lol did I waste my time when I wrote the above explaining it to you?

Basically the rectification side is faulty, what is happening is current is coming via the battery and going through the alternator to the dash light thus illuminating it, usually the field excitation wire usually has 0V when the alternator is stationary, thus the bulb stays off.

When you turn the ignition on a switched feed is supplied to the bulb, not only lighting it but doing the important job of energising the field coils in the alternator itself.

When the engine runs and everything is working correctly the terminal where the bulb connects changes state from being a ground to having +13V, and so as the bulb now has two lives it will extinguish.


Martin