“Where do I put the third wire on my one-wire alternator?!”

Certainly a valid question, and a common source of charging system confusion. Why? Because many one-wire alternators look like this:

By contrast, a General Motors SI series alternator looks like this:

…….see what we did there?

The confusion is understandable, but knowing what makes the two different can clear things up. The SI series alternator is a factory GM part found in various vehicles from 1969 to 1985. It’s available in a variety of amperage ratings, and has become a common replacement for the externally regulated item found in earlier vehicles.

SI alternators use a three wire hookup, with the large battery stud, a brown exciter wire and smaller red voltage sense wire. The brown wire is what would typically be tied to a charge warning lamp, and is responsible for exciting the charging system.

One of the largest contributors to the confusion is the fact that some one-wire alternators are built using an SI series case. This means it could look like an SI alternator, but be modified internally to self-excite at a specific engine RPM. In that case, the additional connector and wires used on the traditional SI are unnecessary.

Our Classic Update kits (and Factory Fit parts modified to work with an internally regulated alternator) work with either style, but it’s crucial to make sure you’re aware what is bolted to the engine before making any connections.

Happy charging!