key on, engine not running i get a normal battery light, Start engine and the battery light dimms.

Key on, engine not running i get 12V on pins 2 and 3 and 5V on pin 1 (lamp)

looking at this diagram


I assume that the alternator provides a ground on pin 1 to turn on the lamp when the engine is off and 5V when the engine is running to turn the lamp off. With the engine running I put my volt meter on pin 1 of the alternator to ground and got something like 1.1V which makes sense why the light dims.