Something is shorting the field control line to ground or it is shorted in the alt itself. Measure the voltage across the field terminals (two small ones) with the engine running and all accessories off. You typically see around 5V, +/- a few. No more than 10V. If you see the 14 or 18 (or whatever the charging voltage is) then it is shorted.
What codes are you getting?
What codes are you getting?