Laptop won't Boot/Post without external monitor

prahlj used Ask the Experts™
I am working with 2 IBM T43P laptops, both had bad displays. Now neither will Post/Boot without an External Monitor connected to the VGA port.

I have purchased a Brand New display, Model is identical to what was installed. I know IBM is very specific about parts that are allowed to be used on their laptops.

I have tested both sticks of RAM is a working laptop without issues, both passed memory diagnostics.

I made sure the switch that detects whether or not the top is closed was not blocked or stuck.

I can get both laptops to boot with an External Monitor, every time I hit FN + F7 (Video Output Select) to switch too the internal LCD output that laptop instantly turns off.

I have switched out the inverters on both laptops also.

I have removed the Bluetooth, Wireless Card,  the PCIMCIA bay, and CD-ROM to make sure it wasn't a hardware malfunction issue.

I have reseated video ribbon on both monitor and board several times making sure it's not a connection issue. Also checked the ribbon for breaks or bare wires.

I've done a complete BIOS reset, removing the CMOS battery, laptop battery, and external power... plus holding the power button for 10 secs.

No matter what I've tried I cannot get the system to boot without an external monitor attached (which works 100% without error)  soon as the output is switched or the external VGA is removed the laptop completely shuts down and will not reboot until all power is removed first.

I'm completely out of ideas at this point.
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Some laptops have video cards that have discrete channels for internal vs. external video.  It might not be the internal panel that is bad, but the internal channel on the video card.  In which case you probably need a new motherboard for those laptops.  

Looking at eBay, there are complete motherboards for about $100, and there is a mobo repair service that might be less.
Since you have done various test and recorded the observations are  suggesting a problem with the motherboard viz the video part.
Sadly the video bit is usually built on the mother board. If you are lucky you may have a separate video card in you system.

I wold agree with jcciv. Is it possible to swap the Mother Board from a good set? it will confirm where the problem really lies. Is it the LCD panel or the Mother Board circutary. Chances are that you may have to change the MB or just live with the external monitor.


Ya, I was hoping it could have been something else since the external monitor worked. Both LCDs only had bad back lights and the laptops still booted, but soon as I switched to a new LCD the laptop just stopped working. Thought it was too much of a coincidence that both laptops did the exact same thing.

I was just hoping I didn't have to switch out the motherboard.

Unfortunately it's a integrated graphics card and not worth it to attempt a switch.

I'll just have to write it up to a lost cause.

I had a similar experience with an IBM laptop, it could not send video to the external port.
The LCD worked fine. The final solution was to replace the mother board. but the cost was prohibitive
So the user lived without the output to external monitor.

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