The APU ammeter will only show a reading when current is being drawn by a load from the APU and not just because it is running. When the main engines are running their generators supply the power so there is no current (Amps) being used so no Ammeter indication.

If the engines are running he will see the "Circuit open" light lit up, if he closes that the APU will generate (you will see a APU Ammeter indication) but will also trip out the other engines generators.