[SimonP]

Martin,
When the alternator starts to charge, the "diode trio" will try to send the field terminal up to 14V. With this shorted to earth, the diodes will be damaged.

Although the alternator relies on about 100mA current into the field via the warning lamp to help it start charging at idle speed, it will likely start to charge at about 3500rpm without the lamp in circuit (or indeed with the field terminal grounded). This is how "single wire" alternators work.

Shane,
Connecting up as Grease Monkey says won't do any harm.
You could also connect up a small (12V 1-2.2W) lamp between the main terminal and field terminal to actually get the alternator to charge. You can then also check it's output.

When stopping the engine, you MUST kill the ignition circuit BEFORE disconnecting the battery, or the alternator will be damaged.

Simon.