Alternator · excitation · voltage sense
Alternator excitation wiring: D+, IG and S terminals
Besides the thick B+ output, an alternator has one to three small terminals. They switch the regulator on, light the charge lamp and tell the regulator what voltage the battery really sees. Names differ by maker, so the alternator’s own pinout comes first.
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What the terminals do
| Terminal | Purpose | Connect to |
|---|---|---|
| B+ | Charging output | Battery + (or the main distribution point) with a thick cable, protected by a high-current fuse |
| D+ (Bosch and others) | Excitation and charge lamp: ignition → lamp → D+; it goes out when D+ rises with the engine running | Charge warning lamp |
| L (lamp) | Same role as D+ on many Japanese and US alternators | Charge warning lamp |
| IG | Switched ignition feed that wakes the regulator | Ignition +, through a fuse |
| S (sense) | Voltage the regulator aims at, measured where it matters | Battery + or the main fuse box feed |
Why the lamp matters on D+ alternators
At start-up the small current through the charge lamp excites the field. With the lamp bulb blown, or an LED that passes too little current, many D+ alternators do not start charging until revved high — or not at all. A resistor of about 50–100 Ω in parallel with an LED lamp restores the excitation current.
Why S is worth wiring
Without a sense wire the regulator measures at its own B+ terminal. With 60 A flowing, a long B+ cable can lose 0.3–0.5 V, so the battery gets 13.8 V while the regulator thinks it is giving 14.3 V. The S wire carries almost no current, so it reports the true battery voltage.
See it in WireForge
- Open the “Starting and charging” demo: the generator has B+, EXC and GND, with a charge lamp on EXC.
- Set Engine speed above 0 and simulate: the generator charges the battery and the charge lamp goes out.
- Click the B+ cable to read the charge current and the voltage lost on the cable.
- Set Engine speed back to 0: the lamp lights again — with the engine stopped, current flows from ignition through the lamp into D+ and the field.
Questions
What happens if I swap IG and S?
Often the regulator still works but aims at the wrong voltage, or not at all on some designs. Check the alternator’s pinout before power-up.
Does a one-wire alternator need any of these?
No: it self-excites once it spins fast enough and has only B+. It is simpler but has no charge lamp and no remote sensing.
What fuse goes on B+?
A high-current fuse rated above the alternator’s maximum output, sized for the cable, near the battery or distribution point.