If you’ve ever worked on automotive electrical systems or replaced an alternator, you’ve likely encountered the mysterious letter codes on the alternator plug: LI, IG, and AS. These standard terminal markings are used globally by major manufacturers including Denso (Japanese) and Bosch (European), and mastering their functions is essential for correct wiring, troubleshooting, and preventing costly electrical damage.
In this comprehensive guide, we’ll break down exactly what each terminal does, common plug configurations, and how to quickly diagnose common alternator issues using these terminals.
Core Terminal Meanings: LI, IG, and AS
These three terminals form the backbone of most modern alternator control systems, each serving a distinct and critical role in the charging process.
1. L / LI (Load / Lamp Indicator)
Primary Function: Charging warning light control and initial excitation
- Connects directly to the battery warning light on your vehicle’s dashboard
- When the alternator is not generating power, this terminal is internally grounded, completing the circuit and illuminating the warning light
- Once the alternator starts producing electricity, the voltage at this terminal rises to match the system voltage, turning the warning light off
- Critically, this terminal also provides the initial excitation current needed to kickstart the alternator’s magnetic field (without it, the alternator won’t start generating power even if the engine is running)
- Typical Voltage: 0V (warning light ON, no charging) → 12V+ (warning light OFF, normal charging)
2. IG (Ignition)
Primary Function: Ignition switch-controlled power supply
- Connects to the IG/ON position of your vehicle’s ignition switch
- Only receives 12V power when the key is turned to the ON position
- Supplies power to the alternator’s voltage regulator and rotor excitation coil, activating the charging system
- Automatically cuts power when the engine is turned off (key to OFF position), preventing parasitic battery drain
- Typical Voltage: 12V (key ON) → 0V (key OFF)
3. AS / S (Sense / Battery Sense)
Primary Function: Battery voltage feedback for precise regulation
- Connects directly to the positive terminal of the battery (B+)
- Continuously measures the actual voltage at the battery, not just at the alternator output
- Sends this real-time voltage data to the voltage regulator, which adjusts the alternator’s output accordingly to prevent overcharging or undercharging
- If this wire is broken or disconnected, the regulator will default to a lower output voltage, leading to chronic battery undercharging and premature failure
- Common Aliases: S, B Sense, Voltage Sense
Common Alternator Plug Configurations
While the LI-IG-AS trio is standard, manufacturers use slightly different plug configurations depending on the vehicle make and region.
3-Wire Plug (Most Common)
- Configuration: L + IG + S (AS)
- Used by: Denso and most Japanese manufacturers (Toyota, Honda, Nissan, Subaru)
- This is the simplest and most widely used configuration, covering the vast majority of passenger vehicles on the road today.
4-Wire Plug
- Configuration: L + IG + S + FR (Field Monitor)
- Used by: Bosch and many European manufacturers (BMW, Mercedes-Benz, Volkswagen)
- The additional FR (Field Monitor) terminal provides feedback on the alternator’s field current to the vehicle’s ECU, allowing for more advanced charging system control and diagnostics.
Quick Wiring Guide & Troubleshooting
Proper wiring of these three terminals is non-negotiable for a functional charging system. Follow these guidelines and troubleshooting steps to resolve most common issues.
Essential Wiring Rules
- L terminal: Connect to the dashboard battery warning light control wire
- IG terminal: Connect to a 12V switched power source that only activates when the ignition is in the ON position
- AS/S terminal: Connect directly to the battery positive terminal (B+). Use a short, thick wire with minimal voltage drop for the most accurate voltage sensing.
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