The circuit for the alternator light is rather simple although the internal circuitry of the dash is full of diodes that can complicate matters.

Generally speaking power from term 15 ignition goes through the light bulb to the 61 term of the alternator. This power goes through the regulator, through one brush through the armature through the other brush to ground lighting the light with key on engine off. When the alternator turns and power is generated the trio diode pack feeds power from the output to the same 61 term input. The regulator then regulates the current flow through the armature but the light now has battery voltage on both sides of the bulb filament and thus the light goes out.

If the light is on there is a voltage across the bulb. To just glow at night this only needs to be a volt or two. The voltage regulator can cause this also. Resistance in either side of the circuit will lower one side of the circuit.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician