The ballast resistor allows the ignition system to run at a lower voltage until the engine is able to start, then regulates the voltage going to the ignition system to prevent system damage. A vehicle's ballast resistor is typically connected to the ignition coil. When the starter motor is on, the current flows through the ballast resistor and the ignition coil. The ballast resistor limits the current flow to the ignition coil, which prevents the coil from overheating and being damaged.