I have an early 90s Giant Yukon MTB that I'm trying to get back into good working order after I gave it to my son for most of the last decade. It has a Shimano Alivio 3x7 drivetrain. The only thing I've not gotten working well is the shifting. The front shifter will only shift between the two smallest chainrings. The back shifter will only shift between the three smallest cogs. When I try to shift further on either it just won't click or engage. No stuck cables; all moving freely. Derailleurs seem fine; I can shift them by hand.

Are these shifters serviceable or should I just go get a new pair?

if you could spend the money, get a new set, thats what i did on my 80s giant. but there's a solution for the ft der. if you are not using trigger shifters, and you shifter has a twister, or a single lever(normally old bike) if you hold the lever or grip as hard as you can toward the 3, itll eventually shift.

Hose out the shifters with WD40.
Find derailleur setup and adjustment instructions at www.parktool.com. Do all of the steps in proper sequence.

When's the last time that it worked right? Having the same problem on both shifters, to me, is a puzzlement.

Hoseing the shifters with WD-40 is good advice. It's cheap to do and there is a better than 50% chance that it will work. If that doesn't work, I wouldn't waste very much time on it. Complete sets of shifter/ brake lever combinations only retail for about $30.00. Just make sure the brake levers match the style of brakes on your bike.

I cannot say when the last time they worked. My son has had the bike for many years. If there is a reasonable chance it will work, I'll give it a try. Thanks.

Sounds like the maintainence was skipped, now you play catch -up.. replace the cables and Housing is a good start ..

the QBP stuff that is re drawn so the outside of the cables are smooth is a nice improvement

as is the teflon tube lined housing..

This seems to have done the trick. The right shifter improved almost immediately. The left shifter only improved after a few days. I guess it took a few days for the oil to penetrate the gunk. All cables and housing have been replaced. New tires. I think I just need to replace these ancient brake pads and it'll be good to go.

good job

Not to me. Shimano "Rapidfire" shifters are known to gum up after a couple years, especially if they sit around unused. Spraying the insides with WD-40 works, but real spray oil lasts longer. This restored my old set of XTR shifters to usefulness when they stopped working.