Ultegra Di2 rear shifting randomly stops working

Question

I've been chasing this ghost for a past 2 weeks now. I own a road bike with Ultegra di2 and rear shifting stopped working (kind of) 2 weeks ago. You press lever, lights on junction box illuminates indicating battery life left, but nothing happens. I started charging it, afterwards it didn't work but then randomly it started to shift like nothing is wrong. I rode 100km in the evening, went to sleep, woke up the next day- doesn't shift once again.

At this point I call my mechanic and schedule an appointment, I put my bike in my car, drive there, take it out, bring it to a shop, I start telling what is the problem and it shifts. I click like 10 more times and it shifts flawlessly. Guy looks at me like wtf, but we agree to keep bike at his shop for a day, maybe it will stop working. Spoiler alert- it didn't, worked all day, I bring it back home, ride another 80kms on it, go to sleep, next day it doesn't work anymore.

Anyone seen something like this? It seems that it stops working only if you leave it unattended for like 2+ hours. If you keep it in the room and keep fiddling with shifters once every couple of minutes- it never fails. I've been away for the past 3 days and I left it not working, I came back today and it still didn't work, but I cycled like 7kms with a single gear to my friend's house, moved my saddle and pedals onto his spare bike and boom, my bike started to shift. After 2 hours bike ride once we came back, it was not working once again. I put it in a car- it works again. WTF?

Like a month ago, I had a crash in a race, I fell on right side and broke off rear derailleur hanger, I got it changed. Battery was replaced ~year ago.

If it doesn't work and I connect it to Etube, it recognises every component including rear derailleur, although I don't have sufficient cable to run error checks. Well at least I cannot do that, you click on error check and nothing happens.

At this point in time I am a little bit desperate. I cannot take it to mechanic, because after car ride it starts working out of the blue.

Answer 1

I can't comment about Di2, but when the symptoms you describe arise in an electronic circuit between two boards, there is seldom anything wrong with the boards. The problem is invariably a poor connection.

With time dust slowly makes its way between surfaces at connections that are neither soldered nor bolted, introducing a discontinuity or a high resistance.

The solution is surprisingly simple. Just pull out each connection and re-insert it five or six times. The two surfaces will act as mild sandpaper against each other, re-establishing contact. If you spot dust accumulating at the surfaces, that might be a sign that the connection is not sealed well enough—and, unless you solve that problem, the process will likely need repeating. It appears though that you will be unable to see the metal—at least on one side of the connection (EW-SD50; EW-SD300).

In other words, ChrisH's and Noise's diagnoses are correct, but it's unlikely that having an extra loop affects the connection, as turning the handlebar presumably doesn't tug on the cable.

If you can spot two exposed points from which to debug the problem using an Ohmmeter, that would be simplest. (Did Shimano anticipate this sort of common-tools diagnosis by providing exposed connections at either end?)

Answer 2

The last time I saw this type of problem (intermittent front shifting), it was due to a poor connection at the shifter. Shimano say the cable at the brifter should have a little loop to relieve stress but this bike had the cables basically taut behind the bar tape and after a bump (minor crash), it was loose enough to work only sometimes, just like yours.

It could also be an internal problem from crash damage.

Answer 3

This same thing continues to happen to me. I thought it was the battery, so I bought a new battery. That wasn't it. The LBS found that one of the wire channels was not working, I am told there are 3, they moved the harness to channel #2 and it works. Except sometimes it doesn't. About once every 2 weeks it just stops working. I was on mile 22 yesterday and I had to ask my group to give me a minute. I pulled the battery and then touched the harness and it came back. It took the bike shop 3 weeks to figure it out. They ordered new derailers but that was not the issue etc... Very hard to figure out the issue if they don't know how to fix it. When everybody moves over to electronic shifting next year maybe the shops will get better at diagnosing these issues. Over the winter I am going to ask them to yank the internal wiring and replace it. I can't be 30 miles out & get stuck with one gear for the return ride.