Coil Failure

[dvd]

Just got back from my SF Bay trip and everything went well with the engine except I apparently burned up the coil.

I had installed electronic ignition a few years ago and had subsequently replaced the original coil with a Petronics Flame Thrower coil. It had been on the boat for a year or two and I never noticed any issues with it. The engine, after running for several hours would act like it was running out of gas, die and not start for about 20 to 30 minutes. For some reason I checked the coil as I wasn't sure what was causing the shutdown. When I touched the coil it was really really hot. So hot I could not put my bare hand on it. After melting a handful of ice on it and waiting about 20 minutes the engine would fire up and I could continue for about 30 minutes and it would happen all over again.

Luckily I still had the old original coil as a back up spare. I was pretty sure it was the coil after several ice down and waiting sessions but I had no way of checking the spark by myself. I thought is may also be a fuel filter issue and I also had a spare polishing filer. So when I finally made it to that days destination I replaced the flame thrower coil with the old original coil and replaced the fuel filter. (which it turned out was not clogged) After that she ran excellent with no problems.

So I now realize I ran afoul of a major principal when I replaced the original STILL WORKING coil with the pretty flame thrower. If it aint broke.....

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[ndutton]

You may still be on borrowed time depending on two factors: the old (now back in service) coil's internal resistance and the voltage between the coil + post and the block at cruising RPM.

It could take a little time for enough internal damage accumulation to cause the coil to fail. We recently got into this pretty deep in this thread. The common denominator is electronic ignition.

[Kelly]

I have to second Neil's thoughts here as your symptoms of a failing coil fit into a growing collection of "classic" coil destruction scenarios. I lived through your exact description of engine failure with subsequent restart 30-40 minutes later- and this over the lifespan of three different coils (all +/- 3.0 ohms resistance).

Only time will tell but the preventative treatment developed and described in the thread Neil references could help a lot of us avoid this exact problem.

[dvd]

?????

So is the the final ruling that we need to get different coils 1.5 or 4 ohm? Or do we need to install a ballast resistor??

[ndutton]

So is the the final ruling that we need to get different coils 1.5 or 4 ohm? Or do we need to install a ballast resistor??

The goal is to get the ignition system (with Pertronix EI) to operate at less than 4 amps. To determine what works best in your particular case you can do the calculation yourself or you can use the calculator provided in the Drawings and Schematics category on the forum (we've done the math for you).

Here are some examples though. The formula is R=V/I where V is volts measured between the coil positive terminal and a solid ground like the engine block with the engine running at cruising RPM, R is the internal resistance of the coil measured between the two small terminals on the top. The goal is for I to be less than 4 amps but not too far less.

If your V is say, 13.2 volts and the goal of I is to be 4 amps, then R must be 3.3 ohms. If your coil as mentioned in your question measures at 1.5 ohms, you'll need to add a ballast resistor of at least 1.8 ohms.

If your V is the same 13.2 volts and your coil measures 4 ohms as posed in your question, you don't need an additional resistor because your coil has sufficient resistance.

But look what happens if your coil is a 3 ohms, now you need a 0.3 ohm resistor at a minimum to bring the system resistance up to 3.3 ohms as calculated.

Note that the need or lack thereof for a resistor is dependent on your particular wiring system, alternator output and measured coil resistance and those factors are different for all of us so there's no flat answer to whether or not a certain resistor or coil will work in your case. It has to be measured and calculated for each boat separately.

All of this algebra can be eliminated with the calculator mentioned in the first paragraph. The calculator targets the amperage at 3.4 amps, 15% less than the recommended maximum, intended for a margin of safety.

I should add that just because a coil is sold as a particular resistance, it's not necessarily so. We've seen measured resistances differ from the nameplate ratings (if they had a nameplate) by as much as 10%. If you're close on the amperage, 10% could be significant so it is important to actually measure the coil resistance rather than assume its rating is accurate.

[dvd]

Math - Huh

Is there any sure way of resolving this issue without having to utilize math or measure volts, etc. Call me stupid but I just don't get this stuff. Is there any Ohm coil or resistor that if installed would resolve this. The petronics flame thrower coil that failed was purchased from moyer. For some reason I think it was epoxy filled but I could be wrong. I am also under the impression that it was a 3 ohm coil. With these facts in mind what ohm coil/resistor should I use. I dont mind spending a little money if I can avoid math.

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[tartansailboat]

Neil, just a simple question, the measurement of coil voltage at cruising speed is somewhat arbitrary. The voltage is an AC voltage and therefore you need to set your voltmeter on AC not DC. What you really are after is the RMS voltage which is the voltage used for heating calculations and I think that most digital voltmeters do indeed measure RMS but the shape of the wavefrorm (due in part to the dwell factor) has some influence.

[ndutton]

DVD,

As long as your alternator output is greater than 12 and less than 16 volts (it had better be), a 4 ohm coil should keep you in the good graces of the electronic ignition gods. A combination of coil and resistor that adds up to 4 ohms is the same thing. Moyer's coils are 4 ohm and checking one on their shelf measured at 4.5 ohms. My previous reluctance in making this type of statement is going too far the other way. More resistance isn't necessarily better, there is a sweet spot. Note that Don has recommended 4 ohm coils for electronic ignition for several years and as far as we know, without incident.

edit: one more thing: it's my opinion that oil filled coils are the way to go. I've learned from several sources they are far more tolerant of heat than epoxy filled.

Tartan,

The reason for measuring the voltage at cruising speed is to be sure the alternator contribution is in full swing. The difference between voltages with the engine off and at speed can be as much as 4 volts. That affects the amperage significantly and in truth it's when the engine is running at speed for hours that really matters.

Regarding measurement, the diode bridge (rectifier) is supposed to convert the alternator's AC to DC output although it is certainly pulsing. Though not truly AC, this pulsing mimics AC somewhat and drives digital meters nuts. I recommend an old school analog dial type meter set on the DC scale. The movement of the needle alone is sufficient damping to get a decent voltage measurement.

One more thought to support that the output isn't true AC, we shouldn't be charging our batteries with AC should we?

[joe_db]

With a normal points setup or the common Pertronix or Indigo setups, the voltage at the coil + should be DC and measure close to the battery voltage. I checked one of my spare coils and it is 3 ohms, just FYI. I used to go through coils at a rapid rate until I went back to points and ditched the Indigo system. Never a problem now
Anyway, here is my theory:
Looking at the graph, the points ignition has a greater time of no current (aka less dwell) than the Pertronix. The Ignitor is essentially "super points" that open and close quicker.The Pertronix system provides more area under the graph, i.e. more heating time.
I'll admit I'm guessing from the literature for the MSD graph. According to the written description, they store energy in capacitors in the unit and not inductively by powering the coil. Thus there spark is created by pulsing high voltage from the capacitors into the coil, not by interrupting a constant current through the coil. Note also it sparks more than once, thus the name. They describe less coil heating as an advantage of this system. I may install one as a winter project.

EDIT: I just called MSD. They confirmed that there is no constant current and that at 2,000 RPM, you'll get 3 sparks. So for a guess, I got it pretty much right on They also confirmed that the Pertronix Ignitor will work well to trigger the MSD system.

[Dave Neptune]

A question

Joe, I am usiung the Indigo ignition and hearing of the problems you have encountered I wonder. When you were using the Indigo and replacing coils were you using oil filled coils? Oil filled coils are for runnig more voltage/power and are designed for the additional heat they build up. Mine has run flawlessly for 9 years now and I do a lot of early morning motorng on the 6 hour ride to my Catalina mooring. I do this 18~20 times a year so I am runnig for quite a while. I am using a Flamethrower and am thinking of going to one with a lesser rating.

Re the MSD it is a great system. It is mostly for hi overlap performance oriented engines that do not atomize fuel well until the get the "flow" goin. It may help an A-4 with an idle problem but I don't see any advantrage on a good running one, unless coil heat is a worry. They also work well on F/I engines that run right on the "lean edge" as that is a fire that is harder to light and also requires a very large gap.

Dave Neptune

[joe_db]

My Indigo system devoured all kinds of coils. Some made it a year and some less. Even the ones I got that were for the Indigo kit died sooner or later. I have a suspicion that as coils fade away from any new car use, the quality has slipped. The original coil from 1973 lasted until about 1988 and did THOUSANDS of hours on points. I discovered accidentally - by getting shocked - that the Indigo had a weird failure mode that had it constantly pulsing the coil. I also discovered it is a relabeled Crane system that dates back to the 70s. See <http://bernardembden.com/xjs/ingamp/index.htm>. I don't want to give the impression that Indigo is bad, I got a lot of help from Tom and bought a lot of other things from there.
So now I am back to points, the tach doesn't have fits and jump around every now and then, the coils don't die, and all is well except maybe cold and damp starting. You need to make sure the choke is absolutely 100% pulled out for a quick start. I have an urge to go MSD just for the fun of trying new tach as well as better performance. 3 or 4 sparks at startup has to help. AFAIK any modern car pulses the coil like MSD to fire the coil(s). I really want to set up dual MSDs and use a spark combiner to be able to switch coils underway If only my wife didn't pay the bills

EDIT : Something I learned the hard way. If your engine NEEDS electronic ignition to operate, you are masking a problem that needs solving.

Joe, I am usiung the Indigo ignition and hearing of the problems you have encountered I wonder. When you were using the Indigo and replacing coils were you using oil filled coils? Oil filled coils are for runnig more voltage/power and are designed for the additional heat they build up. Mine has run flawlessly for 9 years now and I do a lot of early morning motorng on the 6 hour ride to my Catalina mooring. I do this 18~20 times a year so I am runnig for quite a while. I am using a Flamethrower and am thinking of going to one with a lesser rating.

Re the MSD it is a great system. It is mostly for hi overlap performance oriented engines that do not atomize fuel well until the get the "flow" goin. It may help an A-4 with an idle problem but I don't see any advantrage on a good running one, unless coil heat is a worry. They also work well on F/I engines that run right on the "lean edge" as that is a fire that is harder to light and also requires a very large gap.

Dave Neptune

[Dave Neptune]

Yep!~it is

Joe, Crane/Allison were the ones I used many years ago and had no problem It was an Allison XR700 that I adapted to my A-4 now 27 years ago. When the Indio became available with the led indicator I decided to switch it out just to have the indicator for trouble shooting. The Allison I removed is still working on my dune buggie.
Perhaps you just got a bad unit(?) from them. Did you send it back for any kind of analysis, Tom may be interested in that. I have also installed many other types of EI conversion "kits" with almost no trouble and only improvement in "tune longivity".
I do have my original points still mounted to the plate (an extra one as the adv-mech. in the dist' when I bought her was rusted tight from being submerged in salt water) in my didy box with a coil, just in case. It has sat there for 27 years now. I do like the benifit of quicker starts with any kind of electronic set up, however my biggest reason was reliability and not having to do tune ups again. Just change the cap and rotor every 4~5 years and plugs every 2 or 3. The only times I have toudhed my distributor is when making jetting adjustments or when trying a different prop.
I just love to tinker with engines and props.

I bid you fair winds and good luck with your beastie.
Dave Neptune

Dave Neptune

[dvd]

4 OHM Coil??

Ok. So really no matter what the issue is with the electronic ignition, it appears that if I install a 4 OHM oi8l filled coil I should be fine.

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[sastanley]

Yup..you should be. With a 4.0 ohm coil, & a 'normal' charging system that is in the 14.1-14.2v range...14.1v / 4.0 ohms = 3.52 amps of current thru that circuit.

[dvd]

Ok I'll try it and see I didn't have to do any math.

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