How do I test a ballast resistor/wire

I haven't used an ignition system that used a ballast resistor since probably 1984 so I'm ignorant of this crap.

If I need to check a ballast resistor, do I put 12 volts to one side and check the voltage at the other side, or does it need to be checked in ohms?

Also, what about a resistor wire on a GM application? If I turn the key on and get 12 volts at the end of the wire is it a resistor wire?

I ASSume a resistor cuts back voltage. I'm lost.

yes a resister lowers voltage by turning the energy into heat.. You can use a ohm meter but you need to know the specs of it, but I don't know how to do it that way.. But , yes you can apply the cars/bat volt to one side and measure the difference coming out of the other side to see if your getting the desired lower voltage for your component.

edit, and yes current needs to be flowing through it to measure the voltage drop.

Thanks. Just so I'm clear on this...let's say I put 12 volts into one side of the resistor, I should get maybe 8-9 volts out of the other side. Is that correct? I realize we are using rough numbers, but I'm trying to get a handle on this. I have a guy saying you can get 12 volts out of both sides of a resistor and it working (same with a resistor wire). He says you have to check the ohms, because that's all you care about.

I'm just trying to get a grasp on this.

The voltage won't drop unless there's current flowing. The 12 volts applied to one side will show 12 volts on the other side until you send the other side to ground. the circuit needs to be closed to measure the voltage drop across the resistor. Measuring in ohms works but remember to isolate the ballast from the rest of the car when measuring in ohms, and keep your fingers off the leads.

The one negative about ohm's testing is it doesn't verify if the circuit, or ballast, can carry a load. You may measure a correct resistance, but when current is applied the ballast may fail. A voltage drop is the best way to test.

with the ballast unhooked if you put 12V to one end you will get 12 on the other end. the reduction does not start until it is hooked up/current starts flowing in operation. With it unhooked just ohm each end & post the number you get/what resistor you are working with/what coil etc & I'll see if I have some numbers (ohms) that apply to it & if we can improve it for your app (if a 4 terminal ballast).