What did you do, convert a diesel from 24 to 12 volts or what??????? What alternator you have??? You at 12 volts Neg ground now???
Unlike in a gas engine where an idiot light or resistor or diode is used to prevent ignition feedback and run on when ignition is off, such isnt obviously a concern in a diesel.
In the gas conversion some used a resistor in paralell with the idiot light so the alternator could still be excited even when the light burned out.
If its a 10SI family of 3 wire alternators it needs excitation current and an incandescent idiot light or a 10 ohm resistor either one would allow enough current to flow HOWEVER if it was an LED lamp its resistance may be too high and theres insufficient excitation current!!
If its a 10SI 3 wire and the idiot light is an LED you might try a 10 ohm resistor instead.
Not knowing what you have or what was done I just cant whats the problem or how to cure it.