OK,I suppose I lapsed a bit into EE speak.Here follows a description of the ignition system found in the HT engines:
In the engine is a generator coil,the flywheel magnet induces a large voltage in it when it comes by.One side of it is connected to the metal in the engine and a ground wire is brought out,the"black wire".The output of the coil is the "blue wire" it carries a high ac voltage,over 100Volts,a short a high amplitude ac wiggle.This output goes to the CD unit which uses this input to charge a capacitor as thee nergy storage device.At the right time the energy stored in this cap is dumped into a step-up pulse transformer,(rather like an ignition coil), which then fires the plug.
To get back to the generator coil,it has a tap on it fairly close to the ground side,the "white wire".It acts as a low voltage output of the generator coil.The coil is what we in the trade call an auto-transformer (it has only one tapped winding instead of normally 2 or more).Shorting the white wire to ground puts a heavy load on the coil and reduces the output at the Blue wire substantially and with it the output spark voltage, keeping the plug from firing.This is one theory as to what the intended purpose of the WW is.It maybe correct I don't really know for sure.The other is that it is an output to run lights with,I'm inclined to rather doubt this.Using a switch to ground on the WW is a safe way to kill the engine,preferably carry the ground return back to the engine and use a high quality water proof switch
So far so good,now how to kill the spark.One way to do this is to short the WW to ground,(hopefully) killing the output of the generator coil.This is fine but not as sure fire as disconnecting the CD unit from the Blue Wire or shorting the Blue wire to ground.
I think it should work reliably, but it is conceivable that at high speed (high output from Blue wire) the reduction is not sufficient to keep the plug from firing.If the engine keeps running you have a high current in the WW and you could fry that part of the coil ,then the coil has no ground return and no output on the Blue wire,but providing that return with the grounding switch would accomplish this.That was the "paradoxical" situation I referred to previously.(This has actually been reported & I figured out what had happened).
Only a 20 volts or so are on the WW so that's pretty safe to fool around with,not so with the Blue wire,this output is definitely not benign it has over 100 V on it and carries a substantial current (a few amps at least during the ignition interval) and is loaded with all sorts of EMI crud.It's not a safe output, anything over 50 V is not safe for humans under wet conditions.As far as EMI is concerned the longer the wire and the more current it carries the worse it gets.If a series switch in the Blue wire is used to ikeep the Blue wire output from getting to the CD unit the current&voltage from the generator coil is carried to the switch&back to the engine ALL the time,if the switch is on the handlebars,this is the worst situation.The current in the wires create a magnetic field which is difficult to shield against (Normal RFI shielding is ineffective),the best thing is to keep the wires close together and twist,them.The best thing is not to do this in the first place and locate this switch close to the engine as a hidden antitheft device.If the kill switch on the Blue wire is used to short the coil output to ground,the wire to it carries no current normally,so magnetic fields are not a problem,but RFI (radio freq. interference) still is.I should mention that the magnetic field can play havoc with speedos etc by inducing spurious voltages in them.(twist the leads together!!)
In any case if you use the Blue wire to kill the engine,use a waterproof high quality switch,for your own protection but also to obviate leakage to ground which can mess up the ignition.The notion that shorting the coil output to ground can injure it is nonsense,while the engine comes to a halt, high currents are in the shorted coil,but that lasts just a few seconds at most.The amount of heating resulting from it is completely negligeable,I figured it out,making reasonable assumptions.I hope this explains things.
