How a Starter Circuit Works — Relay & S Wire Explained

What actually happens when you turn your key to START? In this video, I use a demonstration board to explain a basic automotive starter circuit—including the ignition switch, starter relay, and the small S wire that signals the starter solenoid. We follow the circuit from battery power through the switches and relay so you can see how the parts work together. In this lesson: • Battery power, ignition power, and grounds • How the ignition switch controls the starter relay • Relay coil terminals 85 and 86 and switched terminals 30 and 87 • What the starter’s S wire does • What’s coming next on the electrical training board CHAPTERS 0:00 What does the starter’s S wire do? 0:28 Meet the starter circuit board 1:17 Ignition power and the start button 2:09 How the starter relay works 3:21 Following the complete starting sequence 4:29 Upcoming troubleshooting lessons 5:19 Expanding the board and your feedback This is a simplified teaching setup. As explained in the video, the demonstration wires are not sized for an actual starter installation, and the starter’s main motor power is disconnected. Next, we’ll introduce faults and work through testing. I’m also planning to add more circuits to the board, including a fuel pump, blower motor, and lighting. What would you like me to explain next? Drop a comment—even if you need something broken down further. Subscribe to TrueTech Mobile Repairs with Jeremy to follow the series. #StarterCircuit #AutomotiveElectrical #StarterRelay