Are ballast resistors either good or bad? If I have one out of the car, can I just put a continuity meter on it and determine if it's good or bad? Lastly, if the resistor is in the car and the ignition is switched on, I've heard from one terminal to ground should read 12v, the other terminal to ground should read 5.6v. Correct?
The ballast resistor is part of a feed-back system the keeps the coil running at the correct voltage for a given engine rpm. At idle the coil has plenty of time to saturate and there is a lot of current running through the resistor and the coil. When this occurs, the resistor heats up. This causes the resistors own resistance to increase limiting the current available to the coil. This keeps the coil/points/ignition module from overheating. If you measure the voltage on the resistor of a running engine you can see this working, the voltage will go up as rpm increases because the resistor is cooling down. It will heat back up when the rpm drops.
For starting this whole system is bypassed, full battery voltage is applied to the coil to get the engine started. This is partially because when you are starting the battery voltage can drop below 10v.
Back in the 70's I tried making my own low cost hot rod ignition improvement on my Charger. Had a toggle switch on the dash that shorted across the ballast to give the coil full voltage. I would flip the switch just prior to a race. Probably did more harm than good, but it seemed like a good idea when I was 17.
