Shimano sora shifter no longer shifts On my commute yesterday, I noticed that my right shifter was no longer 'clicking' when shifting my rear derailleur up into a larger sprocket. It still shifted fine, but I noted that I should look into it this weekend. On the commute in this morning, it stopped shifting altogether.

Symptoms


Chain/derailleur is on the smallest sprocket. It is positioned fine, no noise.
When I press the brake lever inwards to shift the derailleur up, it moves a bit until I can feel the shift cable tightening ever so slightly. The shift lever stops and feels locked at that point. (It will not go through it's full sweep inwards)
The thumb lever on the sora shifter feels loose and has no tension. (Normal for when it is on the smallest sprocket)
If I pull on the shift cable itself (cable is routed along the top bar of the bike) the derailleur moves smoothly through the gears on the cassette.


When it first happened, I played a little bit with the cable tension, but that effectively did nothing. 

This has never happened to me before, does anyone recognize these symptoms and should I attempt a repair or are the internals likely broken in the shifter?
I've experienced 2 problems on the Sora shifters- one condition was the cable wear caused strands of the cable to break- eventually all broken strands resulted in functional loss &amp; cable replacement needed. When I disassembled the shifter, I found small pieces of the broken cable strands jamming the ratcheting mechanism. You may be experiencing a similar condition if the shifter cable has been previously replaced w/o a complete overhaul of the shifter. As the strands started breaking- the shift action deteriorates, and no click latching was one of the symptoms.  A second problem found on another Sora shifter that had similar shift deterioration was a result of a tensioning spring used for the return action- the 90 degree bend point of the spring was broken. Without an available replacement part, I finally mechanically "unwound" 1 loop of the spring to create a new 90 degree bend for the part. It successfully has been working for several years now on that bike.