Mercedes-Benz 300SL Ballast Resistor Replacement at your home or office.

What is the Ballast Resistor all about?

Modern ignition systems use electronic circuitry to control voltage during engine starting and operation, but it wasn’t always this way. Older vehicles with points and coils did not have the benefit of circuit boards and computers, so another way to control voltage was needed. The ballast resistor coil was the solution. Think of it as a giant fuse in a way, although it serves a much greater purpose.

The ballast resistor coil plays several roles in your engine and charging system. In terms of position, it’s located between the ignition switch and the positive battery cable. During engine operation, the ballast resistor is responsible for lowering voltage applied to the coil to ensure that it doesn’t overheat and burn out, or damage the battery. During cranking, the ballast resistor allows normal battery voltage into the coil to ensure a good, strong spark for starting the engine.

The ballast resistor is subjected to significant amounts of heat during operation (the resistor’s resistance level increases or decreases with the application of heat and voltage). Over time, it will eventually wear out and need to be replaced.