How do I test a ballast resistor? The one in the car reads 12v on both sides. The extra used one I have reads 0 ohms resistance. With it being a "resistor" logic dictates these are bad. Am I right? What are the correct readings?

No way to tell from what you stated.

Disconnect both leads so it is not in the circuit and use a multimeter on each blade.
Depending on the resistor type, it could be as little as 1.5 ohms on up to 4 ohms.

If your using a Pertronix system, get their 3 ohm coil and eliminate the resistor. I did that a few years ago. One of the best mods. I have done.

Look at my set up by clicking on the red link.

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Aggressive Ted

http://cid-32f1e50ddb40a03c.photos.live ... %20Swinger


74 Swinger, 9.5 comp 254/.435 lift cam, 904, ram air, electric fans, 2.5" HP2 & FM70 ex, 1920 Holley#56jet, 2.76 8 3/4 Sure-Grip, 26" tires, 25+MPG

Most ballast resistors change their resistance with temperature. That is why they are mounted in a ceramic block. When cold they have very little resistance. Once the ignition has been pulling power thu them for a while they are hot and their resistance has changed considerably. This is so that the coil sees a full 12V during starting, but gradually it decreases down to the ~9 volts recommended for long coil life.

I have seen one that when removed from the block was mechanically connected, but electrically non-conductive. Still don't understand that one!

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Thom

Cross-threaded is tighter than Lock-tite

A resistor that has the property of increasing in resistance as current flowing through it increases, and decreasing in resistance as current decreases. Therefore the ballast resistor tends to maintain a constant current flowing through it, despite variations in applied voltage or changes in the rest of the circuit.
The ballast action is obtained by using resistive material that increases in resistance as temperature increases. Any increase in current then causes an increase in temperature, which results in an increase in resistance and reduces the current.

prototype pcb assembly

If you are running electronic ignition one of the best things you can do for the car is bypass the ballast resistor and use an ignition module with active current control like the GM HEI. Less series resistance allows the coil to charge faster and the active current control keeps the coil from overheating.

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Joshua

Short form is that you need to check voltages at both terminals of the ballast while the engine is running- that gives you an accurate result of what it's doing.

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63 Valiant Wagon
225 - 4 bbl