Did you flash software after installing the new access 7? Typically if the screen does not display anything it means one or more of the modules has the wrong software version and are not compatible.
If that is not the issue, then either the access 2 or 3 is bad depending on the access 7 flash code.
723 - Access 3
751 - Access 2
The only other thought I have is possibly the can circuit is grounding to the truck frame causing a communication issue.
But yes the flash code is the same. For 751 it will flash 7 fast, pause, 5 fast, pause 1, pause