Triumph Repair: Sudden Ignition Failure, electronic control system, rotor arm

Question
I own a 1979 US model spitfire 1500.  I use it as a daily driver here in Seattle.  A couple of weeks ago, I started it up and let it idle to warm up, after about 5 minutes it just shut off.  I could not get it to restart.  After trouble shooting, it is not getting any spark.  Here is what I have done so far. Pulled plug out and turn engine, no spark. It has the original Lucas electronic ignition.  With Key in the on position, I was getting 12.4V (battery voltage) at the positive, negative and distributor lead of the coil.  All voltages stay constant when I attempt to start the car. (dropping to ~11V from the starter of course).  I replaced the coil and still have the same problem.  I cleaned all contacts, new plugs and wires.  Pulled wire from coil off the distributor and checked for spark to engine block while attempting to start, no spark.  Before I go replacing more expensive parts I wanted to get a more professional opinion on where I should look, or what else to replace. (ie: new electronic ignition, distributor)  Any help would be much appreciated.  Thanks    -josh

Answer
Joshua,

Let me give you some basic ignition theory then we'll dive into your system.

The coil in a traditional ignition system is a transformer.  There are two sets of windings, the low voltage (primary circuit) and high voltage (distributor circuit).  Normally the "hot" side of the electrical system is connected to the coil and the other connector goes to the distributor where it's grounded, either by the mechanical points or an electronic control system.  When the points break contact the current in the primary circuit stops.  It's built up a magnetic charge in the coil, and with nowhere else to go it creates a high voltage surge in the distributor circuit.  The distributor itself has moving a contact (rotor arm) to control which spark plug gets the spark.

Considering the car is 28 years old, that's not bad for the Lucas ignition to have run that long, considering the Lucas reputation.  In reality, some people have claimed to have opened up a Lucas control unit from the late Spitfires and found General Motors HEI electronics.  Very possible, and a possible source if you want to rebuild your Lucas system to look "stock".

The other options are to go to an aftermarket electronic system or to get an earlier mechanical points distributor from a pre-75 Spitfire 1500.  Personally I'm a fan of the mechanical points, they're easy to troubleshoot.  The downside is that mechanical distributors do wear so a used mechanical distributor may not be much better than your current (non running) system.

Cheers,

Jim