How to monitor an Alternator exciter wire with MCU?
What happens at the indicator terminal is that there's a trio of diodes from the winding phases that provide the current for the field coil. When the alternator starts up, some current is needed through the field winding (on the rotor) for it to generate enough magnetic field to produce an output, so that's why you needed a low resistance to replace the bulb. Once the alternator is producing an output, the trio brings that terminal up to the same voltage as the battery, since the drops across the trio and main diodes are similar, so there's no potential across the bulb, and the trio is providing the field current.
 With the alternator not turning, the bulb (or bias resistor) current is still being dropped across the field winding, which is a few ohms so the voltage at the terminal is not zero.
 You could try putting a couple of diodes, (or a zener) in series with your 4k7 and another pulldown to ensure that the input is below the low state threshold on the MCU input.

Other alternators have a regulator that switch the field internally based on the output of the windings under the residual magnetism of the rotor, these just have a transistor (usually a FET) that pull the indicator output down. These would function the same - they don't need the bulb or any other current source to aid starting up.