A ballast resistor is a component in the ignition system of older vehicles. If you drive a classic, you are familiar with coils and points. You don’t have an onboard computer, and obviously you don’t have circuit boards that can control the voltage when you start up your engine. This is where the ballast resistor comes in. It is actually something like a huge fuse that is located between the positive battery cable and the ignition switch, and it works to lower the voltage that is delivered to the coil so that it doesn’t burn out. When you crank your engine, the ballast resistor ensures that the normal battery voltage is delivered to the coil in order to start the engine.
If the original ballast resistor is still working in your classic car, then you are a very lucky driver. Because the ballast resistor takes so much heat during normal operation, it is vulnerable to damage and will eventually wear out. How often you drive could be a factor, but there is no specific “best before” date. The ballast resistor could last for many years, but it does take a lot of wear and tear, and it could fail suddenly. Your ballast receiver needs to be replaced if the engine cranks but stalls as soon as key is returned to the “run” position
If your ballast resistor fails, you will have to have it replaced. Resist the temptation to listen to well-meaning classic car fellows, who may suggest jumping the resistor. If you do, your points will ultimately burn out, and you will require expensive repairs. A professional mechanic can replace the ballast resistor and have your beloved classic running properly again.