All about the 3 Wire Alternator

1976 Camaro three wire alternator

One of best inventions in the early 70s, is the three wire alternator. The voltage regulator is built into the alternator. This makes it very easy to wire up and it also has the added benefit that the voltage regulator is no longer a mechanical device but now semiconductor based - no adjustments!

There is a common misconception about alternators and the term one wire alternator is often thrown about. Most people mistakenly identify a 3 wire alternator as a one wire alternator (I know, I was one of those). A one wire alternator was only used for limited applications and has distinct disadvantages. The one wire alternator has no voltage sensing lead which means that it can only regulate the +12V (actually +14V) at the alternator. Due to voltage loss through the wiring, you will actually lose voltage by the time it gets to the firewall and distributor!

The 3 wire alternator is a proven design and has been used for the past 30 years starting in the late 60s, early 70s. It has an external voltage sensing lead that is normally connected to the firewall at the +12V junction under the dash. This ensures that the proper regulated voltage arrives for all the electrical components.

3 wires are actually used plus chassis ground

There are 3 wires needed to run it and the case provides the path to chassis ground Even though there are 3 wires, it is very easy to wire up. Here's a desciption of the wires:

  • Battery positive (Bat+)

    • connect to starter +12V battery wire
    • use 10 ga wire (thick)
    • Screw on terminal on the back of the alternator

  • Voltage sensing line

    • connect to the Bat+ close to the firewall
    • Senses and adjusts the +12V were it is needed
    • use 14 ga wire (thinner)
    • Pin 1 of the push in connector on the alternator

  • Ignition On input

    • When the ignition switch is turned on, power from the ignition switch turns on the alternator
    • Pin 2 of the push in connector on the alternator
    • use 18 ga wire

In the above pictures, you can see a black wire that is connected to the alternator mounting bracket. I ran an extra ground wire from the firewall directly to the alternator's mounting bracket just cause I'm paranoid about bad grounds. Normally the engine has a ground strap between it and the firewall which provides an excellent ground. I've painted all my parts and want to make it 100 percent sure. Most likely it is not needed as millions of cars work perfectly fine without it.

Between the starter and the battery is a short piece of wire about 1" long that is a smaller gauge than the main wire. It is a fusible link. The idea is that if there is a catastrophic short in the wiring harness, the fusible link will burn up before the rest of the wiring harness. It operates on the same principle as a fuse.

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Copyright July, 2011 Eugene Blanchard