A lot of early- to mid-1990s mountain bikes are starting to come in with Shimano Rapidfire shifters that seem to have lost their memory.
At our shop we used to call the early onset of this condition shifter amnesia and the later one shifter Alzheimer's. However, exploratory surgery has disclosed that the problem is a simple and treatable one.
The grease Shimano used in the shifter pods stiffens up over time to the consistency and color of ear wax. Because the temperamental shifter units depend on smooth operation so that the hapless rider can actually get one or more usable gears, the ear wax gumming up the little ratchets causes annoying problems. These range from inaccurate shifting to no shifting at all.
Well-prepared by Shimano's own propaganda, we dutifully replaced a number of these units when a forceful douche with Teflon spray lube did not blast things free. We didn't know yet exactly what was going on in there and didn't really want to take the time to delve into something we could simply huck and replace. In other words, we did just what the manufacturer wanted us to do.
Shimano's own attention deficit disorder led us to rip a shifter apart one slow day. We couldn't get parts anymore and I'd always been challenged by the notion that these things could not be serviced at all. I don't like them in the least, but I don't feel that the customer should be punished for buying what was available at the time.
The older units have fewer parts than the newer ones. Since they are also the more failure-prone, it works out well. I did unstack the whole mechanism once or twice, until I figured out where the critical areas are. Just remember exactly how everything came apart.
Pro Link is the new wonder lube for all your shop needs. It turns out to dissolve ear wax and free up shifters, with a little help from a willing technician. I still take the shifter unit off its bracket and pull the covers off it. I just don't unstack any of the mechanism. Just pick away the visible ear wax and drip Pro Link into the little ratchet pawls. Work them back and forth to help the lube penetrate and displace any ear wax down around the little pivot pins. It should not be necessary to pull the teeny-tiny circlip and slide either pawl off its shaft.
Some patients respond better than others. Knowing what I have to do and how to do it, it only adds a few minutes to a typical tuneup when needed.
To pull the covers off the shifter pods, you need to pull the shift cables out and unthread the cable adjusters, if present.
When Rapidfire first came out, one of our favorite custom jobs was to saw the tabs off the brake levers and slap on a nice set of thumb shifters. With that no longer an option, the least I can do is keep a few of these Crapidfire units out of the landfill for a while longer.