E6420 display backlight flickers/goes out

Question

when i boot my E6420 with AC adapter connected, the backlight doesn't come on. (i can see the screen image with a flashlight so the LCD is working).

with the computer booted, when i remove the AC, the backlight comes on at full brilliance but it flickers and flashes. (flicker--very quick dimming; flickering--very quick alternating between LCD on and totally off).

then when i reconnect AC, the screen dims to the level it would be on battery (dimmer than on AC) but still flickers--but the flashing is gone.

if i boot with AC not connected, the backlight works properly for about 10 seconds, then the backlights flashes between on and off every 1/2 second. after a couple of minutes, the frequency of the flashing on/off goes to a couple of seconds, and when it is on it flickers. so---flicker flicker flicker flash off flicker flicker flicker flash off etc.

i ran the pre-boot diagnostics. it flags the LCD cable as bad, but the display is passed as OK. I can easily replace the LCD cable, but any other ideas?

i thought maybe the DC in jack/cable was bad, but i replaced that to no avail.

thanks in advance

ken

faithie999 · Answer

in the second sentence, i meant to say: flickering--very quick alternating between backlight fully on and then dim but not off;  flashing--quickly alternating between backlight fully on and fully off

ejn63 · Answer

Attach an external monitor -- if it displays OK, no flickering -- chances are it's the display panel that needs to be replaced.  If it flickers as well, it's the GPU -- meaning a replacement mainboard is needed.

DELL-Rishi · Answer

Hi FAITHIE999.

Reseat the LCD cable and check.

If you know the right part# of the system configuration - you can find the LCD cable here - http://bit.ly/2d31ie1

If not, then as advised by ejn63, either the LCD assembly or the motherboard needs to be replaced.

Also, please click my DELL-username and write me a private conversation with the service tag and your contact details(Name and Email) for case records.

Let us know if you have any other queries.

faithie999 · Answer

graphics work properly on an external monitor.

i will re-seat the LCD cable but i did that yesterday when i replaced the DC-in jack thinking that might have been the problem, since plugging and unplugging the power supply affects the screen's flashing/flickering behavior.

can you confirm that the inverter is built into the display panel? i would not want to buy a new panel only to find out that the inverter is built into the motherboard.

thanks

ejn63 · Answer

These systems do not use CCFL backlit screens -- there is no inverter (the backlight is LED and built into the display panel).

No one can guarantee you that it's the screen that's faulty - but it's the next part to replace. If it does not solve the problem, it's a video chip issue and the mainboard would need replacement.

faithie999 · Answer

thanks for confirming that there is no inverter.

both the vga and HDMI output are flawless, mirroring the display while the display is doing its flashing/flickering/dimming thing. i conclude that it's not the gpu.

i think i will try an LCD cable before replacing the display. $4 vs $50 so worth the gamble IMHO.

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E6420 display backlight flickers/goes out

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