DIY

How to Wire a Starter (With Example Diagrams)

Reading Time: 4 minutes

The starter motor cranks the engine to get it going—you know that much. But do you truly understand how the starter and the rest of the starting system work?

If you’re like most drivers, the answer is no. To get you up to speed, we’re going to cover the basics of the starting system operation. We’re also going to include some handy diagrams along the way.

How a Starter Works

starter motor diagram with labeled parts
A typical direct-drive starter motor diagram.

Nearly all internal combustion-powered vehicles (with the exception of some hybrids) use a starter motor to crank the engine. The starter, which operates with the help of a solenoid, can generate a significant amount of horsepower for a limited time. On most vehicles, the solenoid is mounted on top of the starter.

There are two common types of starters—direct-drive and gear-reduction—used in automotive applications. Both designs operate in a similar manner:

  • When the driver turns the ignition key to the “start” position, the solenoid engages a plunger, which, in turn, acts on a lever fork inside of the starter.
  • The fork then pushes the starter’s pinion gear into mesh with the ring gear on the engine’s flywheel or flexplate.
  • Also, the solenoid’s plunger pushes a disc against a set of contacts. Once the contacts are connected, current flows from the battery to the starter.
  • Current then passes through the starter’s insulated brushes, which ride on the commutator portion of the armature, before entering the field coils and the armature windings.
  • The current flowing through the field coils and armature creates a magnetic field that causes the armature to spin.
  • The commutator continuously switches the polarity of the circuit to keep the armature spinning in the same direction.
  • If the starter has a direct drive, the spinning armature turns the pinion gear directly. But if the starter has a gear drive, the armature drives a set of gears that turn the pinion gear.
  • In either scenario, the starter’s pinion gear rotates the engine’s flywheel. Because the flywheel is bolted to the engine’s crankshaft, the internal engine components (i.e., pistons, camshaft, etc.) are set into motion.

Once an air-fuel mixture and spark are added to the equation, the engine begins to run. The driver can then move the ignition key to the “run” position to disengage the starter motor. If the driver fails to do so, the starter has a one-way clutch that will allow it to freewheel. That way, the starter doesn’t get damaged by the spinning engine.

The video below demonstrates how a direct-drive starter operates:

How a Typical Starting Circuit Works

a typical starter solenoid circuit
An example of a typical, non-computer controlled starter wiring diagram.

Now you know how a starter motor works. But what about the rest of the starting system?

Starting circuit operation is fairly straightforward. When the driver turns the key to the “start” position in a typical starting system, battery voltage flows from the ignition switch to an underhood relay.

As long as the neutral safety switch is in the park position (or the clutch safety switch is closed), the relay closes, allowing voltage to flow to the starter solenoid. The solenoid then engages the starter to crank the engine.

modern starter solenoid wiring diagram
An example of a modern computer-controlled starter solenoid wiring diagram.

It’s worth noting that, on many modern vehicles, the ignition switch and neutral safety switch (or clutch safety switch) are not wired directly into the starter circuit. Instead, these switches act as inputs to control modules—usually the body control module (BCM) and the transmission control module (TCM).

The BCM and TCM communicate the position of the switches to the engine control module (ECM) over a data network. If the conditions are correct, the ECM then operates the starter relay to supply power to the starter solenoid.

What are the Different Starter Solenoid Terminals?

starter solenoid terminals with labels
Starter solenoid terminals: B = battery terminal, S = start terminal, M = motor terminal

Many DIYers wonder: What wires go to the starter solenoid? Typically, a starter-mounted solenoid has three terminals with three connections:

  • The “B” or “battery” terminal: The terminal that connects the solenoid directly to the positive battery cable.
  • The “S” or “start” terminal: The terminal that receives power from the ignition switch.
  • The “M” or “motor” terminal: The terminal that attaches to the cable that connects to the starter motor.

When the driver turns the ignition key to the “start” position, voltage is sent to the starter’s “S” terminal. That voltage energizes the solenoid’s electromagnetic windings.

The windings then create a magnetic field that pulls the starter’s plunger, forcing it to push a disc against a set of contacts. Those contacts attach to the solenoid’s “B” and “M” terminals. Once the two terminals have an electrical connection, current can travel directly from the battery to the starter motor to crank the engine.

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