# Ignition coil question

• posted

My son asked me and I didn't know the answer so I thought I'd pose it here. His bike has two ignition coils. The coils are not the same, they are from different years of manufacture for different models of motorcycle. The timing is not the same for each cylinder, but should be the same. The coils have different resistance. My son was wondering if the difference in timing could be because one coil takes longer to build up enough voltage to cause the spark plug to spark. The coils are connected to the same control box. So I'm wondering if the voltage rise from two ignition coils could be that much different. Eric

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Rl is the time constant. Likely the heat of the spark in one cylinder is needed to be more or less. Timing is critical and heat is also.

Martin

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Yup, the coils need to have the same resistance. Your son is correct in the reason the timing is off.

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When he says the timing is off, how is it being determined and how far off is the timing? If he is checking with a timing light on the secondary it might be BAREL:Y noticeable but the coils should be matched for other reasons. The build-up has nothing to do with timing

- the spark happens when the magnetic feild collapses.(unless it is a capacitive discharge system -which is a totally different kettle of fish)

• posted

On Wednesday, June 4, 2014 9:15:55 PM UTC-4, snipped-for-privacy@whidbey.com wrote: My son was wondering

I doubt it. Assuming this is a Kettering type of ignicion, the way it wor ks is that when the points are closed the current builds up in the coil. W hen the points open the magnetic field collapses which creates the high vol tage. The condenser across the points keeps the voltage from arcing at the points. You might swap the condensers and see if that changes things. If it does one of the condensers is bad.

Dan

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Chances are that the two coils were the same at one time.

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Greetings All, I guess it's sort of a Kettering ignition. It doesn't use points though. Some type of inductive sensor tells a box when to make the coils spark. My son says he checked the timing with a timing light, which is why he knows when each coil is firing. Eric

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==================== two types of ignitions

Kettering, where the coil "fires" when the points open. This is because the magnetic field in the coil collapses faster than it builds giving a "hotter" spark.

CD [capacter discharge] where a capacter discharges a higher surge of current/voltage with a faster rise time into the coil and it fires as the magnetic field builds up. If you have a "pointless" system, which is likely as you mention a module rather than points, it may well be a CD. Is this a computerized system where the advance is controlled by the module, and the sensor or points are just index location signals?

Coils are not that expensive, and he should use a matched set of used coils, if not new a new set.

here's a few places to start

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Dan sez:

We stopped using the term "condenser" in the 1960s

ANYway... that's not all the capacitor does. It also controls the decay time of the field in the coil. And that depends upon the inductance of the coil -- the old "LC constant".

If the coils are noticably different in resistance, there's a really good chance they have pretty large differences in numbers of turns. That vastly affects the inductance of the coil.

If you wish to have both coils "fire" at the same time, you must have them inductively matched, and have the same capacitance across both sets of points (gotta be two, unless they were really dolts and paralleled two coils!)

Don't forget point gap. If there are two sets, then if they're not gapped exactly the same, the timing will change there, too.

Two new coils, two new capacitors, and two new sets of points are the best way to go. (might as well throw in plugs and HT wires, too...)

LLoyd

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"Lloyd E. Sponenburgh" fired this volley in news:XnsA343B638E5CD8lloydspmindspringcom@216.168.3.70:

That's on the assumption that both coils fire at the same time, but of course, if they don't then why are there two coils?

Lloyd

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"F. George McDuffee" wrote in message news: snipped-for-privacy@4ax.com...

My truck's electronic ignition is somewhat like a motorcycle's, with one coil firing two plugs on every crank revolution, one before compression TDC and the other before exhaust TDC.

"The spark plug on the compression stroke uses the majority of the coil's stored energy, while the other spark plug, on its exhaust stroke, uses very little of that energy."

The ignition module acts like points and fires by interrupting the coil current. "When the switch opens, the power is interrupted and the primary field collapses, inducing high voltage pulses into the secondary coil windings."

-jsw

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"Jim Wilkins" fired this volley in news:lmqq4k\$leh\$1 @dont-email.me:

No! Really? (C'mon, Jim...) Lloyd

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On my truck there are two independent sets of coils and plugs, on opposite sides of the engine. Except during starting they fire at the same times. I could add a Mag Drop switch.

-jsw

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"Jim Wilkins" fired this volley in news:lmqqta\$q4t\$1 @dont-email.me:

And points, no? If only one set of points, it's a system destined to be unreliable. Even minor changes can affect the timing and voltage of the spark on one coil.

Lloyd

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Really, Lloyd!

I've got an ignition module open here. The coil driver "switches" are

5mm square MOSFETs (comb-like gate pattern) on a ceramic hybrid substrate, with something that might be the voltage clamp to ground next to each. It's potted in clear silicone which I won't disturb just to answer you.

-jsw

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The ignition is not a capacter(sic) discharge system. It has some sort of electronics package that controls the ignition timing and uses a transistor to switch the coil power. So the coil does get the high voltage from a collapsing field. As regards the need for matched coils you are right, he should be using the correct matched coils. I have told him this. He continues to half ass the work on this bike and I continue to give him shit for it. But his question got me wondering so that's why I posted it here. Eric

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Look at figure 2. SPOUT is Spark Out, a logic signal which mimics the closing and opening times of points for the correct dwell and timing. The DIS module connects the (-) sides of the coils to ground like points, but electronically, and sends back something relevant to how the coil fired on IDM, Ignition Diagnostic Monitor. IDM was flaky on the module I replaced, giving a code though the engine ran well enough.

It's a computer controlled Kettering system, not CDI.

-jsw

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"Jim Wilkins" fired this volley in news:lmqv4f\$jc3\$1 @dont-email.me:

It still has two sets of "points", whether they be big MOSFETs or mechanical.

Lloyd

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snipped-for-privacy@whidbey.com fired this volley in news: snipped-for-privacy@4ax.com:

"Electronic ignitions" have been around for decades. Simple. They just replace points with large transistors, and do the sensing of when to fire magnetically or by a controlled-reluctance coil and reluctor mass.

Lloyd

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Actually it has four MOSFET switches, two for the right side coils and two for the left side coils, two plugs per cylinder. They replace the point contacts and the computer replaces the distributor point cam and the centrifugal and vacuum advance.

-jsw

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