Shimano Altus C50 rear adjustment
 

I have read the forum and the suggestion was that the cable needed tightening.
The cable has been tightened, but the trigger simply does not fire and I am stck in high gear.
If I could figure out how the front lever does what it does, I would adjust it.
However, cover is off and I do not see what is wrong.
Ideas???
Thanks,

Post this in the Mechanics forum. I'm sure the guys there could be of service.

Drench the mechanism with something like WD-40 (or similar) and hope!

This is VERY common for shifters from around '95 or so. Grease and dirt clog the shifting mechanism, take spray lube and shoot it into every opening you see until it drips out, and I mean a lot. Make sure the cable is disconnected and start shifting, and I mean a lot. Keep this up until hopefully things free up. A good bike shop can fix this for you.

Disconnect the shift cable from the derailleur. Then hold onto the shift cable with your fingers and try to cycle the shifter in both directions.

If you can feel cable movement with each shift, that's good. Cycle the shifter into it's highest (smallest cog) position, reconnect the cable and do your final derailleur adjustment.

If the shifter doesn't want to click into the other gear positions, you have a bad shifter. You can try to make it work by spraying some WD40 into the innerds and blow it out with a blast of compressed air. For no more than an Altus shifter costs, however, I'd just buy a new one and bolt it on. It'll come with a new shift cable already installed too.