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Ok so I'm confused I have a 53 chevy truck it has been converted to 12 v so I have a ballast resistor on it and it is putting out 5.58 volts should I get a 6v coil instead of the 12 v coil I have on it now . I'm using the original distributer .. Is there a ballast resitor that I need to use if I'm using a 12 v coil ?

The only time you'll see voltage drop at the coil is when the points are closed and there's a ballast resistor between the ignition switch and the coil. Points open- - - -you should see full battery voltage. Points closed, you'll see approximately 9 volts if the points are in good condition.

Try checking the voltage at the distributor side of the coil with the points closed. It should be almost zero. If the points are dirty or burned, you'll see a measureable voltage upstream of the points, which will cause the input voltage to the coil to be incorrect.
Jerry

With the points closed I used a light tester and I didn't get no light at the wire coming from the negative side of coil going to distributer . Now with the points open I get a light at the same spot why is that

With the points open the test light becomes the ground. That's normal operation- - - -points open, light burns. Points closed, light goes out.

WHAT VOLTAGE DO YOU GET AT THE COIL (-) TERMINAL WITH THE POINTS CLOSED?

This test is essential to diagnosing dirty or burned points. If you're unable or unwilling to do this test, I'm wasting my time on this thread.
Jerry

Zero

OK, it looks like the points are not the source of any unwanted voltage drop. It's possible you're trying to run an internal-resistor coil in combination with a ballast resistor, which will drop the current so much that the ballast resistor isn't functioning at the correct voltage drop. If that's the case, your coil performance will be severely compromised. It requires a basic understanding of some very simple Ohm's Law principles to make an ignition system work properly, but most people are unwilling to learn anything about basic electricity before they start shotgunning parts at an ignition system. Then they really jump the shark and install some horrendously expensive electronic ignition system with proprietary parts that will strand them in Lower Slobbovia at the worst possible time when all that was needed was the proper original-equipment parts to begin with.
Jerry

Never been to Lower Slobbovia but have visited Upper Slobbovia on a couple of occasions and its not a pretty place (they nailed me with the ugly tax both coming and going). If you believe the brochures they are the worlds leading manufacturer of muffler bearings and headlight oil.

So for the untrained ear what does the ballast resistor do exactly? Sounds like it reduces the voltage but does it do so at a fixed level or does it reduce it by a percentage of the input? Say you put Sal's ballast resistor on a 12 volt line would it reduce that to 9v? If I put the same resistor on a 40 volt line would it reduce that to 9v? Or some other voltage?

Thanks - Bruce

A resistor is one leg of the 3-legged stool that Ohm's Law is all about. Voltage divided by resistance equals current flow in amperes. Let's take a hypothetical situation where a coil primary winding has 4 ohms of resistance. If we apply 12 volts against the winding, grounded by the ignition points, there will be a current flow of 3 amperes- - - -12 divided by 4 = 3. Simple mathematics. Now let's install a lower-resistance coil winding- - - - -2 ohms. With the same 12 volts, we now have 6 amps of current flow, which is too much for a set of points to handle without overheating and burning. If we add a 2 ohm resistor in series with the coil for a total of 4 ohms, we're back to 3 amps of current flow, and the points are happy campers.

OK- - - -I can anticipate the next question- - - -why use a coil that can overheat and burn the points? Simple answer- - - -the battery doesn't always have 12 volts available. When starting a very cold (or very hot) engine, the load of the starter might drop the battery down to 8 or 9 volts. With a 4-ohm coil winding, the current flow in the ignition system suddenly becomes 2 amps or less- - - -not enough to provide a hot spark. The engine fails to start because of a weak spark at the plugs. To assure a hot spark under difficult cranking conditions, let's install a coil capable of firing the plugs properly at very low voltage, and then add a resistor to prevent point burning under low load/high voltage conditions. Just add a circuit to bypass the resistor while cranking. General Motors built several MILLION vehicles with this system, and they worked well. Then some self-appointed experts come along and tells us those design engineers were all wet and they wasted untold millions of dollars of GM's money on an unnecessarily complicated system. They also think the moon is made of green cheese, BTW!
Jerry