Accel coil ?

[JDK]

I see I can pick up an Accel high output ignition coil (the round kind) for thirty bucks here locally, is there any reason I shouldn't just pick up one of those with the new cap and rotor iv ordered from them ?? Thanks

[joe_db]

Depends a lot on the coil. Do you know the primary resistance?

[ndutton]

Be advised: JDK has electronic ignition.

[sastanley]

My experience has been Accel is junk...Neil eluded to the issue with electronic ignition. Check the internal resistance.

With ample rigging of external resistors in front, it may survive a while, but the extensive discussion we've all had here about EI & coils, etc. deserves review.

This is my personal experience with an Accel coil..it is possible I subjected it to extreme voltage before understanding the underlying effects of new wiring and EI.

There are many threads on this subject...here is one of them.

http://www.moyermarine.com/forums/showthread.php?t=5845

[JDK]

Well now totally stumped......all that talk about resistors and wiring and fiddling has me confused, have I always misunderstood how an ignition system functions ? It seems to me that now something simple is totally overcomplicated all of a sudden......I thought a coil always draws voltage when it is switched on, the points or electronic module sets the exact spark duration, and adjusting the distributor gets the timing to be spot on......by using a "high voltage coil" I am merely upping the amount of power firing down the spark plug wires......iv never heard so much talk when dealing with the ignition on an old Benz or BMW motor.....I never would have thought that the simple stationary A4 motor can be so complicated.....or did I miss the point entirely ?

[sastanley]

No..you didn't miss it..

My opinion is that when you introduce electronic ignition into the equation I think the increased dwell time of an EI unit over points saturates the coil and tends to burn them out unless they have adequate internal resistance, like the one Moyer Marine sells (4.3Ω if I remember?) You can work around this problem if using an auto store coil with lower internal resistance (I think the Accel coils are 1.5Ω) adding resistors in front of the coil to reduce input voltage and reduce the tendency for it to burn out when coupled to an EI module. A coil does not need 12v to fire a spark plug..With some added resistance in front, I think the input voltage on my coil is somewhere in the 10.9v range.

In fact, many coils survive for years and years on the motor traveling thru 30+ year old wiring, especially if they have points in the distributor. I am almost sure the coil on my boat was original from 1977 and worked just fine for 33 years (it was painted the same color as the motor)..until I added an EI module in 2010 and it lasted for about 50 minutes. I further exacerbated my particular problem by re-wiring the entire ignition system a couple years later with properly sized tinned marine wire, with no connectors or plugs, & properly crimped marine grade fittings, further reducing voltage loss and pumping more voltage into the coil.

It can get more complex (maybe convoluted is a better term.. ) than this, but I am trying to stick to just the subject you are asking about.

[Dave Neptune]

JDK, simply put it is the dwell that feeds power to the coil. With the EI instead of 30~35 degrees of dwell (saturation duration) you have with std points you have over 70 degrees of dwell or almost twice the saturation time, thus the heat and premature coil failures.

The higher charging voltages with some battery systems even add to the issue above.

Dave Neptune

[sastanley]

What Dave said, in a way less complicated manner than my rambling.

[JOHN COOKSON]

Originally Posted by JDK

sudden......(1)I thought a coil always draws voltage when it is switched on, the points or electronic module sets the (2)exact spark duration, and adjusting the distributor gets the timing to be spot on......by using a "high voltage coil" I am merely upping the amount of power firing down the spark plug wires......iv never heard so much talk when dealing with the ignition on an old Benz or BMW motor.....I never would have thought that the simple stationary (3)A4 motor can be so complicated.....or did I miss the point entirely ?

(1) There has to be some resistance in the primary coil winding. Otherwise it is a dead short to ground. Would make a good momentary arc welder. Voltage is the push . It is not "drawn" into the coil. The measurement is amps - the amount of electricity that passes a given point. Remember Ohm's law. Amps are directly related to volts and inversely related to resistance. So the two critical measurements are voltage at coil + and total resistance both external to the coil and inside the coil. To many amps through the coil and it over heats and dies.
(2) The spark has no "duration".* It's momentary. The critical parameter is when it occurs not how long it occurs. The secondary winding is energized while electricity is flowing through primary winding. The moment when there is no longer any electricity going through the primary winding the secondary winding belches a high voltage bolt of electricity. The electricity to the primary winding switched on and off by the points or EI.
*What does have "duration" is how long the primary winding is energized. This is referred to as dwell which is expressed as the number of degrees of angular rotation of the distributor cam.
(3) This is where it gets interesting. To many amps through a coil over a period of time will overheat and destroy the coil. With points the dwell can be set by opening or closing the point gap. As a matter of fact if a properly gapped set of points will produce a correct dwell reading (measured with a meter). If this is not the case something is wrong that needs to be investigated.
Now for the fun part. With an EI the dwell cannot be varied. It is fixed in the EI module and is something like two times the dwell seen with points. It has been discovered that the increased dwell time will cause some coils to over heat and die because to many amps are going through the coil over a given period of time. Quiz: With the dwell period a fixed value how do you decrease the amps going through the coil when the dwell is a fixed value? Answer: Remember amps are inversely proportional to resistance. So increasing resistance will decrease amps. Practically this is done by buying a coil with the correct internal resistance or attaching a resistor before the coil.
If you buy your coil from MMI it will come with the correct resistance for the EI .

TRUE GRIT

Edit: Others were typing while I was typing.

[ndutton]

Wow guys, those were outstanding posts. Five star thread rating from me.

[JDK]

now it makes more sense to me, thanks......

yeah i guess i might as well just order a coil from MMI, whatever works best.....

[Easy Rider]

John, That was a superb explanation of the ignition system. Well done and thank you.

[joe_db]

A friend burned out his coil by leaving the ignition on. He took it to the local auto parts store, they looked at it, and then asked what it was
A Moyer coil was then ordered