Alternator Lamp/Indicator: Testing and Inspection

The indicator lamp glows when field relay fails to close. When ignition is in the On position, battery current flows through the charge indicator lamp and a parallel resistor, and through regulator to field, and the lamp comes on. Vehicles with electronic voltage regulator have a 500 ohm resistor. On all others the resistor is 15 ohms. When the alternator builds up enough voltage to close the field relay the charge indicator lamp will go out. Place ignition switch in the Run position with the engine stopped. The lamp should light. If not, the bulb is burned out or indicator lamp has an open circuit. On vehicles with electro-mechanical or transistorized regulators, an open resistor wire in the alternator charging circuit will usually cause the indicator lamp to remain on until engine speed is increased to approximately 2000 RPM. In some cases the lamp will remain on above 2000 RPM. The charge indicator lamp may be tested using a test light containing a No. 67 or 1155 bulb. Disconnect regulator wire connector from regulator, then place ignition switch in the Run position. Place one test lamp lead on regulator wire connector ``I'' terminal and other lead on regulator base. Test lamp will light if circuit is in proper working order. If 15 ohm resistor or circuit is open, indicator lamp will operate at full brightness and test lamp will not light.