Coil mod

[D0wn5h1ft]

You want a relay with a minimum rating of 15 amps; higher would be better. There's a inrush current factor that can give current spikes of up to 300% of the 'rated' current.

Although I agree with Davinci; if you have a serious voltage drop to the coils, you have issues in the harness and this simply bypasses it, it doesn't fix the problem that is probably indicating other issues that you may not be aware of.

Is the inrush current coming from the condensing capacitor?

[DAVINCI]

Hey, before you go thinking your coil voltage is too low, and commence jury-rigging your ignition coils, be sure you're accounting for the ballast resistor.

Just sayin'

Is the inrush current coming from the condensing capacitor?

There is none

[b.walker5]

What are the electrical specifications of the relay? 12v but what should the rated max current be?

Any bog standard relay for accessory driving lights, or a horn etc from Autozone, etc will do. The coils draw mimimal current, so any you don't need to go hunting for some beefed up, special relay, but I too agree with DaVinci and Steve. Look for why, and fix the problem, don't just cover it up.

If I was going to do something like this I would make it a booster circuit and feed the relay from the starter solenoid, so I had full voltage to the coils at start up, but once started the circuit would drop out and coils would continue to be fed from the normal source.

[b.walker5]

Is the inrush current coming from the condensing capacitor?

No. Inrush current is just a normal electical phenomenon that occurs when something is switched on. What's on the end of the circuit determines how much, and things with high inductance, like coils and motors etc, have higher inrush, or startup, current than things like lights etc. it's not enough of a factor in your case to be of concern when selecting a relay.

[D0wn5h1ft]

I have dynateks on my '79 that's why I was thinking about doing this. The coil circuit is meant to be run with the ballast resistor (which is obviously out now with these coils) and I'm thinking this might help things out.

[b.walker5]

The coil circuit is meant to be run with the ballast resistor.

No. The oem coils were meant to run with a ballast resistor. The circuit itself doesn't care if its there or not. Your Dynateks are designed for full voltage so the resistor isn't needed, and without it you should have full voltage. The fact that you haven't says that something else is wrong. I would be checking, and cleaning all connections and grounds before changing anything.

[TomB]

So a standard set-up should provide 12V rather than 10V??

I know it sounds like a stupid question but not being an electrickery expert I wasn't sure if good condition OE wiring would inherently have resistance that would drop the volts at the coils.

Years ago I had an old Ford which ran great but on cold damp mornings would always start a lot easier if hotwired with a wire put direct from the battery to the coil, removed once started. I still have a ballast resistor wire from one of those Fords on my 2H9

[D0wn5h1ft]

double post, sorry.

[D0wn5h1ft]

No. The oem coils were meant to run with a ballast resistor. The circuit itself doesn't care if its there or not. Your Dynateks are designed for full voltage so the resistor isn't needed, and without it you should have full voltage. The fact that you haven't says that something else is wrong. I would be checking, and cleaning all connections and grounds before changing anything.

At that point, I may as well just rewire that whole section of harness because it's more likely that the voltage drop is coming from the cumulative effect of several degraded connections rather than at a single point. I'd rather bypass the issue than playing peek-a-boo with 30 year old circuitry because at the end of the day cleaning all of the connector is probably only going to get me 1 or 2 tenths of a volt more at the coils.

I would contend that the ballast resistor does in fact affect the performance of the circuit. Ignition coils are essentially an RLC circuit. With OEM coils and the ballast resistor your coils see 6v with 1.5ohm coils resistance. With dyanteks the coils see 12v at 3 ohms. All of this is looking at only one coil at a time. Assuming capacitance and inductance stay constant between the two circuits there will be a SHARP increase in peak current. Now assuming that inductance does play a role and it roughly doubles with dynatek (i'm going to assume it doubles since i don't have the equipment to compare the inductance of OEM to dyantek) the peak current of dyantek coils is still much higher than OEM. This is because series resistance affects peak current much more than series inductance since the impedance of an inductor is a function of inductance AND frequency which is dependent on series capacitance as well.

If we increase the peak current with dynatek coils we're taxing the circuit more than what was considered during the design process. If you're charging circuit is weak or if your regulator is muffed up then demanding more from a weak circuit is going to result in a degradation of performance in my opinion.

Unless I'm way off on one of my assumptions this makes sense. The extra power that the dynatek coils use would coincide with people's reports of having a "hotter" spark.

[natemoen]

At that point, I may as well just rewire that whole section of harness because it's more likely that the voltage drop is coming from the cumulative effect of several degraded connections rather than at a single point. I'd rather bypass the issue than playing peek-a-boo with 30 year old circuitry because at the end of the day cleaning all of the connector is probably only going to get me 1 or 2 tenths of a volt more at the coils.

I would contend that the ballast resistor does in fact affect the performance of the circuit. Ignition coils are essentially an RLC circuit. With OEM coils and the ballast resistor your coils see 6v with 1.5ohm coils resistance. With dyanteks the coils see 12v at 3 ohms. All of this is looking at only one coil at a time. Assuming capacitance and inductance stay constant between the two circuits there will be a SHARP increase in peak current. Now assuming that inductance does play a role and it roughly doubles with dynatek (i'm going to assume it doubles since i don't have the equipment to compare the inductance of OEM to dyantek) the peak current of dyantek coils is still much higher than OEM. This is because series resistance affects peak current much more than series inductance since the impedance of an inductor is a function of inductance AND frequency which is dependent on series capacitance as well.

If we increase the peak current with dynatek coils we're taxing the circuit more than what was considered during the design process. If you're charging circuit is weak or if your regulator is muffed up then demanding more from a weak circuit is going to result in a degradation of performance in my opinion.

Unless I'm way off on one of my assumptions this makes sense. The extra power that the dynatek coils use would coincide with people's reports of having a "hotter" spark.

But in the original system you have a 1.5 ohm resistor and then a 1.5 ohm coil equaling 3 ohms. Now with the new coils you just have the 3 ohm coil again equaling a total of 3 ohms. The circuit has the same voltage before and after changing coils so basically it is still the same system just with newer parts.

[D0wn5h1ft]

But in the original system you have a 1.5 ohm resistor and then a 1.5 ohm coil equaling 3 ohms. Now with the new coils you just have the 3 ohm coil again equaling a total of 3 ohms. The circuit has the same voltage before and after changing coils so basically it is still the same system just with newer parts.

But I'm pretty sure it's about the voltage the inductor sees not just the resistance of the whole circuit. OEM coils sees 6v (voltage drop over resistor is 6v and drop over the coil is 6v) and the dynatek sees a 12v drop over the inductor.

By changing where the voltage drops occur we are changing the transient response of the circuit. Again, I'm relatively new to this black magic and smoke since I've still got a semester of my bachelor's left.

[James England]

instead of adding another point of failure, I would correct the wiring deficiencies

JMHO

The only way to correct a 25+ year old wiring harness would be to replace it with a new (unobtanium) one though, surely? Ok, it's possible clean or replace the block connectors but the wiring itself is usually very old on these bikes. The copper core degrades over time, right? The biggest deficiency on an old wiring loom is the wiring loom, usually.

The problem with the "adding a point of failure" philosophy is that it effectively negates any workaround or indeed improvement to what is an old design. Those 'bobber' bikes generally remove lots of " points of failure" and look at them. And, using modern relays and components, the possibility of failure is reduced.

Personally, short of finding a new old stock wiring loom, I think the idea mentioned in this thread is excellent. A lower voltage reaching the coils via a circuitous and confused route of wiring, decades old is thereby easily corrected. I'd do it.

[natemoen]

The OEM coils with the resistor see more in the 9-10 volt range after the resistor.

I agree that there are more factors to it but I think you are over thinking the issue here.

[D0wn5h1ft]

I think you are over thinking the issue here.

That could very well be the case as it has happened more than once before.

[crazy steve]

The only way to correct a 25+ year old wiring harness would be to replace it with a new (unobtanium) one though, surely? Ok, it's possible clean or replace the block connectors but the wiring itself is usually very old on these bikes. The copper core degrades over time, right? The biggest deficiency on an old wiring loom is the wiring loom, usually..

Copper wire should last indefinitely as long as the insulation isn't breached and the wire corrodes and/or if it's not overheated. Almost every electrical problem these bikes have are caused by poor connections.