The S terminal is the Start input and the I terminal is the ignition/input side that together energize the solenoid coil to engage the starter. In typical wiring, applying 12V across S and I activates the coil, which then bridges the heavy power path from the battery to the starter motor.
Wiring conventions can vary by make and model, so it helps to understand the general roles of S and I and then consult the vehicle’s manual or solenoid diagram for specifics. Below is a practical breakdown of how the two terminals function and how they’re commonly wired.
The S terminal: Start signal
The S terminal is the primary control input that carries the start signal from the ignition switch. When you turn the key to the Start position, voltage is applied to S, energizing the solenoid coil and pulling the plunger to connect the battery to the starter motor.
Before delving into details, here is a quick outline of how this terminal is used in typical starter circuits.
- Function: Delivers the start signal to energize the solenoid coil.
- Source: Usually wired to the ignition switch's Start position or to a dedicated Start relay.
- Current path: 12V is applied to S; the coil current returns via the I terminal or an internal connection, depending on the design.
Conclusion: S is the key-start input you energize to trigger the solenoid during cranking.
The I terminal: Ignition/input side
The I terminal provides the other side of the coil circuit or serves as a secondary feed in some designs. In many configurations, I completes the coil circuit by linking to ground or to another part of the ignition/start circuit; in others, it may be tied to battery positive or used by a separate control relay. The exact wiring can vary by manufacturer and model.
Here’s a concise look at how the I terminal typically fits into the circuit.
- Function: Completes the coil circuit or acts as a secondary ignition feed depending on the design.
- Source: Usually connected to a relay, the battery, or the ignition switch, as dictated by the system.
- Current path: Voltage across S and I energizes the coil; the heavy-duty path from B to M is controlled separately to drive the starter motor.
Conclusion: I is the companion to S for energizing the coil; its exact role depends on the specific solenoid design, and some units may use I differently or not at all.
Practical takeaway
For troubleshooting no-crank conditions, verify that the ignition switch supplies voltage to the S terminal in Start, inspect related wiring for corrosion or loose connections, and check the I terminal wiring according to the solenoid’s diagram. Remember that the B terminal feeds the battery to the starter via the M terminal, while S and I control the solenoid coil itself.
Summary
S and I are the two low-current control terminals on many starter solenoids. S stands for the Start (signal) input, while I represents the ignition/input side that completes the coil circuit. The exact wiring can vary by model, so always consult the specific wiring diagram for your vehicle or equipment to wire them correctly.