What makes this light work? All my lights are fine in the dash except the ALT!

 

Current from the regulator.

 

12 volts is applied to one side of the light, in the instrument cluster. The other side of the light runs out to the I terminal of the voltage regulator which is pulled down to ground at first. When you turn the key to RUN but do not start the engine, the light illuminates. When you start the engine and the alternator begins to produce current, the field relay inside the regulator jumps 12 volts over to the I terminal of the regulator. Since 12 volts appears on both sides of the light, no current flows through it and it shuts off to indicate that the charging system is functioning properly.

If the light does not work, try replacing the bulb. However, some aftermarket, non-OEM electronic regulators will not let the bulb come on at startup to begin with.

 

Got it working. Thanks again. Simply changed bulb and the little black clip that holds the bulb and it came right on!