I tried this a few years back with my previous 180c (which had a busted display cable)...anyway, there is no way to enable "clamshell" mode on these old machines, clamshell mode didn't come in until at least the PowerBook 500 series. No matter what you do, if you have an external monitor attached, the PowerBook will always assume that there are two displays, even if the internal is completely disconnected. The only way I was able to get mine to run with just the external monitor was to boot it off a floppy with just the System and the PowerBook Display control panel, open the Control Panels folder entirely by keyboard shortcuts, and then select and open the PowerBook Display control panel entirely by keyboard shortcuts, and then try and guess where the control panel is so that you can drag it onto the external. Its a terrible way to do it, but with these old gals its the only way, since they don't support clamshell mode or anything along those lines. This is made even more difficult by the fact that they don't have any switches to detect when the display is closed unlike newer machines.