Now, what if the light is faint or flickering?
The bulb is like any other 12V/1.2W bulb on the dash except for the wiring difference I’ve already mentioned. Whenever the battery is being charged the voltage at the battery is higher than if the engine is off. It might be 12.8V when off and 13.8V when charging. Of course there is going to be a small inherent voltage drop at the bulb because of the very long wire that runs from the engine to the dash. This drop can be approximated from the gauge and length of wire. If the wire has say 0.0214 Ohm/m of resistance and there is 10 ft. of wire the voltage drop will be 6mV. For an experiment, I took an instrument light bulb out of my bus and hooked it up to a DC power supply. There is no discernable light output from the bulb until about 1.1V are driven across the bulb. At 13.8V the light is very bright and it draws about 90mA (13.8V x 90mA = 1.2W). At in between voltages the light output drops off quickly (technically at 85% voltage, the light output is half). Above 6V the bulb is bright e
