Marine Solenoids

Solenoids are electrically operated switches consisting of a coil of wire around a metal plunger. When electricity flows through the coil it becomes a magnet, and pulls the metal plunger into itself. This pulling action has all sorts of purposes on an engine: it can. for instance, be used to shut off the fuel supply, allowing the engine to be stopped from the dashboard, or to operate the heavy duty switch that allows current to flow between the battery and starter motor.

On most small petrol engines, that is all the starter solenoids do, just allow current to flow to the motor. The physical connection between the motor and the engine is achieved by an arrangement known as a bendix, made up of a cog (called a pinion) that can slide along a spiral groove machined into the motor's shaft. While the motor is stopped, a spring holds the pinion down towards the starter body by a spring. As the starter starts to turn, the pinion takes time to catch up so for a fraction of a second it is turning more slowly than the shaft on which it is mounted. As a result, the spiral groove in the shaft screws its way through the pinion, forcing it away from the starter body to engage with a circle of matching teeth (called the ring gear) on the engine's flywheel. As soon as the engine starts, the ring gear drives the pinion faster than the motor is turning, so the opposite happens: the pinion screws its way back down the starter shaft to disengage itself from the ring gear.

Diesel engines generally have a more rugged set-up called a pre-engaged starter. Here, solenoids do two jobs. The first part of their travel is used to push the starter pinion into engagement with the ring gear: only when the pinion is engaged can the plunger complete its movement to allow current to flow to the motor.

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