Hall Sensor Unresponsive/Magnetic auto-standby not working

I had a water damage with my Nexus 7. I brought a broken device from ebay for replacement parts and luckily I could scrap together a working unit. However the magnetic auto-wake function isn't working properly.

I get a reading with a sensor app (from ~200 micropotatoes to ~800 when I press the case magnet to the device) Occasionaly the wakeup even does react, but it occurs arbitrary and most of the time it's not.

I would to know if there is a software method to lower the threshold. Also, I would like to know where is hall sensor is located within the device and whether there is an easy fix.