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Discussion Starter #1
I was running a 350 Scrambler without the battery and right off the altenator using an AC coil and phasing the altenator so that the impulse would trigger at the same time the points opened. It worked but I went back to a recifier - battery system for more reliable lighting. I kept the same AC coil on it but am having some starting problems. I re-phased the altenator to original spec. What is the difference between an AC and DC coil? They seem to be simple devices but I don't know if this is causing some of the problem.
 

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the alt makes ac current, 250 n/case scr and 250mk3 ac coil
Sebrings,monzas,mach ect. use batt ign dc current, dc coil, one coil won't work for the otherCapt Paul
 

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Discussion Starter #3
Your reply is at odds with others who have posted that all coils are DC and that it doesn't make a difference whether the coil is called AC or DC.
 

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ducati coils

I'VE OWNED ABOUT 20 DUCATI SINGLES, THEY USE A DIFFERENT COIL FOR BATT IGN & AND MAGNETO IGN.THE PART # FOR MAG MODELS,0100.31.300.
FOR BATT IGN PART #0400.31.303.
If I use the wrong coil, they wont 'work, that's all I can say,If you know better, then you know better.
They use 2 different spark advance units, for batt & mag
 

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Coupla things...
When you say "starting problems"... what does that mean exactly?

If you are suspecting the coil - we don't know how you've re-done the wiring, but did you check a) the rear stop-light switch and it's bulb, because together they form the ground circuit for the HT side of the coil.

CaptPaul is exactly right.... and in support - without wishing to upset anyone - both ignition coils are exactly that - just plain coils of wire - but wound with different nos. of turns, with different resistances. The stumbling point is that Ducati - or the industry itself chose to name them by the electrical source of the bike to which they're fitted. Duh.

I don't know who here knows what about electrics, and I don't wish to tread on any toes, but - for the total novice - these coils are both basic transformers with two windings - a primary, low voltage winding, and a secondary, high-voltage winding, which is connected to the spark plug.
Assuming a -ve ground, when the points are closed (with the "DC" coil) there is a continuous direct current flowing from the battery negative, through the frame to the points, across the closed points, up the points wire to the coil, through the coil to the ignition switch - and then to the battery positive terminal. (Electron flow is from negative to positive). This flow builds up a whacking great magnetic field in the primary coil, and is maintained until the points open: at this time that current stops, the mag field collapses, and in doing so - it collapses also through the secondary windings, and this induces a current in that winding: however - this winding has many more turns than the primary, so a much greater resistance, and the result of that high resistance (actually called impedance in a coil) is that a greater voltage will rise across the output connections of that secondary winding - somewhere in the 15 - 25 thousand volt range - sufficient to jump across the plug gap in the gasoline-rich atmosphere in the combustion chamber. That collapsing field is forcing the current to travel, and the more windings, the greater the impedance and the higher the voltage. There's lots written on this on the internet.

Bear with me: In the case of the "AC" coil - there is not a solid 6 - 7 (or 12 - 14) volt battery supplying 1-2 amps of current to the "AC" coil - it is getting a varying-frequency (ac) current that is anywhere between perhaps 4 and 40 volts, and at a lower current than that battery can give during a kickstart. This has to mean that the resistance of the AC coil has to be different, the winding resistance changed to match the lower performance of an alternator at kickstart rpms, and so a lower output can be expected (when you're kicking the bike). If you put the "DC" coil on the bike that doesn't have a battery - then you will have "issues". If you put an "AC" coil on a bike with a battery - you might burn the coil out. I have never tried this, and not having a battery ignition coil to measure against an "AC" coil - I can't give a definitive comment.

In the OP's post - it could be the coil, I suppose, but it could be a number of other things, too, such as points, condenser, ign. timing, grounds, etc.. Those things need to be sorted first. To see if it's the coil - with a known good battery connected, the plug sitting on the head, you can just open the points by hand, and you should get a good, fat blue spark. If not - then go digging.

Sorry if I got long-winded.

Cheers,
 

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AC and DC coils

I have tested coils that I know are for the AC models and coils for DC models (coil run off of a battery). I could not find anywhere how to tell the difference, when the green and red labels are off and no visible Ducati part numbers. The numbers stamped on the bottom seem to be somewhat random (in looking at abour 7 coils).

Here is what I found: The AC coils measure 9K - 10K ohm resistence between the secondary coil and ground and the DC coils measure about 5K-6K ohms for the same. I have a full magento scrambler and both coils work, but the AC coils seems to start/run more reliably. The DC coil in a full-mag system seemed to only work occasionally.

FWIW,
Tony
 

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Mag coil vs bat coil

Now there's one more factor, the spark advance units, are completely different, one has the points lobe 2/3 round the circle, the other less than 1/3 of the way around the circle, one ign needs the points closed longer to build current, the other only needs a small amount of build time to get the same results.
mag coils are different than battery coils
 

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Sounds rational.
casual observation makes me think the components are different?
 

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points lobes

Yes, I tried starting an DC coil in an AC bike with the 2/3 lobes points cam and I could only get a very weak spark. I put the 1/3 points in (after observing the 2/3 points cam are on my wide case bikes) and even with the DC coil, I got it started. When I put the AC coil in with the 1/3 it started much more reliably. I think the parts books ID's the points with different number for the NC bikes, which I believe all use the AC coils and the, and the WC bikes, which all use the DC colis and 2/3 lobe point cam.

This was my experience...
 

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Discussion Starter #10
Interesting stuff. I didn't know that the points lobes were different on a bike with a magneto. It makes sense. Thanks for the information. My charging system is a bastard version in that I'm using an aftermarket rectifier, very small battery and an 'AC' 6-volt coil from a BMW. There is no regulator. The only electrical equipment besides the ignition system is a HI-Lo headlight, a taillight, brake light and horn. When the bike starts it runs well but I've yet to check the charging system with a voltmeter. Without a regulator, the system may overcharge or with the tiny rectifier and the way it is wired, I'm not sure the charging system is working. With only a kill switch, I know the battery drains down with the points closed so I'm installing an on-off switch before the battery. I'm trying to make a simple electrical system that works which is why I tried to run the bike AC even though it was made with an altenator - battery system as original. I believe that my starting issue may have more to do with the lack of a compression release hooked up more than anything else. Without it, I don't think the crank goes through the proper revolution at compression needed to ignite the mixture. We'll see.
 

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Some of the bikes, like the 1967 250 scramblers have an AC coil, but run the lights off of a battery. There is no rectifier, just a power diode that creates a trickle charge. The plunger switch disconnects the battery from the circuit. There are wiring diagrams that explain it better than I am.
 

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Discussion Starter #12
Bike is running and charging, maintaining 6.2 volts at the battery leads. It seems the compression release was the cause of some of the starting issues. I'm looking for the proper compression release mechanism since I had to wedge the mechanism on the rocker cover down manually. If anyone knows where I can find one, please let me know. I kept the wiring simple. I placed a toggle switch between a very small 6 volt battery and one lead to the coil and the other lead to the rectifier and to the fuse block working the lighting and horn.. The two yellow leads from the altenator go to the rectifier and the red lead goes to ground. The rectifier I'm using is smaller than a matchbook cover. I'll post pictures of the bike. I've converted the Scrambler into a roadster of sorts and I like the way it looks. Thanks for all the info you guys supplied.
 

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6 volt charging

To cause a 6 volt battery to accept a charge, you have to generate 6.6volts of dc current, 7 volts is better. A car 12 volt battery needs more than 13.6 volts to charge any thing less and nothing happens, each cell in a lead acid battery,is 2.2 volts 6volt is 6.6 volt, 12 volt is 13.2. My sebring charged well because it had a 60 watt alt. But the earlier 28 watt and 40watt alternators,
got by,till Florida passed the lites on law, and I would turn off the lites,when no cars were in site, or the battery voltage would drop below 6 volts and motor would sputter. My son rewound the stators with better wire and more than the factory did, and could make good charging performance,and even more reworking produced 12volt systems,
Nowadays, guys in oz make great stators that correct all of the ducati misgivings and charge like Jap bikes.
 

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I think the parts books ID's the points with different number for the NC bikes, which I believe all use the AC coils and the, and the WC bikes, which all use the DC coils and 2/3 lobe point cam.

This was my experience...
Not strictly true-450RT is a widecase engine but uses AC only-no battery and will provide both sparks and lights-I know since I have one.

John
 

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Yes you have magneto lites, they don't affect the eng,But battery ign needs power, if it does not get enough battery re charge to run lites & engine the engine suffers. Your RT is like my bronco mag ign mag lites runs without a battery
 
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