First thing you need to do is determine whether you have a ballasted or
unballasted ignition system. You could assume that the old coil was correct
and measure the primary resistance of that, but having been cooked that may
well not be giving the correct readings anyway. However if it still runs the
engine it will probably be good enough for checking for a ballast. Turn on
the ignition, make sure the points are closed or connect an earth/ground to
the points wire (should be the coil -ve), and measure the voltage on the other
coil terminal (+ve). If you see battery voltage it is unballasted so you need
a 12v coil. If you see significantly less than 12v, typically around 6v with
the correct resistances in coil and ballast, then you need a 6v coil. When
getting a coil measure the primary resistance, ideally before paying for it!
Ignore what it says on the can, the box, or what the salesperson may tell you,
labelling can be very confusing if not downright incorrect. A 12v coil should
measure about 3 ohms for a standard coil, about 2.4 ohms for a 'sport' coil.
A 6v coil should measure about 1.5 ohms for a standard, about 1.2 ohms for a
sport.
The other thing is that the same amount of heat was being dissipated by the
ballast resistance as the coil, so that may have suffered damage as well. The
only external visible signs of that would be if the harness wrapping across
the front of the car between the headlights shows any signs. Fortunately the
ballast wire is spread out along a run of the harness and not concentrated as
in the coil, so damage should be less likely.
And finally in case you are wondering why the coil gets damaged like this when
it doesn't in use, when the engine is running the points are opening and
closing and disconnecting the current through the coil for nearly half the
time, so there is almost double the heat being generated by the coil (and
ballast) when the ignition is left on like this than when the engine is
running.
PaulH.
----- Original Message -----
Any recommendations for a replacement? - I had a Lucas Sports Coil think!
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