Phil says: As you say, the 12V supply is in the back of the coil in the armored cable, where it is not accessible. The "negative" terminal (a misnomer in common usage) is accessible and connects to the points and condensor, and the high-voltage lead connects to the distributor cap. The coil's case is the ground.

Rather than check resistance, first check for voltage at the "negative" terminal with the ignition in "run" position. With the points open, you should read some low positive voltage; with points closed, you should read no voltage -- the terminal grounds through the points. If you get no voltage with the points open, disconnect the terminal and try again. If you still get no voltage, either the coil has gone open or is not being supplied with 12V through the cable (bad ignition switch?). If you do get voltage, there's a problem with the points, condensor or distributor insulator.

If the above checks reveal no fault, pull the high-voltage lead out of the distributor cap and hold it with a dry rag or other good insulator (potential exists to get a painful shock -- be careful!) so that the loose end is about 1/8" from a head bolt or other good, unpainted, ground. This should produce a fat spark upon cranking the motor. If it does, the problem is the distributor cap, rotor or plug wires. If not, you can be pretty sure the coil is bad.

The stock coil is sold as an assembly with the cable and ignition switch. This may still be available from Volvo (through GCP?), but it will not be inexpensive. There is no aftermarket replacement, and we'd recommend this only if originality is important to you. The usual fix is to mount a conventional coil (Bosch "blue" is good) and wire the 12V supply from the "switched" 12V terminal external to the cable on the ignition switch.