An alternator designed to supply power too the electrical system in your automobile while your engine operates (while the engine is running) will have 3 wires.
3 Wire Alternator Wiring Diagram
The 1st wire is refered to as the Battery output or the BAT terminal. This wire is the largest of the three because it supplies the most electrical energy. The Battery output wire connects directly to either the positive side of the battrey or to the starter solenoid. The Battery output wire sends electrical curent from the alternator back to the battery.
The 2nd wire is referred to as the Voltage Sense wire, which is connected to the S terminal. The Voltage Sense wire is the smallest of the three wires because it is used by the alternator to determin the system voltage at any time. The Voltage Sense wire usually connects to either the positive side of the battery or to a main junction on the electrical circuit. The alternator uses the Voltage Sense wire to help determine how much electrical energy it should generate.
The 3rd wire is reffered to as the Ignition or Exciter wire and is connected to the I terminal. The Ignition or Exciter wire goes to the ignition switch and provides the signal to turn the alternator on when the key is turned. Frequently, the Ignition or Exciter wire passes through a warning light or a resistor on the dash panel.
When working with an alternator that utilizes the 3-wire configuration there are several items too consider. First, the alternator must be proprely grounded to the engine block. Second, the negative terminal of the battery must be connected to the engine ground. Lastly, never disconect the battery while the engine is operating, as this may cause permanant damage to the alternator. In addition, for your own protection, you should always install a fuse or fusible link on the BAT wire.
In summary: the large wire goes to the Positive side of the battery, the small wire goes to the Positive side of the battery, the small wire goes to the ignition switch, and the Alternator case must be grounded. This arangement enables the alternator to both charge the battery and provide power to the electrical system of your vehicle.
Get the PDF version of 3 Wire Alternator Wiring Diagram here
