hi everybody
the last couple of days i have had a problem using my Di2..
when i shift from the 21 to the 23 cog(11/23 cassete) the rear derailleur after few seconds move down again to the 21cog. That happend with either the big or small chainring.Everything else works fine
Any ideas?

does it stay in 23 when the wheel is off?

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2012 Scott Foil Premium

Have you checked it in the stand to see if the motor in the derailleur is actually moving back to the 21 spot? If it is, then you have some kind of programming error, which you will have to solve by contacting your LBS Di2 expert or Shimano. Otherwise, it may be a problem that is normal for all derailleurs (electronic or not.) It could be that your LOW limit screw is slightly too tight, and therefore the derailleur stays in between the 2 cogs and therefore and your chain moves down when you encounter a bump or something that makes it move a little bit.

This problem is especially relevant with Di2 because the motor actually shifts slightly past the point it is programmed as the center of the cog. There are 30 clicks in a Di2 brain, every shift it should move 3 for a 10 speed cassette. However, it moves 4 clicks to ensure that the chain moves to the next cog, and then shifts back one, to keep it centered on the cog.

Therefore, your limit screws may be out of adjustment, or you "electronic cable tension" may be off (this means that you need to calibrate where the "center of the cogs" is in the Di2 brain.) I would recommend checking your limit screw first, because it is a much easier adjustment, and involves no monkeying with electronics.

I hope that was clear

GOOD LUCK!

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My Bike:
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Yes as pointed out. Likely the limit screw. Release it that wee little bit such that the chain will remain in place after shifting.

It even happens on mechanical sets!!

Also, do follow the tuning and setup instructions word for word. It usually will sort out little issues with shifting due to its unique nature as many do not follow them and use the traditional mechanical setup methods and minor problems arise as a result.

when the bike is in the stand with the wheeloff it return again down to the 21 indeed

problem solved
The low was to tight !!!
thanks a lot guys