No display except when I turn it off

The LCD illumination button illuminates the top LCD just fine, but doesn't do anything to the primary LCD.

Do you tend to twist the display up or around? Possible cable issue.

The display on this particular unit doesn't rotate. I tried plugging it into HDMI, and I get no video. It seems to be a graphics driver problem.

Hi @kyle ,

I don't know if a EOS 5D Mk.III is too much different in a lot of things from an EOS 5D but perhaps you could try a complete power refresh and check if that resolves the problem.

According to the EOS 5D service manual there is a CR2016 Lithium coin cell date and time backup battery in the camera. It maintains a few other settings as well.

If your model is the same perhaps you could try removing the main battery and the backup battery, and operating the camera's Power On button for 20 seconds to dissipate any residual charge in the camera in the hope that this may reset the camera back to its' default operating condition in case any settings were corrupted and preventing the camera from fully functioning correctly.

Suggest that you also test the voltage of the backup battery while it is out. It should be 3V DC. If it is <2.6V DC replace it.

There are similar batteries (RTC or CMOS) in desktop and laptop computers and when the voltage falls on these it can cause problems such as no startup, no boot etc because if the BIOS has been corrupted somehow the battery then maintains the corrupted settings etc. This doesn't seem to be too prevalent with batteries that have the correct voltage. Also these are non rechargeable and only last about 5-7 years.

If it does work, you will need to reset the date and time in the camera

Worth a try anyway. Apologies if you've tried this already