Macbook only works on external screen

Hello,

I'm having trouble with my (black) Macbook3,1
The previous owner installed WinXP on the machine. I removed it, cleaned the disk an reinstalled Leopard on the machine.

However, I can only use it while using an external screen.
If I remove the screen, the internal screen appears for a few seconds and the Macbook goes into sleep mode. When I press any key or the power button, the internal screen pops up for a second or 2 before going to sleep again.
Once I plug in an external screen, It operates again. It is connected to the power supply.

I installed all the updates. Think I tried all the reset options like removing the battery and holding the power button for 5 secs etc etc..
Swapped the internal memory with another Macbook, used the Sysinternal suite, hardware tests ok.

Could anybody help me with this issue?

could you try going to System Preferences>Display.

You could try clicking on DetectDisplay.

if you reformatted the MacBook should ideally have gone back to Leopard Default but if you did an "over" install the settings might have been saved from when an external monitor was used