Coil input information

[smosher]

Yes, lower voltage dropped across the coil.

fwiw its a ratio between the coil and the coil + the ballast resistor,

3.3/4.1 in my application = 80% of the supplied voltage will be dropped across
the coil

Steve

[egmonster]

Confused on Coil

I have an A4 installed in a 27" C&C Sailboat. The engine has consitantly run for hours without problems until last week when it began to run rough and then died. I was unable to restart.

I suspected it was a coil problem and began research on this site which has provided some great info however I am now confused.

The engine has an electronics ignition package and I assumed from the info here that the coil would have a ballast resistor or an internal resistance type coil.

I removed the old coil #407, manufacturer unknown, other labeling unreadlable except the words "use ...R...", however could not locate any resistors in the circuit. I measured the primary coil resistance at 3.9 ohms but no sure if this would be accurate on a failed coil.

I am not sure I can find a replacement coil based just on the part number.

Will I damage anything if I replace the unit with a new coil with internal resistor or should I replace with a normal external resistor type coil.

Any help would be greatly appreciated.

[sastanley]

egmonster,

some of us are using both an internally resisted coil AND and external inline resistor to assist the internal ballast coil...edit- what the in-line resistor is really doing is reducing the input voltage to the coil to hopefully keep it from burning up.

What I think is most important is the input voltage the coil is seeing..This is measured on the (+) on the coil post, and the ground on your meter to a good ground like the engine block (the (-) on the coil is NOT ground.) - this needs to be checked while the engine is running, and while the alternator is charging (i.e., normal cruise RPM)

Then you can use Neil's calculator somewhere in this thread to determine if you think you need an external resistor.

3.9 Ω is pretty good, and higher internal resistance than most. But, if your input voltage to the coil is too high, it may not be enough to keep from frying the coil.

Hope that helps clear the muddy water a bit..I think we need more info from your set up for further diagnosis.

Just for a reference point..I have a 3.3Ω internally resisted coil, and a 1.63Ω external resistor in-line in front of the coil (if my memory serves me..) This gives me somewhere in the 10v range of input voltage to the coil.

[roadnsky]

...I am not sure I can find a replacement coil based just on the part number.
Will I damage anything if I replace the unit with a new coil with internal resistor or should I replace with a normal external resistor type coil.
Any help would be greatly appreciated.

A really simple answer would be to buy the coil sold on this very site by our sponsor.
It already has the proper resistance. No fussing with an external resistor.
Just a straight R&R and you're done!

Give Ken a call (610.421.4436) in parts and he'll get you all set up.

[sastanley]

Jerry..of course you are correct. I think the coil sold by Moyer is over 4 ohms of internal resistance.

[BobMcD]

Flamethrower coil from Moyer

Hi all,

Just to throw a monkey wrench in the proceedings, I have a Flamethrower coil that I got from Moyer, that when measured shows only 2.9 ohms across the - and + terminals. In fact the case of the coil says 3.0 ohms, yet it was originally order from Moyer as the internally resistered coil. It has not been installed as yet, and I was wondering if I should be adding a external resistor to is as well?

[ndutton]

Actually Bob, your situation is what this thread's about. To answer your question of resistor or not, it depends on your voltage between the coil + terminal and the engine block ground at cruising RPM. Divide that voltage by the coil resistance and make sure the result is under 4. In the calculator I factored in a 15% safety factor reducing the quotient to 3.4.

At first blush I'd say you should add a resistor but without the coil input voltage we won't know what size.

The calculator can be found in the first post of this thread.

[roadnsky]

Just to add a little more to Neil's comment...
Yes, MMI used to sell the 3Ω Flamethower on their site.
(In fact, I also bought one)

This was before Neil did some research (a lot actually) and discovered that their claim (PERTRONIX not MMI) of a perfect match to their EI unit was, in fact, bogus.
If I understand correctly, once we'd made the discovery of needing additional resistance to avoid overheating (sorry, you'll have to actually READ the thread)
that was when MMI stopped offering the Flamethrower coils.

The coil now offered is at the necessary 4Ω requirement.
As I said, give Ken a call and he'll make sure you're set up proper if you have any doubt.

Just wanted to clear up any confusion to Bob regarding the Flamethrower originally offered on the site.

[sastanley]

Jerry, that might be a couple of paint chips there around the coil on your museum piece!

Better get on that!

[roadnsky]

Jerry, that might be a couple of paint chips there around the coil on your museum piece!

Better get on that!

I know.
I'm outta paint!

[ndutton]

Credit where credit is due

Jerry mentioned the amount of work that was done on this issue to reach the solution we have and he is correct, it was quite a bit but in fairness I had a ton of help from several forum members. It was a collaborative effort. Thatch, Shawn and a bunch of others were involved. Reading through this marathon thread you'll see the variety of contributions.

We started with Pertronix themselves, found contradictory information in their literature and when we provided identical failure data to two different techs we received wildly differing responses. We knew then and there we were on our own, flying without a net so to speak.

And the rest, as they say, is rock 'n roll history.

[BobMcD]

Thanks all, I was wondering about the flamethrower coil, in fact I got it a long time ago from Moyer as a spare and was planning on installing it ..just because I had it and the original coil is rather old.. The existing coil reads 3.9 ohms across the terminal (disconnected), and so I was concerned about installing the new coil. I read about 13 volts across the coil + to ground when the engine is running at cruising speed, so to use the new coil I will need to add about 0.9 ohms resistor to be safe... Thanks for the info.

[sastanley]

Bob, I think you have the right idea.

I am of the opinion that you put as big a resistor in front of the coil as you can, but keep the motor from being hard to start....i.e., too much resistance in front of the coil while cranking does not allow enough voltage to the coil to fire the plugs.

I have enough confidence in this arrangement now, that I've removed my 'dual coil' mounting. I have a 2nd coil aboard, and it is another Flamethrower, but now stored in a locker with the other engine spares. I have 3 of those little $8 ceramic resistors aboard, 1.82Ω being the middle range I think, and if I recall correctly, a 2.2Ω and a 1.1Ω, so that I can tailor my input voltage to the coil if necessary.

I started mine today after a three week rest. My current setup is the 1.82Ω resistor in front of my 3Ω (3.3 actual) Flamethrower and it fired instantly. I have not put a meter on it in a while, but if I recall, the voltage to the coil (+) post while running is in the low 10v range, and I have had zero trouble with that set up from an ignition system standpoint.

[BobMcD]

Shawn,
Thanks. This makes sense. Did I see somewhere that you all got those resistors at NAPA stores ?

[sastanley]

Bob, Affirmative...I had to do a little digging for the specifications, but I picked them up at my local NAPA between work and home.

I'll be happy to re-research when I have time to provide the data, but I think I found most of it on NAPA's site. Also, as I did with the internally resisted coils (a 3.0 ohm Flamethrower really being 3.3), I always check them myself with my own meter before installing..I have found that about 9 times out of 10 that the actual resistance is a little bit higher than advertised...which I think in this case is a good thing, as it lowers the overall voltage into the coil.