There are two main possibilities, the pickup coil that surrounds the distributor shaft, and the ignition module, the 4-pin electronic device mounted under the distributor cap. The pickup coil can be tested with a digital voltmeter. Unplug the 2-wire harness going from the pickup to the module, and attach the leads of an AC voltmeter to the wire terminals. Crank the engine, being careful not to snag the wires with the rotating distributor shaft. The voltage generated by the pickup coil should be about 1/2 to 1 volt AC as long as the shaft is turning. The module requires some dedicated test equipment that most shade tree mechanics don't have- - - -if the pickup coil is generating a signal, swapping the module should put you back on the road. Ignition coil failures on this system are extremely rare, so swapping the coil up inside the cap is usually not necessary. One other thing- - - -the very early design rotor buttons had a habit of burning a hole under the center connector and routing the high voltage spark to the center of the distributor shaft, but all of those rotors probably failed and got replaced many years ago.
Jerry
