'83 300SD Voltage Light.

Another issue I haven't been able to resolve is the voltage light being persistently on. I have checked both the alternator voltage at idle, which is in the mid 14's and the battery voltage after running, which is in the high 12's with a surface charge. The car isn't draining the battery, and all of the other electrical components function normally. No dim lights and wipers, and heater work at seemingly normal speed.

When I start the car, the voltage light will be on, but only dimly. If I rev the engine the light will go to full brightness and stay that way. Any other change in charge or load will do the same, such as turning the stereo or heater on and off. So my questions are: Is there a diagnostic unit in the instrument cluster that can fail? Is it possible the voltage regulator is failing in some way I can't see?