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On V-twin coils, with the diodes in the kill wires, how does that diode being bad cause issues with the coils? If that wire is just a ground, how could the coils interfere with one another?

Diode only allows current to flow in one direction. With both coils hooked to the same kill wire, the diode will not allow one one coil to back feed the other coil. Think of it as a one way street, everything needs to flow in the same direction.

Rivets said:

Diode only allows current to flow in one direction. With both coils hooked to the same kill wire, the diode will not allow one one coil to back feed the other coil. Think of it as a one way street, everything needs to flow in the same direction.

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i understand HOW the diode works, What i don't know is the WHY, if that wire is a ground circuit when the key is shut off, they both go to ground? What causes the back feed from one coil to the other?

The voltages that trip the Hall Effect Chip is minute in the microvolts region
When a coil fires you get a thing called a ripple . a tiny voltage that gets sent down the kill wire which causes the other coil to attempt to fire
Ripples are wonderful things every time electrical device turns on or off it causes a ripple and every one is different
If you set up a ripple filter in your house you can use it to tell what your neighbours are doing when they turn each light on or off.
It is the way a lot of law enforcement finds growing houses ( pot ) or backyard meth labs .
Diodes are funny things electrically and tiny voltages can pass strait through depending upon exactly what type of diodes and their construction
So the clamping diodes used in the coils will allow a tiny negative voltage through before they go open circuit
Anda lot of them are temperature or voltage dependent
The Zenner diode used on British motorcycles for decades is closed from 0V through to 9V so under voltage is dumped into the frame , goes open from 9V to 14V so the power can pass into the wiring ten closes above 14V so again the power goes into the frame
They are not the simple one way valve under all circumstances that most think they are

Got it.
Thanks bert.

Ripple technology was another of those great Australian developements that our idiot government would no support so it got sold to the USA .
The other big user is railways & power grids
There was a plan to use power lines for the internet but wi-fi ( another Australian developement ) put an end to that
I think you can still get ethernet connectors that plug into a wall socket
If it is above line voltage , it is called a surge or spike
If it is below line voltage it is called a ripple

Zener diode (also known as breakdown diode) are used primarily for voltage regulation.

They get used a lot for switching
We have a system to blow branches off power lines where a high voltage capacitor is charged till it gets to the zener voltage when the zener cuts in / out and a high voltage DC pulse is sent down the wires that blows the debris off the power lines
There is one just down the road a piece from me and the first few times it fired I thought there was a lightning flash or a dead short

BE careful posting links like that some my go there and actually learn about electrickery

I don't think our electrical system here uses anything like that but I would get with the FPU engineers and ask them. But getting them off their high horse is a problem. One of our biggest problems is tree overgrowth. A lots tree trimming goes on here. They came by my place to take about two thirds of the tree tops off. I ask them if it would simpler just remove the trees. They said yes and agree to take out about 24 of tree under the lines.

The Bradford pear trees are known to be brittle trees. Besides who planted directly under the power line was being an idiot. I do need the windbreaks but trees I replanted are no where near the lines and are good strong trees. It will another 10-15 years before they big enough to do their wind braking but by then I be too old to care.

Most DC power supplies I built uses linear regulation so only one Zener is needed and one or more slave transistors depending the current level I need. Inefficient in one way so all over voltage is wasted as heat but they are high frequency noise free. Now switching regulators are a whole different beast. Some uses two Zeners back to back To pulse width modulate the voltage and current, A lot filtering has to done to use them in sensitive electronics circuits to remove the high RFI components. I kinda the DC and to DC convertor as they are royal pain to repair as when they fail usually you got to replace at least half their components. Getting the oscillate at the right frequency is one of the problems.

Remember this is Australia the home of the gum tree renown for dropping branches of all sizes
We used to employ real people to drive the power lines to check for this sort of stuff.
Possums and flying foxes also seem to like to bridge high voltage wires as we run 240 to the houses , 415 for 3 phase & 20kV feeders to the pole tops which reduces the number of sub stations but make for a spectacular light show then a pole top blows and of course means we can use lighter gauge wires
The grid here is not particularly stable as I regularly get line voltages as low as 210 & high as 280
Hopefully I will be off grid in the near future as I am part way through making 4 wind turbines from old washing machines then some solar panels .
Already have a UPS good for running the technology for 24 hrs & a couple of generators to run the freezer .
The major tree problem is lack of forest & verge maintenance so after the 2019 bush fires probably 10,000 to 30,000 new trees have shot up and these are now pushing 40' tall
Most are at 1' to 2' spacings so impossible to get in & thin

Scrubcadet10 said:

i understand HOW the diode works, What i don't know is the WHY, if that wire is a ground circuit when the key is shut off, they both go to ground? What causes the back feed from one coil to the other?

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I haven't read all the comments, but as you know, when one is firing, that mean the other is not. In that micro second, it could be getting juice back to it.

Now, I'll go read the other comments and see how I'm wrong.

StarTech said:

I don't think our electrical system here uses anything like that but I would get with the FPU engineers and ask them. But getting them off their high horse is a problem.

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I think they do in some places. But only on those big lines.