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When the Shifter clicks but it's not shifting gears - Anyone know the cause of this ?
It's as though the shifter is not changing the cable tension at the Rear Mech end
When the fault occurs it's stuck in the same gear despite the shifting. The shifting is doing nothing.
It does this fault all the time when the Bike's on the stand , yet only does the fault occasionally when riding
Change the inner cable is the first thing.
APF
I've changed it quite recently and it's not high mileage
Outer cable damaged?
I've got this on one bike regularly and changing the gear cable sorts it most times, either just the inner with a flush of the outer or a whole new set. Occasionally squirt GT85 into the shifter itself and that helps too. A small drop of lube on the mech pivots is also a good idea. The bike that does it regularly (every 2-3 months) spends a lot of time at bike parks where it gets very little shifting, one or two gears used all day, and spends lots of time on uplift trailers getting covered in gritty crap. Give it a few runs out on all-day rides and the shifting stays fine for ages. It's just a fact of the life the bike leads, little shifting to clear the cable of grit and it spends most of it's time filthy.
From what you're saying, it sounds a lot like you've got a loose/worn componenent in the shifter, and when you operate the shifter from a different angle to normal, it allows the ratchet/pawl to disengage instead of shifting.
does your stand clamp around the cable, by any chance ?
Changing up or down?
Check the inner is seated in the shifter correctly
Check outer cable ferrel are seated correctly on the outer and in the stop ends
Undo mech cable clamp, hold cable with one had pulling on cable and operste shifter with other had. You should feel cable pull and release with each click depending on which lever your pressing
Change both inner and outer if the shifter works OK after trying the above
If all else has failed, then as scary pants says, it could be a damaged internal part in the shifter, so you'll need a new one
does your stand clamp around the cable, by any chance ?
+1
Also, what shifter? SRAM ones can 'jump out' of the cable end stop internally if not enough spring/pull on the cable.