The three terminals you’ll typically find on a standard automotive alternator are B+, S, and IG (also labeled L on some models). B+ is the main output to the battery, S is the sense line used by the regulator, and IG/L energizes the field and drives the charging-indicator lamp.
Identifying the three terminals
Most three-terminal alternators in passenger vehicles use labels B+, S, and IG (or L). The B+ terminal is the large power post, S is a smaller sense lead, and IG or L is the ignition/indicator connection that controls field excitation and the dash charging light.
- B+ (Battery positive) – The main output that delivers charging current to the battery and the vehicle’s electrical system. It is typically the largest terminal.
- S (Sense) – Feeds back the battery voltage to the voltage regulator so the alternator can regulate its output to maintain proper system voltage.
- IG or L (Ignition/Lamp) – Connects to the ignition circuit to energize the regulator when the engine starts and to the dash charging lamp. The lamp usually lights briefly when starting and goes out once charging begins.
These three terminals work together to start charging when the engine runs, monitor system voltage, and inform the driver if charging isn’t occurring. Note that some brands or models may label the terminals slightly differently, and a few designs include additional terminals for specialized functions.
How each terminal functions in practice
B+ terminal
The B+ terminal carries the high-current output from the alternator to the battery and the car’s electrical bus. It is the primary path for charging the battery and powering accessories when the engine is running.
Sensing terminal (S)
The S terminal feeds a sample of the battery’s voltage back to the internal regulator. By comparing this sensed voltage to the regulator’s reference, the alternator adjusts its field current to hold the system voltage around the manufacturer’s target (commonly about 13.8–14.8 volts).
Ignition/Lamp terminal (IG or L)
Testing and wiring considerations
Before attempting any checks or wiring work, ensure the vehicle is parked, the engine is off, and safety precautions are followed. When testing, use a multimeter and refer to the vehicle's service manual for model-specific wiring and voltage targets.
- Identify the three terminals on the alternator (B+, S, and IG/L) and inspect for corrosion or damaged insulation.
- With the ignition on and engine off, confirm that B+ shows battery voltage and that the IG/L terminal has voltage (or the dash lamp is driven) as specified by the manual.
- Start the engine and measure the voltage at B+ relative to ground. A healthy system typically reads about 13.8–14.8 volts; significantly lower or higher readings indicate regulator or wiring problems.
- Check the dash charging lamp function: it should light with ignition on but engine not charging, and go out once the alternator starts producing voltage. If the lamp behavior is incorrect, inspect the IG/L wiring and regulator.
If readings are outside the expected ranges or there are symptoms (dim lights, battery not charging, or a constantly lit charge warning), consult the vehicle’s service manual or a qualified mechanic. Wiring colors and connector shapes can vary by manufacturer, so model-specific diagrams are essential.
Summary
In most cars, the three external alternator terminals are B+ (battery positive output), S (sense input for regulation), and IG/L (ignition and lamp feed for excitation and dash indicator). Understanding these roles helps diagnose charging problems and ensures the alternator maintains proper system voltage. For model-specific details, always refer to the manufacturer’s wiring diagram and service documentation.