Voltage at coil

[V12T]

70 Challenger. 512 stroker, 10.3 compression, stealth heads, ultralight crank, sixpack, modest street/strip Comp hydraulic lifter cam, headers (unknown brand), Mopar electronic ignition, 440 Source dist, new coil, new ballast (4 pin), 727 trans.
Engine running fine but trying to understand why I only have 9.5 volts at the coil. I have put a bypass wire on the ballast. The blue "run" wire has 11.8v when disconnected. When connected to the ballast (and therefore directly to the brown coil wire) the voltage falls to 9.5v at the coil. The blue wire is twinned with a light blue/yellow wire that goes to the ign. module. If I disconnect the ign. module plug, the coil voltage goes up to 11.8v. (not running now of course). Can anybody explain why the presence of the ign. module in the circuit cause the circuit voltage to drop? Is it perhaps meant to do this? Is there an alternative way to wire the ign. to get 12v at the coil which I assume is optimum?

 

[Challenger RTA]

The start brown wire is for staring only. It put full battery voltage to the coil for easier starting. The run blue wire connected to the resistor reduces the voltage. If there is full voltage to the coil all the time it will over heat.

 

[V12T]

As I understand it the brown wire for starting connects directly to the coil. If the ballast is used, the blue (run) connects to the brown through the ballast at reduced voltage. The brown is the only connection to the coil. I have directly connected the blue and brown, in theory passing full voltage to the coil on run as well. But it doesn't! Are you saying that a 12v coil overheats if it receives 12v?

 

[Deathproofcuda]

First off, the ignition module is providing an on/off switching of the ground connection to the coil. When the coil loses that ground connection it generates the spark for the secondary side of the ignition system.

When you are getting a reading of 9.5 volts at coil positive, the coil has a ground connection and there is current moving through that circuit from the battery to the ground connection provided by the ignition module. You are reading the voltage in a circuit under load, and your reading will be based on the resistance in that ground connection. If you bump the position of the distributor using the starter, the resistance of that ground may change and your voltage reading may change.

If you lose that ground connection, either by bumping the distributor to a position where the reluctor triggers the ignition module to switch open, or if you do like you did by disconnecting the ignition module switch, then the circuit is open and current is no longer moving in the circuit. The ground lead of your volt meter is now providing the ground and you are measuring voltage potential at the positive side of the coil. You are now measuring voltage in a circuit that is no longer under load.

Your ignition is acting like it should, you don't need to modify it to increase voltage to the coil.

 

[V12T]

Thanks Deathproofcuda. Great explanation - puts my mind at rest.

 

[Challenger RTA]

As I understand it the brown wire for starting connects directly to the coil. If the ballast is used, the blue (run) connects to the brown through the ballast at reduced voltage. The brown is the only connection to the coil. I have directly connected the blue and brown, in theory passing full voltage to the coil on run as well. But it doesn't! Are you saying that a 12v coil overheats if it receives 12v?

Yes. There are different coils for different applications street or race or other manufacture. It also depends on the trigger that is being used. The brown wire only has power when the ign sw is in the start position. The blue wire has power when ing sw is let to return to run and provides power to the ECU or points as Deathproofcuda explains. The way a coil works is. When power is applied It builds up a magnetic field in windings and stays there until the ground is open. Being it point or ECM or as newer cars it all goes thought the computer. Once the circuit is open the field collapses. And exits through the secondary winding of lest resistance. Hense Spark if everything is in order.
The system went from this to the below.