[rotorwrench]

The generator light function is switched inside the generator control unit or voltage regulator (VR). When the generator starts generating current there is a switch function in the current limiter pole circuit that opens the ground path for the generator light and the light goes out so at least it's showing some function. If voltage is low then the voltage regulator pole is not properly adjusted to a high enough voltage for operation.

Make sure you have good connections with the generator to VR and battery to buss/frame. Grounding of the generator to the frame and in most cases, the voltage regulator to the body have to be good. When there is no ground wire between the generator and regulator then this ground has to come from other sources. This also includes grounds from the engine/frame to the body. If all are good then either try a different regulator or find someone who can properly adjust it.