What Are the Four Terminals on an Alternator?

The four terminals commonly found on many modern automotive alternators are B+ (battery/output), S (voltage sense), IG (ignition/excite), and L (lamp/indicator). These provide the main output to the battery, allow the regulator to sense system voltage, energize the alternator when the ignition is on, and drive the charge warning light. Labels vary by manufacturer and era—some use D+, DF, R, F, A, I, or computer control pins—so function, not the letter, is what matters.

What Each Terminal Does

Although markings differ, most four-terminal alternators support the same four functions. Below is a quick guide to what each circuit typically handles and the common label variants you may see.

• B+ (BAT, B): The main output stud that sends charging current to the battery and vehicle electrical system. It’s usually the large post with a heavy-gauge cable. Some units may also have a rare B− stud, but ground is usually through the case.
• S (Sense, A): A low-current input that lets the voltage regulator “sense” system voltage—often tied to the battery or main junction—to maintain accurate charging under load and lead-drop.
• IG (IGN, I, 15): An ignition-switched feed that wakes or excites the internal regulator/field circuit when the key is on. Without this, many alternators won’t start charging.
• L (Lamp, D+, 61): Drives the charge warning lamp (battery light). It typically provides a ground path for the bulb at key-on/engine-off and goes high when charging starts, turning the light off. On some systems it also provides the initial excitation current.

Together, these terminals let the alternator produce power, monitor actual system voltage, turn on with the ignition, and inform the driver of charging status. Exact labels differ, but these four roles are widely consistent.

How Labels Vary by Manufacturer

Different makers use different letters for the same functions. Use this mapping to translate the “four terminals” concept across common alternator families.

• Bosch (classic European): B+ (output), D+ (lamp and excitation), DF (field control on externally regulated older units), and D−/ground (often the case). On many internally regulated Bosch units, D+ effectively serves the L function.
• Delco/GM (10SI/12SI, CS-series): BAT (B+ output), terminal “1” (L/indicator), terminal “2” (S/sense). Some older or heavy-duty units also have R/F for tach or field. Later CS/AD units can add a “P/L/S/F” configuration with similar roles.
• Nippondenso/Toyota: B (B+ output), S (sense), IG (ignition), L (lamp). This is the archetypal four-pin plug plus the B+ stud.
• Ford (3G/4G/6G): B+ (output), A (sense), I (ignition/indicator feed), S or STA (stator/monitor). The “I” line often interfaces with the warning lamp circuit.
• Chrysler/Mopar (various eras): B+ output and two field terminals (F1/F2) with PCM or external regulator control; the lamp is driven via the PCM. Functionally this still maps to output, sense/control, ignition/enable, and warning.
• Modern smart alternators (LIN/BSS/BSD, “C/COM”): Some late-model vehicles replace IG/S/L with a single communication line (LIN/BSD) or a “C/COM” pin. These are ECU-controlled and don’t follow the simple four-wire scheme.

If your alternator’s letters don’t match exactly, identify each pin by its function: main output, voltage sense, ignition/excite, and indicator/monitor. When in doubt, consult the service manual or the alternator’s datasheet.

How to Identify the Terminals on Your Vehicle

Because labels and connectors vary, use a methodical approach before wiring or replacing an alternator.

1. Check the service manual or the alternator manufacturer’s pinout for your exact engine and model year.
2. Look for the heavy-gauge cable on the output stud—that’s B+; it usually routes to a fuse link or battery junction block.
3. Trace the plug: a constant or battery-sensed line is S; a key-on 12 V line is IG/I; a line that goes to the cluster battery lamp is L/D+.
4. Use a multimeter: with key off/on and engine running, verify which pin is switched power (IG), which tracks battery voltage (S), and which changes state with the lamp (L).
5. Beware of smart systems: if there’s a thin single wire marked LIN/COM/C, it’s ECU-controlled and not a simple L/IG/S setup.

Correct identification prevents miswiring, which can damage the regulator, blow fuses, or cause no-charge conditions.

Wiring and Safety Notes

Charging circuits carry high current and are critical for vehicle reliability. Keep these best practices in mind when working with alternators.

• Never disconnect the battery while the engine is running; voltage spikes can destroy the regulator and ECUs.
• Ensure the B+ cable is properly fused or fusible-linked close to the source and sized for alternator output.
• Confirm whether the regulator is internal or external before applying field power; mismatches can burn components.
• Maintain solid grounds: the alternator case must have a clean, low-resistance path to engine and chassis ground.
• Do not interchange smart LIN/COM alternators with simple four-terminal units without the proper harness and ECU strategy.
• If adding accessories, consider a remote sense connection at the main distribution point for stable voltage under load.

Following these precautions helps ensure accurate charging, reduces electrical noise, and protects sensitive electronics.

Summary

Most four-terminal alternators provide: B+ (main output), S (voltage sense), IG (ignition/excite), and L (lamp/indicator). While different brands use different letters—D+, DF, A, I, R, F, and modern LIN/COM—the functions map to the same four roles: deliver power, sense system voltage, enable charging with the ignition, and report charging status. Identify by function first, label second, and verify with documentation and a multimeter for a reliable, safe installation.