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ignition coils

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moe7404

New Member
moe in wichita ks
i have a 1975 case 446 garden tractor, with a onan 2cyl 16 hp engine. it has one ign coil with 2 high voltage towers one going to each plug. it fires both plugs each time. its whats called a waste spark system.
the question is could i use two different coils? and if t did would i have to put a diode in each primary line? to keep voltage the same on each primary line. thanks
 

crutschow

Well-Known Member
Most Helpful Member
What's the reason for going with two coils? The spark energy is determined by the primary side of the coil which would just be shared between the two coils. You could increase the spark energy by pulling more current from the primary, but that could overload the circuit providing the primary current.

I don't see that a diode would have any effect on current sharing.
 

Boncuk

New Member
To me it seems the ignition circuit is a magnet ignition type, normally used in two-stroke-engines and aircraft radial engines.

Therefor the system can only be changed using a distributor.

Boncuk
 

moe7404

New Member
moe in wichita ks
thanks for the good info. iam thinking what if the coil goes out. a new one from onan is 80 to 90 dollars. reg coils about 20 to 30
 

tonigau

Member
Go to a motorBike wrecker or swap meet, floating secondary (double ended) coils are common on four cyl 4 stroke & 2cyl 2 stroke bikes. (newer bikes would be lower ohms due to elect. ignition, I like the old stuff better)
You didn't mention what drives your coil primary, points ?

Important !! When replacing your coil, get one with same or close primary ohms or use a series ballast resistor.

Parallel ignition coils:
---------------
If you try to drive 2 coils (with half the total ohms of original coil) your points will not last as long. (Maybe use 2 ballast resistors.)
Also the 2 coils in parallel would likely "fight" somewhat in resonance, but then try it & see what happens.
There is plenty of resistance in the primary to enable reasonable current sharing as long as the 2 coils are the same as possible, Any problems would be when the coil is in flyback mode & the coil(s) is resonating.
(often things can work well with the "try it" & see aproach)

----------------------------------
Series connection:
Using 2 identical coils (eg: bosh epoxy potted - should be better matched than 2 old oil filled coils)...
You could try connecting the 2 primary in series, don't see why this wouldn't work, as long as the total primary ohms is similar to original.
There would be some interaction between the 2 coils but I dont think it would be significant.

If I had 2 identical coils handy I would give it a test

Tonig.au
 

Chip_er

New Member
Moe, I did the very thing your thinking on doeing last week. I had two Onan coils the were burnt out on one side, all I did was wire the two positive together and negatives and used the side that sparked. sounds primitive, but it works. I am thinking on using a balast.
Good luck and keep that Onan alive!
 

moe7404

New Member
moe in wichita ks
thanks guys. that is realy big help. when i put it inside for the winter iam going to try to put an electronic ignition on it. iam thinkig of putting a photo beam above the points and let the point arm break the beam, why remove whats already moving up and down? and set up darlington transistors to drive the coil. or use a ecu off of a chrysler in the 70s, i have a few laying around. what think?
 

tonigau

Member
gday moe

Those old chrysler ignitions are quote robust, I have got one on my HQ 30+ years & still going well even though its leaked most of its potting compound. They need a ballast resistor (4.7ohms) that is required for internals (not the usual coil ballast) Standard is the 2 resistor pack one for coil & one for unit.
I am asuming the same ignition moduled as here in Australia.

If you are going to brew your own, you might consider using an ignition IGBT, these are more robust/efficient for ignition coil driving than darlingtons, they incorporate active clamping & are easy to drive.

Just ensure the signal that is turning off the IGBT gate (initiating spark) is actively switched to 0V, a pullup resistor turning the IGBT on is fine.


TG
 

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moe7404

New Member
onan ignition

moe in wichita ks
thanks for the great info guys. but i just had an other question. the chrysler ign box has two wires that go to the hall effect sensor. years ago i found out that if one wire was scraped on ground with the ign on it would fire the coil. question is, could i do the same thing by grounding that wire through a transistor? maybe through a resistor so it doesnt short out. iam going to print out your posts they are so good. thanks
 

rxforspeed

New Member
Here's a thought-maybe you should check out an S4-S5 Mazda rotary primary ignition coil. You can find plenty of schematics on www.rx7club.com for retrofiting them to older systems, and they also use two towers and the "wasted spark" system. Older non-electronic ignitions in S1 ('78/9-'80) RS-7 12A rotary engines use them frequently as upgrades, as the only real mods required are to modify the wiring to the module to bypass the points to fire the S4 ('86-'88 models) primary coil. You can find these plentifully in the classified section of rx7club for around $10-$20 for an OEM used unit in great condition, or also on ebay for around the same price. I'm not 100% sure if this course of action would be your best bet, but it may be worth looking into. I'm sure the output of these rotary engine coils would produce more spark energy then your '75 model coil, resulting in better fuel economy, efficiency, and quicker start-up times. Make sure you find the primary coil if you do go this route, as the trailing coils are two seperate units and don't utilize the "wasted spark" method. Hope this can be of some help to you...

-Adam Collins
 

moe7404

New Member
elec ign

moe in wichita ks
thanks rxforspeed you have a very good idea. i even printed it out so i can put it in the electronic ignition folder. i was looking under the hood of the case tractor and found some wires melted to gather, luckly i do have a wireing diagram. so guess ill be doing some wire spliceing this week. thanks for your help. " if you are smart when you get old you will have wisdom, if you are not smart when you get old you will just be old"
 
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