246gt Low voltage at coil

12.4 is only 80% charged. 13V at alt. is low. But those numbers should work. It is possible to have a battery look ok 12.4V but still be dead not able to put out the current. Poor connections can make this all worse a dino like 60 years old. I would pull a battery out of another car and just try that first. If that does not work check wiring connections especially high current ones. Then check components like fore/aft V at ballast resistor etc... 8-9V at coil is ok because ballast resistor drops V down. But at crank that number goes up because power bypasses ballast resistor from solinoid to get more juice to coil under cranking. I think that's right. Points and coils are a very long time ago for me.

 

With the ignition on and the points closed you will never get full battery voltage at the coil. There are many componets that result in a voltage drop: the contacts of the main power relay, the fuse block contacts, the fuse itself, the ballast resistor, the main winding of the coil, and the points themself. Collectively, these are known as a voltage divider. As a first check remove the positive lead from the coil and measure the voltage at the terminal end. If it's very close to the battery voltage, the relay contacts, the fuse block, and the fuse are OK. You then have to check the remaining components one by on to determine if any one of them are the source of the problem. The starter solenoid and the starter are a completely different circuit. I suggest working on one at at a time; otherwise, you won't be able to isolate the problem.

 

It's normal for the voltage to drop a volt, or two, when the starter motor is actually cranking, but it should only drop a relative small amount with just key "on". You must have an unwanted resistance somewhere in the path from battery-to-coil so just check the voltage at all the connections in that path with key "on" until you find where the voltage is falling below the battery voltage.