Is my alternator failing?

My ammeter is reading negative and if i put on lights, brakes etc it reads even lower.
I only get 11.5 volts when i put a test meter across the main feed wire and the alternator body.
I have disconnected the fields wires to the regulator, but cannot get over 11.5 volts at tickover speeds.
Is my alternator failing? I had a dead short circuit a couple of weeks back (se other threaes involving fire!) and I wonder if i have damaged a diode in the alternator.
Just need to be sure before i spend $.