Hi Martin.

I do it different to you. Here is how I do it:

Battery - TO ISOLATED DC BUS
Co-Pilots AC Instruments      - ESSENTIAL AC  BUS
AC INST & FVM CONT - ESSENTIAL AC BUS
GTC Door Switch - OPEN
GTC Main Switch - START
Wait for the GREEN light...
Bleed Air Valve Switch - OPEN
ATM Control Switch - ON
ATM Generator Switch - RESET then move to ATM GENERATORS

But... I followed your instructions, and the external lights worked just the same as they do when I do it my way. I don't know why it isn't working for you. :-/