They are B+, L (or D+), S, and I (also labeled IG on some models). B+ is the main battery output; L/D+ links to the charge-indicator lamp and provides initial excitation; S is the sense terminal that helps regulate the charging voltage; and I/IG supplies ignition-switched power to energize the field. These four connections enable the alternator to charge the battery and power the vehicle’s electrical system.
In most modern cars with an internal regulator, these four terminals connect the alternator to the battery, the dash warning lamp, and the regulator’s sensing and excitation circuits. Labels can vary by manufacturer, but the fundamental roles—output, lamp, sense, and ignition/field—remain consistent.
Terminal-by-terminal overview
The following explanations apply to the four common external terminals found on many automotive alternators with internal regulators. Terminology can vary between brands, but the functions are consistent across designs.
- B+ (Battery positive): The main output to the battery and electrical system. It carries the charging current from the alternator to the battery and is typically the thickest wire connection.
- L or D+ (Lamp or Indicator/Charge Lamp): Connects to the dash warning light and provides the initial excitation path. When the engine is off, the lamp is lit by the ignition switch; once the alternator starts producing voltage, this terminal helps regulate and the lamp usually goes off.
- S (Sense): Feeds a voltage signal to the regulator so it can maintain proper charging voltage. It typically senses the battery voltage and helps adjust the field current accordingly.
- I or IG (Ignition or Field): Supplies ignition-switched power to energize the rotor’s field when the engine starts. It provides the excitation current needed to begin charging and is controlled by the regulator.
Note: Some models combine lamp and sense functions on the same terminal (for example, D+ may serve as both the lamp feed and regulator sense). Always check your vehicle’s service manual or a wiring diagram for the exact labeling on your specific alternator.
Model variability and labeling
Not every alternator uses the same labels. Some common variations include L and D+ being the same terminal, or the sense wire being internally connected to the battery side of the circuit. In some designs you might see a dedicated R or E terminal for remote sensing or auxiliary control. The four-terminal configuration described here—B+, L/D+, S, and I/IG—covers the majority of four-terminal alternators in contemporary vehicles.
For diagnostics or replacement, it’s best to confirm the exact terminals with the vehicle’s wiring diagram or the alternator manufacturer’s documentation. Misconnecting any terminal can prevent charging or affect dashboard indicators.
Testing and diagnosing basic four-terminal connections
Before performing any tests, ensure the engine is off and the battery is disconnected if you’re inspecting wiring directly. Use a multimeter to verify voltages and continuity across the terminals. Here are general pointers to confirm proper operation.
Step-by-step checks help you verify that each terminal is functioning as intended and that the regulator is regulating the system voltage around 13.8–14.4 volts when the engine is running.
- B+ terminal: With the engine running, measure voltage between B+ and ground. You should see the system voltage (usually around 13.8–14.4 V). If it’s significantly lower, there may be a wiring issue or a failing alternator.
- L terminal: With the ignition on and engine off, the dash lamp should be illuminated if the alternator is not yet generating power. Once the engine starts, the lamp should go out as the regulator begins charging; a continuously lit lamp can indicate a fault in the alternator or regulator wiring.
- S terminal: With the engine running, test that the S terminal voltage tracks battery voltage (near B+). If S reads a noticeably different voltage, the regulator’s sensing circuit may be malfunctioning.
- I or IG terminal: This should receive ignition-switched power when the key is in the ON position. If there’s no voltage on I/IG, the excitation circuit may not engage, preventing charging.
If you encounter irregular readings, consult the vehicle’s service manual for the specific wiring diagram and consider testing the alternator under load or having it professionally diagnosed, as failures can stem from regulator faults, worn brushes, or diode issues within the alternator.
Summary
Most four-terminal alternators use B+ for main output, L/D+ for the indicator lamp and initial excitation, S for sensing the battery voltage, and I/IG for ignition-fed field excitation. While labels can vary by manufacturer, understanding these core roles helps you diagnose charging problems and interpret wiring diagrams. Proper testing typically confirms that the output voltage sits in the normal range when the engine is running and that the lamp and sense circuits perform as expected.