just pull a plug cap: no harm. An electronic waste spark ignition may tolerate a few brief doses of that treatment, but that's about all; current that would normally go to ground through the disconnected HT lead is going to seek ground through another pathway.

yes thats my point really. Despite the typical warnings ,TCI ignition is not generally at risk when testing for a spark ...........it is at other times when the engine is cranked or run with the HT lead ungrounded,..like the good example you give of running on a single cylinder.
I was just being a bit pedantic I suppose but so often warnings are given without a clear explanation and that can lead to confusion

PamcoPete showed a video demonstration of how he could run the Ultimate (?) coil in open air. It's the dielectric breakdown in the OEM and other coils that causes the risk. The high voltage can leak back to the primary windings, directly to the TCI, or other ignition device. His IGBT transistor, and the TCI driver, are rated for about 500-600 volts. Hi-tension leakage from those coils would exceed that rating. In summary, it boils down to the coil, its insulation quality and construction...

For TCI. Example where a person could overlook not grounding the plug caps through a spark plug would be performing a compression check.

As part of my method, I 'gator clip plug thread to cylinder fin before I attach compression gauge to cylinder head.

My understanding of how the XS650's TCI works is that it's all one system.
The spark goes down one coil HT wire to a plug, through the cylinder head, through the other plug and up the other HT wire back to the coil.
Open the system by pulling either plug cap and nothin'll work. You have to ground the pulled plug to run t'other cylinder.
As to grounding the plug wires when compression testing, I've no idea if not doing so would damage the ignition.

I have started my 1980 with tci, and wondered why in the hell is it still hitting on one cylinder?? Walk to other side of bike and DOH!! WIre off. I don't mean to forget, but sometimes I do.