Both coils gone at once?

I'm afraid that's a bit beside my point. My point is that the coils are wired exactly as shown in the diagram RickCoMatic posted: there is no electrical connection between the secondary winding and the primary winding, nor is there any connection between the core and either of the windings.

 

To electrocute you current has to flow through you. There is no circuit path through you that has anything to do with the secondary of the coils. Electricity will take the path of least resitance to complete the circuit and that is the path I explained earlier.

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Now, there is another experiment you can try if you don't believe me on this one (OK, don't really try this, it's just for illustration):

disconnect the caps from two corresponding cylinders (like 1 and 4)
put spare plugs in the caps
hold the ground part of the plugs in each hand
crank the engine

Now you are taking the place of the cylinder head and the coil WILL electrocute you.
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Another way you can analyze this is to use Ohms law: A good igntion coil might peak out at a .25A or so on the high voltage side. The resistance of the cylinger head between the two plugs is probably about .01 Ohm. .25A over .01 Ohm means about .0025V difference between the two plugs through the cylinder head when the plugs are firing. You'll never feel that.

Also, ground is not some worldly absolute. It's just a convenient way of saying a bunch of stuff is connected together at that point. Current certainly can flow THROUGH ground. Look at the schematic again. There's a potential between the two ends of the secondary coil when the coil fires. That potential results in current that wants to flow from one end to the other of that coil, and it will do so through any conductor, even if that conductor is an aluminum cylinder head labelled as ground.

 

Quite so.

 

ok i think i see the light, it's a isolated system and there is no ground

 

Yup. That's exactly it.

 

No.
That's incorrect.
The Cylinder Head is the Ground.
The schematic shows the Coil's Secondary Wiring to be Parallel.

There is no indication suggesting anything other than BOTH Spark Plug Wires firing simultaneously.
The Path from the Coil shows that the Output from the Coils go only to the Plugs straight to Ground.
There's absolutely no indication other than the Path terminating at Ground.

No additional path indicating that the sequence is in any way a Series.
Isn't that exactly what the Ignition System Schematic shows?

All four paths to the Spark Plugs terminate at Ground.

 

It looks to me like the TCI creates a HV loop on the primary coil. This creates a magnetic field. When the TCI stops the HV the field collapses, inducing another HV in the 2nd coil.

For current to flow there has to be a loop, and it flows in ONE direction in that loop. This is fact, no way around it.

When the field created by coil 1 collapses, a current spike is induced in coil 2. Think of it as pushing all the electrons that are part of the actual wire that makes up coil 2 to one end of the wire. They create a HV potential at that point. The voltage is great enough that it jumps across the air gap of the plug and eventually to the engine block. Now those electrons want to get back to the other side of the coil, so they jump the other gap, thus maintaining the balance that must be. The engine block IS ground, but only for the 12volt circuit that the lights, starter, etc are a part of. This HV circuit is totally separate.

I just learned why these (and most) bikes are so noisy electronically... the entire bike is part of the ignition system. Joy. It is going to be fun if I ever get around to adding a ham radio to the bike.

 

Just to clarify, they do fire at almost the same time, the electrons complete the loop at the speed of light. I guess this means that one plug fires from the electrode to the tip, and the other fires from the tip to the electrode. Interesting.

 

when you say coil 1 & coil 2, do you mean primary & secondary windings ?

 

kd5uzz has given a great explanation. His coil 1 is the primary and 2 the secondary.

Let me add a couple of points. You may not need the loop to fire either plug. Current induced in the secondary coil will flow in one direction when induced by the primary field building and the other direction when the primary field collapses.

I'm not sure how the TCI works but my assumption differs from his in that I believe it pluses the primary - thereby having a building and collapsing field for each "firing".

Back to his explanation - current flows as the field builds and the spark happens, as the field collapses the current flows to the other end and the other spark happens.

So I contend one plug can indeed fire without the other. It may be a weaker spark because the air gap in the plugs in not infinite resistance so in a good system there is some of the looping described which could provide more current. But for our purposes we are only concerned with a spark hot enough for combustion.

I will admit that so far I have only seen fuel issues causing 1 cylinder to not fire but I don't have the wealth of experience others here do.

By the way this is a great discussion!

EDIT - thought of a good illustration. You build up static on a cold day and touch a door knob. You get a spark because of the difference in potential energy. The knob is in a wooden door - no ground - same if you touch a person. If you touch something grounded you get a bigger shock/spark because the potential difference is greater. kd5uzz mentioned the HV build up at the plug - same thing.

A vandegraph (sp?) generator is a similar device - you can have on rubber soles and still draw a spark - again potential energy differences.

 

I should add that this points to one fact - If one side is firing and the other is not the fault is in the Wire, cap or plug not the coil itself. At least IMHO

 

I would check the SIDESTAND RELAY and the STARTER CIRCUIT CUTOFF RELAY

Both stop spark.

 

You're still hung up on a misconception about ground. For the secondary of the coils, ground is just a connection between the bodies of the two spark plugs. Period.

A transformer, such as these coils are, is an isolation device. The fact that one end of the primary coil is intermittantly connected to ground means nothing to the relationship of the secondary coil to ground.

TCI charges and discharges the coils just like a Kettering points system. It simply does it with transistors instead of points.

I assume you didn't actually try the first experiment I suggested. Please do and let me know what you actually see.

What the schematic shows, and you highlighted, is that the secondary side of the coil has only one possible circuit path. Here are four equivalent circuits to the secondary side that might make this clearer (keep in mind that the arrowheads represent the spark plugs):



When the coil discharges, its secondary side has a potential difference between one end and the other, just like a battery has a potential difference between the + and - terminals. And, just like the battery, current can't flow if one of the terminals is disconnected. If you disconnect one of the plug caps, you've opened the circuit and current cannot flow.

Does this progression of equivalents make it clearer why this is a series circuit and not a parallel one?

One other thing to think about is: What happens to th 12V side of the electrical system if you disconnect the ground strap to the battery? Does the starter still work since it's got +12 on one side and it's still connected to "ground"? No. It doesn't work because the electrical source is no-longer connected at both sides (+ and -) to the load (starter).

On the 12V side, when the bike is not running, the battery is the source of electrical energy. Current flows from the battery's - terminal, THROUGH ground, through a load (like the headlight), through a closed switch or relay, and back to the + terminal of the battery. Ground is not a destination, it is just a connector between the - terminal of the battery and one side of the load.

On the high voltage side, the secondary of the coil is the source of electrical energy. Current flows from one end of the coil to the other. Ground is in the middle of this circuit, not at one end.

If you search on wasted spark and "in series" you'll find several articles that describe how it works. Here's one: http://findarticles.com/p/articles/mi_q ... i_n8876015

Paul

 

Bill,
I think you are correct in that the plugs can fire as the TCI 'charges' the coil, but I don't think that 'fire' will be nearly as strong as when the field collapses. And yes, it would 'change directions' based on the charge/discharge cycle of the primary coil.

When the TCI 'charges' the primary coil it does so at a fixed voltage and current (lets just think of it as power), simply because the XJ's electrical system can only supply a specific amount of power. And it 'charges' at a finite rate (don't make me get my circuits textbook..).

On the other hand, when that field collapses there is nothing, except the resistance of the wire, holding that charge back, so it all 'dumps' instantly.

Think of it as walking up a mountain that has a sheer cliff on the other side, with a rock. You walk up, one step at a time. Each step is getting you higher and higher until you are at the top of the mountain. Once there, you drop your rock down the sheer face of the cliff. It drops much faster than you climbed.

 

Fixed it guys . Full battery and a dry, windy day later and everything is back to normal. The coils weren't a problem after all. Think I'm going to spend my next day off putting a blade-style fuse box in and tidying up/winterproofing the wiring under the seat.

Cheers for all your help people, and sorry for leading you off on the wrong trail. I will continue to monitor the condition of the coils, and am gonna do the resistance checks stated in my workshop manual to check that they're up to scratch.

 

Good to hear.

I'd still check the spark plug boots and replace them if they don't look too good. If they do look OK, they may still want a bit of dielectric grease to help keep moisture out.

Cheers,
Paul

 

Agreed - collapse greater than charge given the systems resistance. I was thinking of a standard transformer with no real difference in the charge vs discharge which will have relatively equal but opposite in phase currents.

Given the nature of the TCI (which does not match my original assumption) and the circuit then I have to conceede I don't see a way for one to fire without the other.

Paul's explanation is very good. I was coming from a different prospective, which doesn't make it any less wrong

I learned something new today.

I did pull something out of Paul's reference

The voltage capacity of a DIS coil is high enough, however, to ensure that the available voltage is always high enough to fire the plug with reversed polarity when it's on the compression stroke.

Is it possible with a weak or wet coil that there is enough voltage to fire the "normal" plug but not the reversed plug?

Obviously we are seeing strange things that indicate this and the solution to this problem appears to be a dry day and charged battery....

 

I've always had problems getting my XJ to start after it has been sitting in the rain. That said, I did once make the drive from Stillwater to OKC (~70 miles) during a downpour (my first time to ride in the rain...lets hope it was the last.) The bike ran great, but it was already running when the rain really started. This was after I got 20miles froms Stillwater and had a quick connect ground itself against the altenator cover, getting me stuck on the side of the road without any idea as to what was wrong. An hour+ later, after getting a ride back to STW to grab the truck, gas it up, and get the bike back home, it took 5min to find the problem and get back on the road. Without that delay I would have made it to OKC without the rain. I'm pretty sure that was the last time I drove the truck. It won't start now, the battery is dead.