In an automotive engine, a ballast resistor is a component that allows full battery voltage to reach the ignition coil during starting, but reduces this voltage to about 50 percent once the engine is running.
When the engine is running, the ballast resistor is switched into the circuit, so the battery voltage is divided between the ignition coil and the resistor. This generates less heat within the ignition coil and produces maximum spark efficiency at high speed.
Points distributors are not designed to carry high levels of current. In the absence of a ballast resistor, the distributor breaker points would receive too much current and become pitted or burned.
The resistance of a ballast resistor, measured in ohms (Ω), can increase over time. This affects the voltage available to the spark plugs.
Automobile manufacturers typically include the resistance value of suitable ballast resistors in their service manuals; typical values range from 1.3 Ω to 1.4 Ω. If you are in any doubt about the value of a ballast resistor, you can check it by connecting an ohmmeter across the resistor.
