What are the three wires on a Ford alternator?
In Ford's classic three-wire alternator setup, the three connections carry the essential signals for charging, indicator, and field excitation. The main battery output is the B+ terminal, the dash light uses the L terminal, and a sense/excitation lead (S or I) provides regulator voltage sensing and field current. The exact wiring can vary by year and model, so consult your vehicle’s diagrams for specifics.
How the three wires work
The following describes the typical roles these wires play in controlling charging and informing the driver.
- B+ — The main output terminal. This heavy-gauge connection carries charging current from the alternator to the battery and the vehicle’s electrical system. It is usually fed back to the battery positive through the wiring harness and a fuse or fusible link.
- L — The charge-indicator (lamp) wire. This connects to the dash warning lamp and ignition switch. When the alternator is not generating, the lamp is energized (grounded) to light up; once charging begins, the lamp goes out or dims as system voltage rises.
- S (Sense) or I (Ignition/Excitation) — The third lead provides regulator feedback and field excitation. The sense wire helps the regulator monitor battery/system voltage so the alternator maintains the correct output. The ignition/excitation lead supplies initial current to the field when the engine starts; some models label this terminal as S, others as I/IG, and the exact labeling varies by year and model.
In practice, many harnesses tie the S sense line to the battery positive so the regulator can read true system voltage. The L wire is wired into the dash lamp circuit to display charging status. If your alternator uses a newer two-terminal design, the wiring may be combined or simplified; always verify with a model-specific diagram for your vehicle.
Model-year variations and wiring checks
Variations by year
Ford’s alternator designs evolved over decades. Older three-wire units typically use B+, L, and S (or I). Some later units merge or re-label these terminals, or rely on internal sensing, so the exact wiring can differ by year, engine, and vehicle model. Always refer to the service manual or a factory wiring diagram for your specific car.
How to verify and wire correctly
To confirm which wire is which on your car, consult the exact wiring diagram for your year and engine. When diagnosing charging issues, verify that B+ is connected to the battery and that the L lamp behaves as expected: lit with ignition and off when the alternator is charging. Use a multimeter to check voltages at each terminal with the engine off and running, and avoid contact with live electrical connections.
Summary: The three wires on a Ford alternator are typically the B+ output, the L dash-indicator lead, and a sense/excitation lead (S or I) used for voltage sensing and field excitation. Labels and exact wiring can vary by model year, so always consult the vehicle-specific diagram for accurate connections.
Why does an alternator have three wires?
A three-wire alternator has an internal regulator and uses the fat B+ wire as the voltage output to the battery/fuse box, but has a separate D+ wire to the battery indicator light that acts as the source of bootstrap current, and a separate voltage-sensing wire.
What are the three terminals on an alternator?
The most common terminals include: S terminal: Senses battery voltage. IG terminal: Ignition switch that turns the voltage regulator on. L terminal: Closes the circuit to the warning lamp.
What is the color code for 3 wire connector?
In the USA, for instance, 3-wire cable color code is: Black: Live or hot wire. White: Neutral wire. Green: Ground wire.
How to check a 3 wire alternator?
The three-pin multiple plug has no earth terminal. Switch on the ignition and test the continuity of the leads one by one, by connecting them with the voltmeter to an earth. You should get a reading of battery voltage for each one; if not, there is a broken connection and the alternator cannot change the battery.
