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Based on my experience there are 3 types of failures for rear Di2 derailleurs:
- Clicking/grinding noise - caused by loose bolts inside the servo. This can be fixed 99.9%
- Grinding Noise - possibly caused by a knock or fall. This may resulted in striping teeth on the worm-screw attached the to motor. replacement motor required. Could be repaired but spare parts are required.
- No response to shifting, or several clicks required. This is likely water damage- you have 50/50 chance of repair depending on the water damage. PCB is small SMD components even smaller and with no description, so difficult to source replacement.
- Shifting out of sync (under or over shifting) - caused by a knock, water damage or devil himself. Usually, can be repaired, as there is position sensor (rotation counter) which tells derailleur which gear is selected.
I haven't had the opportunity to repair front derailleur, apart one case of rusty linkage - that was purely mechanical.
I also repair Di2 hydraulic shifters.
Any questions please give me a shout.
P.S. If I can't fix it your item, it won't cost you a penny ;-)
- Clicking/grinding noise - caused by loose bolts inside the servo. This can be fixed 99.9%
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Good work, glad I could be of assistance. Another Di2 saved from trash = Success