Question : Problem: Show Powerbook apple hardware test on external display

I'm trying to diagnose exactly why the LCD panel on a Powerbook G4 is completely white.
The contrast controls work the backlight okay, and the LDVS cable is, with a visual inpspection at least, looking good.
I want to run the apple hardware test disc on it next but need this to display on the external DVI monitor when running. It appears to boot but does not turn on the external display (which works fine if i just boot into OS X).
The f7 key does not cause a switch of displays.

Any help with this would be appreciated, even if it's just a solid confirmation it can't be done, as googling is getting me nowhere.

Answer : Problem: Show Powerbook apple hardware test on external display

Ouch. Thats right. On the other hand, I remember making open firmware commmnds that would hold for the next boot. So you could boot it on the other screen and make changes. The basic command is command option o f to bring you to the command prompt. So once you figure the commands you can reboot with the output forced to your other screen. Does your machine have a serial port to use ttya switch? By the way have you tried resetting the pram just in case your problem is an open firmware corruption issue in the first place.
http://www.dialectronics.com/Words/OF_Part_I.shtml
http://www.geocities.com/pm9600g4/open_firmware_basics.html