di2 woes - the tale of two rear derailleurs

[Was_fast_once]

I have two ultegra 6870 di2 groupsets. One is on a bike and has a short cage, the second which has a medium cage rear derailleur isn't currently on a bike but is wired up. The short cage works on both groupsets, the medium only works on the groupset that sitting on the ground and nothing happens when connected to the group on the bike. I'd like to put the medium cage on the bike. Where am I going wrong?

 

[TerryDi2]

What parts are on the bikes / groupsets exactly? Have you tried updating all parts using E-Tube Project?

I would connect the medium cage derailleur to the bike (so the not-working configuration), and then run E-Tube Project 3.4.5 or 4.0.4. That'll probably tell you what's wrong right after connecting to the bike

 

[Was_fast_once]

I googled and ran E-Tube Project 3.4.5 after failing with 4.0.4 due to wrong charger. Updated everything and it works.

 

[TerryDi2]

Good to see my page can still be found through the magic of Google ;-)