Next in our wiring series, it is time to wire the starter motor. There are a few important things that need to be considered when you do this. In this post, we are just talking about connecting the POWERCELL output to the starter solenoid. In later posts, we will go through the details for wiring the ignition and starter switch.
There is a dedicated output on the front POWERCELL for the starter solenoid. In most kits it is the light-blue wire, which is output 4 on the B connector. When you turn on the MASTERCELL input for the starter, you get power on this starter wire. This energizes the starter solenoid coil.
There are two main types of starter motors. Some have the starter solenoid built into them. Others rely on an external starter solenoid. It takes hundreds of amperes to crank an engine. Your battery is the source of this current. It takes a special solenoid to repeatedly switch this high current. A normal relay can’t carry this amount of current. You need a solenoid. This picture shows you the details of a typical starter motor with a built-in solenoid.
The starter solenoid gets a direct connection from the battery. This will supply the starting current when you turn on the starter. There is also a contact on the starter solenoid that connects to the starter output on the POWERCELL. When you turn the key to the start position, battery voltage is applied to this start terminal. This voltage does two things. The solenoid pulls in which pushes the Bendix out to engage with the engine flywheel. It also closes a large set of high-current contacts within the solenoid that lets current flow from the battery to the starter motor. This current spins the motor, which turns the flywheel.
When you release the key from the start position, the battery voltage is removed from the start terminal on the solenoid. This opens the contacts providing current to the motor, so the motor stops spinning. Also, the Bendix retracts from the flywheel.
This picture shows a typical wiring diagram for a starter motor with a built-in solenoid.
The B terminal connects directly to the battery. We’ll discuss this in more detail below. The M terminal is internally connected to the windings on the starter motor. The S terminal is what connects to the starter output on the POWERCELL. When there is battery voltage from the POWERCELL on the S terminal, the solenoid engages. The Bendix extends into the flywheel and the B terminal gets connected to the M terminal. This provides the current to turn the starter motor. The starter motor gets grounded to the chassis through its mounting plate to the engine block.
There are other starter motors that do not have the built-in solenoid. They use a different mechanism to extend the pinion into the flywheel. They rely on an external solenoid to switch the current to the starter motor. This picture shows an example of an external solenoid.