246gt Low voltage at coil

SO trying to figure out why I have low voltage at coil. Battery voltage is 12.4 and fully charged. With ignition on I get a first voltage between Coil + and Ground of 8volts for about 2 or 3 seconds and then jumps to 10volts? Strange... Then when I crank voltage drops back to around 8 or 9 volts. Therefore creating a hard start issue but if lucky then I can get it to start and once running voltage is back up to charge rate of around 13volts and the car runs fine. Just stumped on 1) Why voltage jumps after 2 or 3 seconds after turning on ignition . 2) Why I have a low voltage at the coil. Any suggestions what to check. Car is a 73 Dino 246gt

 

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.

 

SO the battery is new two days ago and put on charge over night. At cranking the voltage drops which to me means that I have a weaker spark and therefore hard start issue. I have not traced the power to coil cable yet but think it comes via the starter solenoid. I need to check resistance through the ignition key terminals as well. I have also read that the little fuse box on the bulk head aft is also a problem. On my Fiat dino I get the full 12.2 at coil on cranking and it fires straight up. I am narrowing the hard start to lack of volts at coil giving me a week spark. I will run a wire direct from battery to coil to try and start to see if tis makes a difference then I know I need more volts there and need to work my way though each connection point.. Also the Fiat dino starter seems to have a lot more power and turns the engine over a lot faster and easier the the Ferrari Dino starter. Maybe time to remove starter for service and or replacement...

 

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.