737NG DEU Failure?
 

Can someone please refresh my memory as to why the EECs revert to "soft alternate" mode with a DEU failure?

Can't seem to find reference to the reason why, in the books.

:) I am bloody impressed, how did you find out this info? Sure as hell is not in the Boeing books!!!!!

SOPS

Impressive? True! but the reason it ain't in the books is Boeing have decided you don't need to know and just follow the QRH when the lights come on.
If you look at the info in the new A5 AOM's compared to the older A4 versions, there is a wealth of data removed. It's easy to get too complicated and bogged down in gobbledy gook, (remember we became pilots not engineers) but removing the flow diagrams of many of the systems was a retrograde step.
I'll not be surprised if you come up with a lot more similar questions; be it from curiosity or first hand experience.
Boeing are going down the 1 pilot and a dog route.
Problem is when something happens that is not supposed to and has not been covered in the QRH. I guess they reason that if it troubles you and you can't find a solution, LAND. However, there is a world outside the USA and sometimes that is not an option.

"Loss of either DEU results in a loss of signal to both EECs. The EEC ALTN lights illuminate and each EEC reverts to the alternate mode to prevent the engines from operating on a single source of data".

737NG DEU Failure
 

Basically, in a plain english explanation, if the EEC's fail to get their info from the right places, eg Adiru's which go through the DEU's, than, the EEC's have a redundant feature in them that makes them revert to a mode whereby they use their own information. Example is the static pressure port that it will use if the ADIRU's fail to supply ambient pressure etc. this is a soft fault.