Wiring an Alternator

Wiring an alternator

This diagram shows how to wire a Delco (GM) internally regulated 3-wire alternator. This particular model (10SI, used in the 1970s and early 80s) is the one you'll find on the generation of GM cars most often used in demolition derbies. It was also used in a lot of off-road equipment, so they're generally cheap and easy to find.

These instructions also apply to the 12SI series, which is similar but can be distinguished by the water-pump style cooling fan on the front (flat front instead of individual fins).

  1. This is the main power wire that goes directly to the battery. Make sure it is a heavy gauge since it has to carry the full output of the alternator (up to 100 amps).

  2. The leftmost of the two spade connectors around the perimeter of the alternator when viewed from the back (will sometimes be marked "1").

    This is the field wire that energizes the alternator; without it, the alternator won't start charging. It must be switched, or else it will drain the battery overnight. Do not wire this into the coil power wire that you use to switch the engine on and off (a seemingly elegant solution, since it would save a switch), since once the engine is running the alternator will feed power back through this wire, which will then power the coil, and you won't be able to shut your car off.

    If you wire a lightbulb into this line, it will serve as an 'idiot light' that you can put on your dash. Why do that? In a derby you can't actually hear your own car running, so if you're not using an oil pressure gauge or voltmeter, this will tell you when your car has stalled.

  3. This is the lead for the sensing wire that helps regulate the output. For derby purposes it isn't needed so it can be either left off or jumped over to the main power wire.

  4. This is the regulator bypass hole. Sticking a long thin metal object about 2 inches deep into this D-shaped hole causes the alternator to go to full charge. If you do this while it's dark out and the main power wire is disconnected, the whole alternator will emit a cool supernatural kind of glow. I don't recommend that you try this since you'll likely end up toasting your alternator.

If you're trying to troubleshoot a battery that tends to go flat in a day or two for no apparent reason, try this method of troubleshooting.