Does the LEGO "Power Functions Extension Wire" (8886) work in both directions between old 9V and new Power Functions devices?
According to Customer reviews on the official website, this cable enable powering motors in both ways between old and new power functions versions:




  This cable allows you to power the new 9v motors from the old 9v system or vise versa..


Author: heavyD.



But there is an incompatibility between the old battery boxes and the IR receivers:


  This cable also powers new motors from the old battery box, but it cannot power an IR receiver, which requires the new battery box and all 4 wires of the new cable. (This cable only converts the 2 motor-powering wires.)


Author: Technic010.