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.

Keep in mind:

• The ballast resistor is not inspected during normal maintenance.
• The ballast resistor is subject to significant amounts of wear and tear.
• Ballast resistors are not generally found on fuel-injected engines.

How it's done:

• The Ballast Resistor is verified that it needs to be replaced. The battery and battery tray are removed.
• The defective Ballast Resistor disconnected and removed.
• The new Ballast Resistor is installed, secured, and connected.
• The battery tray and battery are reinstalled. The Ballast Resistor is tested for operation.
• The vehicle is tested to ensure proper operation of the Ballast Resistor and road tested to ensure fan operation.

Our recommendation:

Over time and through normal use, the ballast resistor is subject to a great deal of wear and tear, and heat. This can cause it to fail unexpectedly. When this happens, your vehicle will experience unwanted, and perhaps dangerous, operation. It may crank but then stop running immediately after. Have any starting problems immediately diagnosed by one of our expert mechanics.

What common symptoms indicate you may need to replace the Ballast Resistor?

• Engine cranks, but dies immediately when the key is moved back to the run position from the start position

How important is this service?

Without a ballast resistor, your engine will not operate unless you jump the resistor, which is dangerous, as it will eventually burn out the points, and is not recommended. If your engine dies immediately after turning the key back to run from start, likely the ballast resistor needs to be replaced. If the engine runs with the key in the run position, you may have another issue. Have it properly diagnosed by one of our expert mechanics immediately.