On a 3 wire system you have a trigger terminal and also a voltage sensing terminal, plus the main Batt terminal.
First the alternator should not be hooked directly to the battery, it should be attached to the starter battery post. As this is the main junction. Some people just bridge the voltage sensing terminal and the batt terminal together at the back of the alternator. (This is what one-wire system do). This is typically not what you are supposed to do. The voltage sensing should be connected to the where the power feed attaches to the main harness to it can accurately read the voltage load on the system. If you connect it direct to the batt terminal it constantly see full voltage as that is where it is produced, so it is cycling up and down. Also depending on the length of the harness, there is going to be a small difference in voltage between the batt terminal, and the main harness feed.
Now the trigger wire needs a load to trigger the alternator on. One wire units do that internally once a certain RPM is reached. If you have a dash idiot light (GEN light), then this become the load to trigger the system, and no diode is needed. On cars without a idiot light, and diode is needed.
The way the GEN light works is that when the igintion is turned on 12 volts (or 6 depending on your setup) is supplied from the ignition switch to one terminal on the light, and the other terminal is connected to the trigger on the Alternator. When the alternator is not rotating, the Trigger terminal is supplying a ground path. So the light turns on as visual alert that no power is being produced by the alternator. As the Alternator spins, it start to produce volatge and this also causes the resistance to increases on the trigger termial the Gen light gets dimmer and dimmer, once the voltage being produced by the alternaor is equal too or greater than the voltage at the ignition switch, then the light goes out.
For more on 3 wire vs. 1 wire, check this out:
http://www.madelectrical.com/electricaltech/onewire-threewire.shtml They also have other tech acticles on alternators, and 6>12 volt conversions.
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