Latitude C840 and a C800 LCD panel: Compatible?

When my Latitude C800 died recently I found a used Latiitude C840 without a working LCD panel on eBay. I researched and noted that BOTH the C800 and the C840 used the same LCD panel, part number: 1K855. Since my LCD was still operable, and the C840 was priced right, I decided to go for it.

However, when I connected the C840 chassis and my old LCD together to make my completed laptop, the LCD would not work. I can use an EXTERNAL monitor and see the bootup process. I even installed the OS (XP Pro) using the external monitor.

What I see on the LCD is a 2 inch, solid gray band across the bottom ¼ of the screen. I cannot see the BIOS POST or anything other than the gray band. That gray band goes away whenever it gets to the Windows XP boot process. Otherwise, the display works perfectly on an external monitor. All of the drivers are loaded and working properly according to the XP DEVICE MANAGER.

I tried swapping the graphics card from the C800 and installing it on the C840 without any success. I thought I may have figured it out when I went into the BIOS setup and changed the display settings from “docked” to “system.” However, that also failed to correct the issue. I am certain that the LCD is good. I tested it on another C800 and it worked fine.

Anyone here have any ideas on what I might do to correct this issue?