You turn your ignition key and the engine fires up. It’s as simple as that, right? Actually, it’s quite a bit more complicated than it seems. Massive amounts of energy are required to turn your engine’s heavy parts, getting it to run on its own. That’s the role your engine’s starter performs.

A Powerful Electric Motor

It’s the component in your car that draws the most battery power. Your car’s starter is an electric motor that packs a powerful punch. It needs to create enough torque to rotate the engine on its own from a completely dead stop. Here’s how the electricity flows, from the moment you turn the key:

Ignition Powers the Starter Relay

When you turn the ignition key to the ‘start’ position, the starter relay switches to power up the starter circuit. In many electrical systems, the switch is in line with the part it’s powering up. With the starter, though, there’s too much draw for that. The ignition switch wires are much too thin and would overheat and burn up if it was designed in that way. A relay is used so smaller wires can be used. It’s much safer.

Starter Relay Sends Power to the Starter Solenoid

When the starter relay is switched by the ignition circuit, it powers up the starter solenoid. A trigger wire initiates the process where the starter begins its job. It’s a much thinner gauge wire than the starter wire itself, and that’s because it doesn’t need to carry the amperage load the starter will draw.

Starter Solenoid Engages the Starter

When the starter circuit is powered up, the starter solenoid kicks into action. It’s a massive switch itself, ensuring the starter engages quickly and immediately without sparking or dragging.

The starter wire is connected directly to the battery’s positive terminal, although it’s not powered up until the solenoid trigger wire is powered. The starter wire is extremely thick gauge wire to allow for the most power possible to reach the starter. Otherwise, it’s like drinking a milkshake through a coffee stir stick – there’s simply not enough coming through at one time.

  • A plunger inside the solenoid pulls back, moving an actuator fork on the starter motor.
  • When the plunger is pulled back, the electrical contacts for the starter are bridged and the starter receives power.
  • At the same time, the actuator fork pushes the starter gear outward. This motion engages the starter gear with the engine’s flywheel or flexplate.
  • The starter receives continuous and uninterrupted power from the battery until the ignition key is released.

How the Starter Operates

As mentioned, your engine’s starter is an electric motor. It uses an electromagnetic field to create enormous amounts of force, spinning the starter at high speeds and with huge torque.

  • The electrical power passing from the solenoid to the starter starts at the brushes and induces a magnetic field.
  • The magnetic field begins rotation in the starter, and the armature carries on this rotation through the starter assembly.
  • The commutator switches the electricity’s flow every half rotation, keeping the armature rotating constantly in the same direction.
  • The armature has a shaft attached which extends out when the starter is powered up.
  • When the starter is depowered, the solenoid releases the plunger, the actuator fork returns, and the starter gear disengages all in one swift motion.


Starter Gear Reduction

Starters aren’t a one-size-fits-all situation. Each vehicle uses a starter that is appropriate in size to start the engine in the vehicle. Requirements vary greatly based on engine displacement and rotating mass, compression, and engine timing, as well as other considerations.

For a starter to work properly, the right gear reduction must be selected. There are a few different ways this is accomplished:

  • A starter motor can be equipped with a gear with more or fewer teeth. The fewer teeth on the starter motor gear, the faster it will turn the flywheel. An engine has high compression or heavy internal components, or if the flywheel is small, more teeth on the starter can be beneficial.
  • The starter motor can have gear reduction. Between the starter solenoid and the starter armature shaft, another gear engages which reduces the starter’s gear ratio. By adjusting the number of teeth on the internal gear reduction, a starter can be tailored to work at its best.
  • A planetary gear set in the starter can act as a gear reduction. The planetary gear can change the starter’s gear ratio, reducing or increasing the ratio for the best operation.

Another option for car manufacturers is to use a flywheel or flexplate with a different number of teeth. Gear reduction on a starter is a much simpler process for car manufacturers than using a different flywheel; it often doesn’t change the powertrain structure at all, where a flywheel update would require major modifications.