The Stovebolt Site Forums The Shop Area The Electrical Bay Wiring the 3 wire alternator

I'm not the expert on this stuff- I read all the websites I could find, talked to some people and then posted to see if what I came up with is correct. Here's what I understand:

The three wire alternator (delco 10si or 12si?) has two spade terminals and a battery terminal-

The battery terminal charges the battery. Usually a line runs from this terminal on the alternator to the starter and from the battery to the starter.

The two spade terminals are labled 1 and 2. The number one terminal controls how much charger the alternator produces by sensing the voltage in the system. This line is the thicker of the two when looking at the 'da' plug that fits the two spade terminals. If you use a junction box it is best to run the sensing line to the junction box. If you don't use a juncttion box you can just run a line from terminal one to the battery terminal and that'll be enough.

Terminal two is the 'exciter' terminal- It actually turns the alternator on by 'exciting' the electrical field. You must have some juice to this. Normally this is connected to the ignition switch so that when the switch is in the on position juice gets to the alternator and when the belt turns it produces electicity.

The coil also gets juice from the ignition switch. Many times the Coil Wire and the Alternator Exciter wire are connected to the same terminal on the ignition switch. When you turn the ignition switch off you cut power to the coil.

If the Alternator Sensing wire is hooked to the BAT terminal on the alternator or a junction box it is basically hooked up directly to the battery (via the starter). Electricity can flow from teh battery to the sensing wire, through the alternator and back through the exciter wire to the ignition switch. If the coil wire and altenator wire are on the same terminal, turning off the ignition switch will not cut the power to the coil and the car/truck will keep running without the key in it because the flow of electricity to the coil hasn't been interrupted. That is what Alan means by 'run on.' The diode in the exciter wire is to prevent electricity from flowing 'backwards' to the ignition switch, because the right diode only lets electricity flow in one direction- From the Igntion switch TO the alternator, but not the reverse.

I think that is right, but I would be happy to be corrected!