Im working on a 54 Chevy truck that has been switched over to 12 volt. All the reference I can find on it says it takes a ballast resistor before the breaker points. I wired a ballast resistor (one of those white resistors from the 60's) between the cold side of the coil and the breaker points. I then checked the voltage on each end. On the hot side of the coil it shows as it should 12.84 volts, On the points side it shows the same voltage (12.84v). With the resistor in place shouldn't the voltage to points side be 4 or 5 volts? What am I missing?
Or is it the amperage that get reduced?
Your voltage should not change the resistor helps to reduce the arcing of the points when they open causing the field to collapse. the truck will run without the ballast resistor but your points won't last as long.
I believe that your points were open when you measured the voltage on the point side of the resistor. The resistor should be about 3 ohms and you are correct, about 4 or 5 volts is right.
Sorry G.R., what you're thinking of is the condenser, it reduces the arcing of the points.
The ballast resistor won't drop the voltage until there is current flowing through the primary circuit, see the attached video link.
In addition, most cars with ballast resistors have a bypass circuit that routes full voltage to the coil when the key is in "start". When the key is released back to the "run" position, current to the coil goes through the ballast resistor and the voltage to the coil is dropped.
Mark, Am I correct that I put the ballast between the coil and the points or should it be on the hot side of the coil and the cold side goes direct to the points?
Yes, put the ballast resistor in the circuit between the ignition switch and the input side of the coil.
By the way, here is a quick tip about which side is the "output" side of the coil.
The output side of the coil (the side with the wire that goes to the distributor) should match the ground polarity of your car.
For instance, if you have a negative ground car, the wire to the distributor should be connected to the negative side of the coil. The wire from the ballast resistor would then go to the positive side of the coil.
If you have a positive ground car, the wire to the distributor should be connected to the positive side of the coil. The wire from the ballast resistor would then go to the negative side of the coil.
If you wire the coil backwards, the plugs will still fire, but the spark at the spark plug will be going "the wrong way", from the ground strap to the center electrode. This will require more voltage for a successful spark than if the secondary current goes from the center electrode of the spark plug to the ground strap of the spark plug. Why? Because the center electrode runs hotter than the ground strap, so it is easier for the spark to jump from the hot center electrode to the cooler ground strap.
On the chevies and mopars that I've had the resistor was between the ignition switch and the coil, Don.
Kinda' wonder what might happen if you were to "accidentally" and "unknowingly" install a new coil that has a built-in ballast resistor, on a car that already has an "external" ballast resistor on the firewall,.....??? Maybe a pretty weak spark, huh?
How long ago was the switch made? Did you check, (if you didn't make the change yourself), if the existing coil had an internal resistor? I ask because if it's been running like that for a while with no external resistor the coil probably already has one. Unless you're replacing points every week.
I just picked up this little truck. Its a 1940 with a 54 engine. It already fried the points which is why the change. Its a nice little truck, It stills needs some TLC but Im getting closer. Mark, I went and bought a internal resistor coil for it. The engine bay just didn't look right with that ballast in there. Yesterday it gave me a OH SH%@ moment. I pulled up to an intersection and when I hit the brakes the pedal went to the floor. Fortunley the hand brake did the job. The wheel cylinder blew out in the rear. So now I'm waiting on parts. Going to rebuild the whole brake system so the next time I take it down the road I'm sure it will stop! Oh, Side note, The set of points I bought for it from Auto Zone will only close to 23th. Tomorrow I will pick up a set from NAPA and they should adjust to 19 like the book says. Thank you everyone for your help.
Oh by the way;
If you have not been there yet.