Pull the cover off the regulator and check to see if the voltage regulator points (directly behind the Field wire) are stuck together. If not, disconnect the F wire from the regulator and see if the ammeter continues to peg out. It should not move at all with the F wire disconnected unless there's a short to ground somewhere between the generator and the regulator in the field circuit, or a shorted field winding inside the generator. If the heavy current continues with the wire disconnected at the regulator, disconnect it at the generator and repeat the test. When you find the source of unintentional ground, replace the wire, the regulator, or the generator.
Jerry
Edit: It's also possible that the battery has developed a shorted cell and it's the cause of the heavy charging current. Swap a good-condition fully-charged battery into the circuit before you start chasing other gremlins.
