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Old 10-11-2017, 10:37 AM   #1
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Default 3800 ignition coil question

I'm using a 3800 coil on my 2 cylinder waste spark garden tractor.

It uses points to trigger a transistorized ignition circuit that operates at 12v. Before I changed to the transistorized ignition I was using the coil with a ballast resistor and appropriate condenser, the spark was too weak, was hard starting, and still burned up the points over the first year.

With the transistorized ignition I don't have to worry about burning up points, so I can run the coil at the full 12v without the ballast resistor/condensor. Problem is, the coil gets really hot (though it runs really well). The coil basically shorted on the primary windings and took out the transistor that came with the kit after 3 hours of mowing the lawn. So now that I've upgraded the transistor and changed it to active cooling, so it can't also be killed, I need to figure out why the coils are getting so hot.

I guess I thought these coils were designed to work with 12v. So I'm guessing that the ICM is basically doing what a resistor wire on an older car does, supplying full voltage during cranking and supplying a lower operating voltage after the engine starts?

And, no, using the stock ignition coil is not an option. It's discontinued, and doesn't fix the hard starting issue.
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Old 10-11-2017, 11:31 AM   #2
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Old 10-11-2017, 01:20 PM   #3
 
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So 1 coil for a 2 cylinder motor?

If so basically its constantly running then? 100% duty cycle?

No time for cool down.

Any kind of deal to run 2 coils, 1 per cylinder?
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Old 10-11-2017, 03:47 PM   #4
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Haha, didn't even think of that.

I had initially run a pair of standard ignition coils,

But had issues with hard starting, and they didn't last very long for probably the same reason the 3800 coils don't. And it was a mess of wiring and shit. So I figured the OE coil was just one of those coils with two posts, why shouldn't any two post coil work? Probably has a lot to do with the primary resistance.

The problem is I have no way of triggering two separate coils at two separate times off a single set of points. But the motor only runs at, what, 3k rpm to begin with? That wouldn't be much different than running a 3800 coil at 6k rpm. Guess i need to find a way to disperse the heat better, or build a relay circuit that adds in a ballast resistor when not cranking..
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Old 10-11-2017, 09:52 PM   #5
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Is putting the rest of the 3800 into the tractor an option?

Sorry I'm no help.
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Old 10-12-2017, 09:57 AM   #6
 
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Can you feed the coil less voltage? Does the 2 banger need as much voltage output from the coil to ignite ?

Is lack of dwell a problem?

"The dwell on this system is held low at low engine speeds and increases as engine speed increases. The ICM will always supply a little more dwell than is actually needed so the ICM relies on current limiting to prevent the coils from overheating."
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Old 10-12-2017, 12:30 PM   #7
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I would have to measure the dwell (and I probably don't have a dwell meter).

The problem with this thing is definitely starting voltage drop no matter what ignition setup you use, but the compounding issue was coil availability and cost even for a used one.

I would think though that a transistorized ignition conversion wouldn't have an issue with dwell, I'll have to email the place that sold it as it doesn't mention anything about dwell in the documentation (ie: is dwell measured at the points a 1:1 relationship with the circuit output?)

But that statement 'relies on current limiting' sounds a lot like what a ballast resistor does. But this being a cheap garden tractor, it doesn't have a separate supply circuit for starting (to easily use a ballast resistor).

To answer the question, no I don't really need a hotter spark all of the time. The motor has hours on it, leakdown is around 20%, so not having a strong spark during cranking means it's just not going to start. Once it's running it runs like a champ until the ignition coil or ballast resistor crapped out (about every 3 months before changing to the transistor setup).

Actually, fun fact about this tractor, the ignition coil doesn't get power while cranking. So it has to fire at just the right time after it starts to spin down to start. I changed that problem when i put the transistorized ignition in, so with the 3800 coil and no ballast resistor it fires up on the first rotation... I guess I traded off one problem for another.

So I guess I'm going to hunt down a low resistance Ballast resistor and just wire it up to a switch and manually switch it over. I might get bored and try active cooling on the coil using a CPU heatsink and fan. If I can keep it cool, it shouldn't fail. I could sand the plastic off the top to get a metal on metal contact for better heat dispersion.

If that doesn't work, well, screw it. I'll just build a 3800 4x4 chassis for the snowblower and mega-mower. That could be fun.
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