Originally Posted by
jim hughes
t.del, not sure what you mean when you say "I disconnected the cable completely and the problem persisted. ". If you just pull on the cable by hand (not using the shifter) does the derailleur move through its entire range?
+1 With the cable disconnected, you should be able to push the derailleur through its entire range. Or just pull the cable with your hand (leave it connected to the derailleur).
MTB shifters on the other hand can be a can of worms. Sometimes you can revive them, sometimes you will need to replace them. If you are going to do a quickie $20 service, I would have an exception for parts. Worn out or broken parts don't tune up.