Alternator idiot light

[Teamamc <teamamc@xxxxxxxxxxx>]

I have an 81 Eagle SX4.
Yesterday on my way home the alternator idiot light came
on. I pulled the Positive cable with the engine running
and the alternator is still charging.
I notice that when the engine idles really slow to the point of stalling the
light goes out. When I turn the key to start the Car the ALT is out. after
the engine starts the ALT light comes on.

Any Idea  on what would cause the aLT light to reverse itself.
Thanks
Bryan