Alternator Charging Circuit (B, IG, S, L) — Battery Light Explained
All CarsGeneral GuidesAll yearsPower & FusesUpdated 2026-10-10While the engine runs, the alternator makes electricity for the whole car and charges the battery. Many Japanese cars (Toyota, Suzuki, Honda and others) use the same 4-terminal layout: B, IG, S and L. Newer cars may let the engine ECU control the alternator instead.
Pinout — pin number & wire colour
Alternator terminals (common 4-terminal type)
| Pin | Name | What it does |
|---|---|---|
| B | B+ | Charging output — thick wire to battery +. |
| IG | IG | Key-ON power — wakes the regulator up. |
| S | S | Sense — reads the real battery voltage. |
| L | L | Lamp — turns the battery warning light on / off. |
Charging circuit
- B: the alternator's output goes through a main fuse to the battery +.
- IG: with the key ON, 12 V wakes up the voltage regulator.
- S: a thin wire lets the regulator read the battery voltage and keep it about 13.5–14.7 V.
- L: key ON / engine OFF the battery light is on. When the alternator charges, L goes high and the light goes off.
🧰 Tools you need
- Digital multimeter
- OBD2 scanner (ELM327 or better)
- 12 V test light
- Back-probe pins / jumper wires
Common problems & quick checks
| Problem | Likely cause | What to check |
|---|---|---|
| Battery light on while driving | Alternator not charging, broken belt, or fuse blown | Engine running: battery should read 13.5–14.7 V. Check belt and main fuse. |
| Lights get very bright, bulbs blow | Overcharging — regulator fault or S wire broken | If above 15 V, check the S wire first, then the regulator. |
| Battery light never comes on with key ON | Bulb / meter fault or L wire open | Unplug the alternator; ground the L wire — the light should come on. |
Related trouble codes
Powertrainmedium
P0562 — System Voltage Low
The computer sees battery / charging voltage below normal.
PowertrainhighP0563 — System Voltage High
The computer sees charging voltage above normal (usually over about 15–16 V).
PowertrainmediumP0620 — Generator (Alternator) Control Circuit
The engine computer has a problem with the wire it uses to control the alternator.
This is a simplified drawing for learning. Pin numbers and colours can change by model and year — check the service manual for your exact car before repairing.