# Di2 rear mech shifting problems



## temporal (9 mo ago)

I have a problem with my Shimano 8000-series 11-speed Ultegra Di2, which was fitted a few months ago. It has been working fine, but over the past few weeks it’s started to occasionally fail to shift. During my last 90 minute ride I did 155 rear shifts. About 10 times I tried to shift into a bigger/easier sprocket and the shift did not happen. I felt the button click, my head unit (Wahoo Elemnt Bolt v2, fully updated) showed a shift, but after a few second of nothing happening, the head unit changed to report the original gear.

I’ve ensured the battery was charged. Disconnected the battery for 30 seconds. Ran the shifter through the post-crash-protection process. Ensured the firmware was up to date (and even re-flashing the same version). Inspected the rear mech for damage, there is none (I have not crashed and do not recall any case where it could have been impacted). The rear hanger isn’t bent. There’s still plenty of life in the chain.

I took it back into the bike shop and they looked at it, suggested it might be a loose right shifter wire. The following day as I set out it got stuck immediately.

It’s always during a shift to an easier cog. Typically the computer shows a shift, then after a few seconds of nothing it shows the original gear. For example, it reads 11. I hit the button and after a moment it reads 10, then after a few seconds of nothing happening reads 11 again.

I’ve spent a good deal of time thinking this through, what could be the issue. For the computer to show the new gear, then switch back, it must be getting a signal from the E-TUBE junction. That signal must originate from the rear mech itself, because nothing else in the system would be able to sense that the intended gear shift did not occur. The mech must sense the shift didn’t happen and transmit that back across the cables.

I’ve considered these possibilities:


The shifter is faulty. I don’t think so because it must be sending the signal to shift, so that the E-TUBE junction can pick this up and transmit it to the head unit
There is a cable break or loose connection somewhere. it’s my understanding that the “brains” of Di2 is in the battery. All the cables carry a “data” signal, and power. There’s only a few places a break makes sense:
Between the shifter and junction A. This can’t be the case because I can see the shift on the computer, so the signal must at least be reaching the E-TUBE junction
Between junction B and the rear mech. This is also dubious, because
Most of the time it shifts OK
If the shift fails, my head unit switches back to the starting gear. This could only be possible if the rear mech got the signal to shift OK, and was able to transmit the failure.


It’s also possible a cable somewhere has enough contact to send a signal at low voltage, but not power at high voltage. it could be loose so that as I ride it shifts. I have doubts here because it seems to always shift to smaller/harder sprockets. Maybe that takes less power. But also, the front mech is OK. So the only place this could be the case is the connection between the rear mech and junction box B, but that has been checked.
The battery is faulty, not delivering enough power. I don’t think so because the front mech has not failed to shift. Having said that I don’t shift the front so often, so maybe it might.
The rear mech is faulty. If there is an intermittent fault, the mech gets the signal to shift, fails, then senses the gear it is in, and broadcasts that back to the BT connector, which transmits it to the head unit.

Sometimes it does something a bit inexplicable: The head unit is reading “6” on the rear. I try to shift down. After a brief pause, the head unit reads “5”. No shift occurs. After a few seconds, the head unit reads “8” (not 6). I can get to where I want by shifting up twice so it’s actually in “8”, then back down to “5”. This really isn’t fun because the time I really want to shift from 6 to 5 is while climbing. This means easing back on the power so the shift is smooth, losing cadence and speed, then after nothing happening shifting into a much harder gear! The 6-5-8 pattern suggests to me something is wrong with the rear mech. There must be a sensor that works out what gear it is in, and that is somehow malfunctioning.

Any thoughts?


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## JSR (Feb 27, 2006)

have you tried doing the rear mech adjustment procedure? Perhaps adjust one or two clicks toward the easier side.


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## TerryDi2 (May 12, 2020)

temporal said:


> I’ve spent a good deal of time thinking this through, what could be the issue. For the computer to show the new gear, then switch back, it must be getting a signal from the E-TUBE junction. That signal must originate from the rear mech itself, because nothing else in the system would be able to sense that the intended gear shift did not occur. The mech must sense the shift didn’t happen and transmit that back across the cables.


Yeah.. I agree.

Does this also happen at home, on a workstand? If it does... does the RD make any sounds? Clicking, ticking, or nothing at all?
Does it help when you take the rear wheel off?

Usually having to "wake up" a system before it'll shift means that there is a wiring problem, or dirty connectors, or similar.. In those cases I wouldn't expect the display to 'change gear' while the RD stays in place though.. so I don't think this is your problem.


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## temporal (9 mo ago)

JSR said:


> have you tried doing the rear mech adjustment procedure? Perhaps adjust one or two clicks toward the easier side.


I took the bike back to the person who fitted the groupset. I didn't ask specifically if they'd made any adjustments but I assume if it was an alignment issue they would have done that. I personally have not messed about with the adjustment.


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## temporal (9 mo ago)

TerryDi2 said:


> Yeah.. I agree.
> 
> Does this also happen at home, on a workstand? If it does... does the RD make any sounds? Clicking, ticking, or nothing at all?
> Does it help when you take the rear wheel off?
> ...


It does it in a workstand yes. There is some noise, a sort of buzzing/whining, like it's "trying". Not the normal sound when it shifts, much weaker.
At one stage I had it in the workstand trying to work out what to do, just spinning the back wheel shifting up and down and it got progressively worse in front of my eyes until it was stuck in the smallest sprocket. Then it wouldn't shift at all, stopped making any noises. That evening I went back, and was again just spinning the back wheel, hitting the shifter and it came back to life. Bamboozling.
The next morning I was out for about 3.5 hours and it wasn't terrible, probably failed to shift about 10 times or so.

I haven't thought of trying it without the wheel/cassette there. I guess if it does shift perfectly under that condition it would confirm something wrong with the mech, if the resistance of the chain and cassette stops in moving, something is wrong there.


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## faulker479 (Jan 12, 2015)

temporal said:


> It does it in a workstand yes. There is some noise, a sort of buzzing/whining, like it's "trying". Not the normal sound when it shifts, much weaker.
> At one stage I had it in the workstand trying to work out what to do, just spinning the back wheel shifting up and down and it got progressively worse in front of my eyes until it was stuck in the smallest sprocket. Then it wouldn't shift at all, stopped making any noises. That evening I went back, and was again just spinning the back wheel, hitting the shifter and it came back to life. Bamboozling.
> The next morning I was out for about 3.5 hours and it wasn't terrible, probably failed to shift about 10 times or so.
> 
> I haven't thought of trying it without the wheel/cassette there. I guess if it does shift perfectly under that condition it would confirm something wrong with the mech, if the resistance of the chain and cassette stops in moving, something is wrong there.


This is a total stab in the dark, but how is your B Limit adjustment? If the rear mech is too close, it might be contacting the next cog or two up and not want to shift. if it shifts fine with the rear wheel off, that could be the cause. or you may have a faulty rear mech. 

Again, just a stab in the dark.


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## temporal (9 mo ago)

Turned out to be a problem with the cable between the rear-mech and junction B. Replaced some weeks ago and working fine ever since.


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