Recently I was baffled by my distributor and coil system was not producing a spark at the plug even though I was getting power to coil and distributor.  Out of the car and on a test fixture spark was produced.  I tested a few distributors and coils all the same results.  My wiring harness is a reproduction appearing in good condition.i hooked up a headlight bulb to a load on to see if it would light, it didn’t.
 I made some ten foot jumper cables going from the ignition switch to the coil, it lite the head lite bulb



71% of original size (was 710x19) - Click to enlarge. Got spark and determined something amiss in the wire from the ignition switch to coil so I replaced it an all is good.
Point being even though the original wire produced twelve volts apparently there was not enough amps to push it.
Attached is my test fixture 

I like the test fixture. Got some of that strut out in the garage, but wouldn’t have thought of that. Last year I finished fighting a problem like that. A wire that carried enough voltage to read good, but under load had enough resistance to drop the voltage below what a starter solenoid required. Glad you found it.

miker
55 bird, 32 cabrio F code
Kent, WA
Tucson, AZ

Removed duplicate

miker
55 bird, 32 cabrio F code
Kent, WA
Tucson, AZ