Terrano WD21 electrical puzzle.

[nasa]

Terrano WD21 1990 Td 2.7 Auto.

I have a little electrical puzzle on my hands.

The alternator light is telling me lies.

The alternator light comes on with ignition and goes out when the engine is started

From what I have read the alternator light is fed power from the battery and it earths out through the alternator once the ignition is on, thus illuminating the lamp.

Once the alternator produces current it feeds current back to the earth side of the lamp thus canceling out the difference in potential thus the lamp goes out.

This sounds like a foolproof system. It will always make the lamp go out unless the alternator is not working . Sounds good Hey.

Well I have the alternator totally disconnected and still the lamp goes out when the engine is started..

Why is it so.??????