Rear Derailleur wire sags completely in higher gears I have a Shimano 7 speed rear derailleur which used to work properly. Lately I observed I was unable to shift down to gear 7 from 6 but let the problem be. The derailleur would move very little (there wasn't even any grinding sound against sprockets when shifting).
Now the gear won't even shift from 4 to 5. Derailleur works fine till gear 4 but as soon as I shift to gear 5, the movement is miniscule. I checked the wire and it was all slack (literally hanging with 0 tension) in this position. Clicking to shift down to 6 or 7 doesn't even produce a clicking noise and there is no effect on wire/
derailleur.
I have read about wire tightening (per this question) but those steps require chain to be on lowest cog. My wire goes all limp in gear 5 and chain is on fourth sprocket. Can I undertake the tightening procedure in this situation? How do I gauge requisite wire tension in this case?
What you describe suggests friction in the rear housing loop. It fits with the problem getting progressively worse, and with the apparently slack cable, presumably along the downtube or chainstay. This can happen from corrosion, contamination, kinks in the cable/housing, or the housing or cable fraying or coming apart.
It might need new cable and housing. That is normal maintenance and not a big deal if so.
Put the shifter in 7 and leave it there. Pedal with one hand while pushing the derailler inward up to the big cog with the other. Use the slack you've created in the cable to slip the rear loop out of its stop. Try to slip it up and down the cable to inspect for friction. Inspect it for the above. If it's dirty or corroded, get it clean. Put a few drops of oil along it. Shove the derailleur inward again as you reseat the housing in the stop. See if there's any improvement. It would be normal to need to readjust the tension at this point.
If the bike has a housing segment near the seat cluster, it could also be the source of the problem.