Coil failed on the way home from the MOA rally

[mrclubike]

My Coil failed on the way home from the MOA National rally.
I got lucky I was about 6O miles north east of Indy but there was a small independent Cycle and ATV shop down the road and he had a coil with 2.4 Ohms primary resistance.
Was able to wire tie it all together and Get the rest of the way home.
It does miss fire occasionally during acceleration but it did the job

The coil that failed was a 1,5 ohm Dyna coil less than 2 years old
In its defense  I  may have damaged the coil when I was setting up my charging system and  the charging voltage spiked to 16 volts.
 
When it failed the engine would run on either cyl one at a time but not both at the same time.
If I couldn't  have found a wasted spark 2 tower coil  my next plan was to buy 2 1,5 ohm coils and wire them  in series.
Like was used on the 81 thru 84 R100.  (see bottom picture  )

[Bob_Roller]

Was the coil, the gray and black coil

[mrclubike]

No It was a brown Dyna coil

I had thrown that one away when i installed the dyna but i should have kept it
It would have at least worked enough to get me home

I was carrying a complete extra ignition except For a COIL  

[Bob_Roller]

What failed on the coil, primary, secondary circuits ??

[mrclubike]

Not sure
I didn't Ohm it out.
Resistance testing a coil is not very conclusive.
They will often Ohm out Ok but still be bad
But I will check it towmorrow just to see what is Ohms out to.

[Bob_Roller]

Just curious, I asked Rick Jones, the owner of Motor rad Electric, how many confirmed failures he has gotten on DYNA coils in the time he has been selling them, his response, was less than five .

[georgesgiralt]

The problem with these coils is that a very fine wire is put into a pot of some sort of plastic.
If the plastic shrinks one way or another it puts too much stress on the wire which cuts.
I have,  long ago, worked  in a factory producing electronic ignition systems for the automotive industry. The electronic was in an aluminum box filled with pot plastic then baked. We had failure where one capacitor DISAPPEARED from the circuit board because the potting reacted with the plastic case of the capacitor which turned into gas and this was accelerated by the aluminum foil of the capacitor... The quality insurance boys did not believe it at first !
If I were you, I would go to a car's junk yard and ask for a dual tower ignition coil with a close to perfect primary resistance. Why a car coil ? Because these are produced in very far larger quantities than their bile counter parts and do not change that much during the years (the coil part is often identical for a huge number of cars, the support evolves from car to car because of mounting constraints). Being produced in huge quantities they are well debugged and very reliable.
I've a set from a Fiat car which is 0.8 Ohm primary and work perfectly. They are made by Magneti Marelli in Italy. But the coil itself looks like some Bosch I've seen in cars or a Lucas one in a Rover... So maybe they outsource the coil itself or share a common design which is proven to work well ...

[Barry]

If you want to keep the substitute 2.4 ohm coil it will work better when bolted up.  All coils with exposed laminated cores at either end are designed to have the magnetic loop completed by the metal of the mounting bracket. It actually changes the inductance of the coil and makes it work more efficiently.


Other coils like this one have an internal magnetic loop so it's not important

[nhmaf]

Anything can fail, certainly.   One thing that has always concerned me about the Dyna browns is how close the electrical connection machine screws are to the mounting bolts heads when all is fitted up.   I put extra heatshrink tubing over those wires and make sure no stray strands are poking out.

[mrclubike]

Just curious, I asked Rick Jones, the owner of Motor rad Electric, how many confirmed failures he has gotten on DYNA coils in the time he has been selling them, his response, was less than five .

That is why I wasn't  concerned with carrying an extra coil.
In my 35 years of wrenching I have only seen maybe 4 or 5 coils actually fail  
Most are replaced because of the secondary connection in the tower burns up from a bad coil wire or corrosion but are still functioning ok

I suspect I damaged it  when my charging voltage spiked while I was setting up my charging system

[mrclubike]

If you want to keep the substitute 2.4 ohm coil it will work better when bolted up.  All coils with exposed laminated cores at either end are designed to have the magnetic loop completed by the metal of the mounting bracket. It actually changes the inductance of the coil and makes it work more efficiently.

 Other coils like this one have an internal magnetic loop so it's not important

Thank Barry I may try rigging something up to complete the loop

[mrclubike]

What failed on the coil, primary, secondary circuits ??

The Primary is 1.5 ohms
secondary is 13.4 K ohms
Not sure what the secondary is supposed to be.

[Barry]

13.4K sounds about right.

Perhaps the windings are shorting out under load. A megger test at high voltage might show up the fault but as they generally only go up to 5000 volts it's not guaranteed that a megger would expose the fault.

[Justin B.]

Since a Dyna failure is so rare, especially in a fairly "young" coil, maybe they might want it back for failure analysis?  Maybe ask Rick...

[skippyc]

The previous owner had solved the crack in the coil on my bike buy smearing it with silastic. I am in the process of putting a dyna coil on it.
I have a dyna Green on my BSA and it really improved things.