While ignition coils can go south suddenly, an open in the HT circuit
won't neccessarily cause it to be non functional. This is because we
have very high voltages available, and, if it takes a couple of
hundred volts to jump across an open circuit in the coil, it really
makes little difference to overall performance.
Of course if the LT circuit is open, it won't work.
When I worked for Lucas, I was told (but I never verified) that the
spec allowed for 3 breaks in the HT windings -- the ends of the wires
would be twisted together and if there was not an electrical contact,
there was enough voltage to break down (short across) the insulation
around the wire.
Regards,
Simon
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