Is my coil faulty??

Ok, i have two 10E motors here, both running, but, when i measure the resistance of the coil secondary [earth to HT terminal] i get open circuit [or at least higher than 20Mohms] on both..

Cannot see that this is right??

john..

Hi John,

I have measured the secondary resistance of several different coils and found this to be between 4 and 6 k ohms. Obviously the coils in question are giving an output , so if you have a resistance in the M ohms it would suggest that you have a break in the secondary winding (probably where the fine enamelled wire is soldered to the terminal. Because the output voltage is in the order of 20-30kV thie current.will jump the small gap. which is effectively in series with the spark plug, to get to earth. If this is the case the internal break will gradually grow due to erosion and cause the effectiveness of the coil to deteriorate. If you checked the resistance with say a 500 V insulation resistance tester you may get a reading in the K ohms.

Bit of a coincidence that two coils have apparently the same fault! Are they both the same make and vintage?

Regards Dave.

Hi dave, sorry for being slow to reply, my internet has been off for days. Anyway, what I will do, is later on i will try try the coils, or at least the one, with my megger 1731 meter, as i can do 1000v insulation tests with that and see what happens..

I think they are the same age, unless one has been replaced, but the one is hiding behind a flywheel so i have no idea of the make. Both are on bikes that have not moved since the 1970's, so i expect they are both proper villiers coils and not chinese rubbish.

i will try the one now..

john..

Hi Dave,

Just tested the one, and it gave 302 Mohms at a test voltage of 1104V !!

Must be a break... What you think??

john..

Hi John,

From the tests it certainly looks like the coils have broken secondary windings but that the width of the break is such that the coils are still able to produce a spark at the plug in addition to one across the coil break! It appears that the coil break is in air rather than covered by insulation since, from your Megger test, there is no evidence of a carbon track being formed.

Out of interest a basic test on 0.5mm wire using a BM222 Megger on the 1000V range, showed that a gap of 2 thou gave a reading (whilst arcing) averaging 0.22 M ohms, whilst with 3 thou gap there was no breakdown arcing and instrument indicated greater than 999 Meg.

The million Dollar question - How long they will continue to produce a viable spark at the plug?

Dave

How long will it last? Thats easy Dave, it will wait till you are somewhere very far from home, no one you know will be available to pick you up and it willl start to rain, oh and you will have left your wallet at home. Keith.

Usual symptom is difficult starting when hot, which progressively gets worse, then finally ending up as Keith describes.

Sandy

Hi Keith and Sandy,

Sounds like you have both experienced the worst work of the spark gremlins.
On the two occasions I have suffered total loss of sparks (Italian coil and Lucas magneto) they have let me off lightly and I have been close to home.
Better not let it happen a third time then, I hear you say!

Regards
Dave