In a typical automotive starter, the three wires are B+ (battery positive), S (start signal), and M (to the starter motor).
On most vehicles, a starter’s solenoid uses these three connections to control engine cranking: a battery feed, a small control signal from the ignition switch, and a large motor feed that engages the starter windings when cranking.
Three wires, three roles
The following breakdown clarifies each terminal and its function in the starting sequence. Note that some vehicle designs vary, so consult your service manual for exact wiring.
- B+ (Battery Positive) — A thick cable from the car’s battery feeds the solenoid’s input. When the solenoid is energized, it bridges B+ to the motor terminal, delivering battery voltage to the starter motor.
- S (Start Signal) — A small wire connected to the ignition switch or start button that energizes the solenoid coil. This is what tells the starter to engage when you attempt to start the engine.
- M (Motor) — The terminal that delivers battery power from the solenoid to the starter motor windings. When the solenoid closes, current flows through this path to crank the engine.
In short, B+ supplies power, S triggers the solenoid, and M feeds the motor. Together they make the starter function during engine start.
Practical considerations and safety
Testing and troubleshooting
When diagnosing starting problems, verify each connection: ensure the battery is charged, terminals are clean, the S wire receives 12 volts with the key in the Start position, and the M circuit shows continuity to the starter motor. If in doubt, consult a professional.
Summary
Most starters rely on three wires: B+ to feed the solenoid, S to activate the solenoid via the ignition switch, and M to deliver power from the solenoid to the starter motor. Understanding these connections can help with diagnosis, maintenance, and safe repair practices.