Di2 Rear Derailleur Loses It's Mind

[Mike K.]

Looks like my rear derailleur is having a major malfunction as it is alternating from seizing the chain on the rear cassette to allowing it to rotate freely over the teeth. At first I thought it was a rear hub pawl failure, but after switching over to a new wheel the problem continued. I AM PISSED!!!

[pritchet74]

What

The

Hell

?

How did this come about? If you take off the chain and shift the RD does it look like it works correctly? What gears is it doing it in?

[Mike K.]

The derailleur tracks just fine across the 11x23 cassette. The antics occurred in the 39 on cogs 21 and approx two cogs down. I sensed something was off a couple of days ago when I felt an ever so slight hesitation/engagement occurring while pedaling from time to time. I thought the pawl was failing. Then today all hell broke loose. At one point I could not generate any forward motion on a 4% grade as the chain was rotating freely over the cassette. (I assume this is what the chain is doing as it is not between cogs.) I was sure I sheared off the pawl but that was not the case. The chain itself is not jumping off any of the cogs from side to side. The derailleur has had a pampered existence. I've never dropped the bike or yanked the derailleur in any direction manually. Hell I've never even been caught in the rain or hosed it off for that matter. Cleaning has been restricted to wiping with a damp cloth. I hope the seller will just get me a replacement. But overall I am very disappointed that this failure has transpired.

[pritchet74]

I am shocked. Hopefully the Almighty Cow can chime in with some knowledge on the matter.

[Mike K.]

Yes I would like to hear his thoughts. I forgot to mention that for added entertainment value the derailleur locked up a couple of times in the aforementioned gear configurations, preventing me from pedaling. At one point I could see that the derailleur cage was extended forward in what looked like an exaggerated position. Shifting freed things up again.

[maxxevv]

That's odd, do check that there are no damaged chinks on the cable? Take out the battery and replug it in again ?

Tried dis-assembly and re-assembly of the system ?

[Mike K.]

Nope just shipped it back to the originator in the UK for a replacement.

[pritchet74]

Damn. Too bad your local dealer couldn't just swap it out for you.

[Gumgardner]

That's what scares me about the electronic shifting. For this reason, I'll stick with my SR11

[Ypsylon]

Sounds like a chain problem to me.

AFAIK the RD locks itself if something's jammed.

I mean, you are the one who saw it and not me, but why exactly did you think it was the RD?

[pritchet74]

If you haven't sent it off yet it wouldn't hurt to try a different chain.

[geoff]

Hmmmm. Sounds like a bent hanger to me...

[coloclimber]

That is what the Di2 system checker is for- To diagnose the issue.

[Mike K.]

Hanger is true. Can't speak of ability of diagnostic checker to resolve this issue.

[pritchet74]

Come to think of it - and I can't believe I didn't think of it before - but I kind of had this issue a couple months ago. Sometimes when I would shift from the big ring to the small ring while shifting the rear mech too the chain in the back wouldn't catch the cog - it would just spin. I had to backpedal for a second to get it to catch. After it happening 2 times in a month I did a "from scratch" adjustment of both derailleurs - I re-did all of the adjustments as if it was a new build - and it's been perfect ever since. I think it had something to do with all the cassettes I have been switching around. When I built the bike I had a 11-26 SRAM and now I was on a 12-25 Shimano. Also, I was on alloy rings and was now on Fibre-Lyte rings. Just lots of changes in the drivetrain.

So maybe you need to look at all of the things to adjust on the RD and maybe a different chain?

Just a thought.