What are the S and L terminals on an alternator?

The S terminal is the “Sense” input that lets the voltage regulator monitor system/battery voltage, while the L terminal is the “Lamp” control that drives the charge warning light and often provides initial field excitation. In practice, S should see battery positive (ideally at the main distribution point via a fuse), and L connects to the charge indicator circuit so the lamp is on with the key and turns off when the alternator is charging.

How S and L fit into modern alternator/regulator systems

Most internally regulated alternators (common on Denso, Nippondenso, Delco/GM SI and CS families, and many others) use labeled regulator pins such as S (Sense), L (Lamp), IG (Ignition), FR/F (Field monitor), and sometimes P (Phase/tach). Among these, S and L are the two most universal. Understanding them helps with diagnosis, swaps, and wiring repairs, especially when retrofitting an alternator into a different vehicle or dealing with a cluster that has LED lamps.

Terminal S: “Sense” (system voltage monitor)

The S terminal feeds the regulator with a reference of actual system voltage. The regulator uses this reading to adjust the rotor field current and maintain a stable charging voltage across varying loads and engine speeds. Connecting S to a point that accurately reflects vehicle voltage improves regulation and reduces under/overcharging caused by wiring drops.

Terminal L: “Lamp” (charge warning lamp and excitation)

The L terminal interfaces with the charge warning lamp circuit in the instrument cluster. With the ignition on and engine off, the regulator typically provides a ground path at L, illuminating the lamp to show the system isn’t charging. Once the alternator produces output, the regulator removes that ground (or drives L high, depending on design), turning the lamp off. On many systems, current through the lamp (or a parallel resistor) also supplies initial field excitation; without it, some alternators may not start charging at idle.

Typical wiring practices for S and L

The following points outline common, reliable ways to wire the S and L terminals on most internally regulated alternators. Always consult the specific alternator and vehicle service data, as pinouts and behaviors can vary by make and model.

• S (Sense): Connect to battery positive at the main power distribution/junction block via a dedicated fuse or fusible link (commonly 5–10 A). This placement compensates for harness voltage drop better than tying S directly to the alternator output stud.
• S (alternate): If simplicity is paramount, connecting S to the alternator’s B+ output stud is acceptable and widely used by OEMs, though it may be slightly less accurate under heavy loads.
• L (Lamp): Connect to the charge warning lamp circuit that receives ignition-switched 12 V on one side of the bulb; the other side goes to L. The regulator grounds L with key on/engine off (lamp on) and opens it when charging (lamp off).
• L with LED clusters: If the cluster uses an LED (or no bulb at all), add a resistor (commonly 330–560 Ω, 1/2 W) in parallel with or in place of the lamp to ensure enough excitation current. A popular choice is ~470 Ω, 1/2 W.
• Fusing: Protect any added S lead with an inline fuse; keep L wiring in the protected, ignition-switched circuit.
• Grounds: Ensure the alternator case has a solid ground to the engine and the engine to the chassis/battery. Poor grounds cause false lamp behavior and unstable voltage.

Following these practices provides correct regulation, reliable lamp behavior, and consistent charging performance across vehicle operating conditions.

What changes by brand or generation

While “S” and “L” are broadly consistent, labels and behaviors can vary. For example, Denso/Daihatsu/Toyota often add IG (Ignition) and FR (Field feedback) pins; GM SI alternators align “1” with L and “2” with S; later GM CS/AD units still use S and L but may integrate ECM communication. The functional idea remains: S monitors system voltage, L signals lamp status and excitation. Always verify your alternator’s specific pinout.

Symptoms of miswired or faulty S/L circuits

Issues with these terminals commonly present as warning light or charging irregularities. Watch for the following:

• Charge lamp stays on dimly or flickers: Poor ground, weak diode, or L circuit leakage.
• Charge lamp never comes on with key-on/engine-off: Burned-out bulb, open L wire, no excitation (may prevent charging on some units).
• Overcharging or undercharging: S not connected, connected to wrong point, or poor S fuse/link; regulator can’t sense true system voltage.
• No charge until revved: Lack of lamp/resistor on L, so the alternator doesn’t self-excite at low RPM.
• Blown fuses when key on: L shorted to ground or S shorted to ground; misrouted wiring.

If you see any of these, inspect the S lead for correct, fused battery sense and verify that the L circuit has a working bulb or resistor path from ignition power.

Basic checks with a multimeter

You can perform quick tests to confirm S and L are behaving properly before replacing the alternator.

1. Key off: Measure continuity of the charge lamp bulb/LED path; replace the bulb if open (on many cars, the lamp doubles as an excitation path).
2. Key on, engine off: At the alternator plug, L should be near 0 V (regulator grounding the lamp) and the dash lamp should be on. S should read near battery voltage.
3. Engine running: Measure battery voltage; expect roughly 13.8–14.7 V depending on temperature/load. The charge lamp should be off.
4. Voltage drop check: With heavy electrical load, compare voltage at the S tap versus battery posts. Large differences (>0.3–0.4 V) suggest harness or ground issues.
5. If LED cluster: Confirm a parallel resistor is present and measures the expected ohms; add one if the alternator won’t excite reliably.

These tests help isolate whether the regulator is receiving the right signals and whether the wiring and indicator circuit are functioning as intended.

Key cautions

Miswiring can damage the regulator. Never connect S to ground; do not feed L directly with battery power without the proper lamp/resistor path; and avoid running S unfused. When swapping alternators across platforms, confirm the pin functions on the specific unit—same letters do not always guarantee identical behavior across all makes and years.

Summary

The S terminal is the alternator regulator’s voltage Sense input, ideally tied (via a fuse) to a point that reflects actual system voltage. The L terminal is the Lamp control/excitation line that grounds the charge indicator with key on/engine off and releases it once charging begins; it may also supply initial field current. Properly wiring S and L ensures accurate voltage regulation, correct warning lamp behavior, and reliable charging performance.

What is S on an alternator?

S – Voltage regulator pin that senses the battery voltage. L – ‘No charge’ control signal, from the voltage regulator to the ECU, to turn on the warning light. FR – ‘Field regulation’ voltage signal, from the voltage regulator to the ECU, when the alternator is charging.

What are the L and S terminals on an alternator?

The most common terminals include: S terminal: Senses battery voltage. IG terminal: Ignition switch that turns the voltage regulator on. L terminal: Closes the circuit to the warning lamp.

What are the two terminals on an alternator?

Field this pin supplies voltage to excite the alternator. And start its operation. This is the key difference between the two pin alternator. And the single pin alternator.

What is the L wire on the alternator?

R is reference voltage L is battery light That alternator is grounded through the alternator body.