A variety of factors can cause mountain bike shifters not to work, and most are relatively easy to fix. If you can't assess the problem right away, avoid using muscle power. Forcing the shifters can compound typical problems and lead to unnecessary replacements. An inspection and evaluation can help you identify the cause of shifters that don’t work on your mountain bike.
Shifters operate cables that move the front and rear derailleurs to another sprocket when you shift. You inspect the system by first checking each derailleur for damage that resulted from a spill or impact. Next, gently move each derailleur by hand to check the bolts that attach it to the mountain bike frame. Loose derailleur bolts might not cause a shifter to malfunction, but a loose attachment can affect gear changes. A damaged derailleur can cause a shifter not to work and must be replaced. You tighten derailleur attachment bolts using a metric Allen key.
Shifter cables consist of a stranded steel cable that runs through a flexible housing. Inspect the cables to make sure the flexible housing is not cut through or that a section of the cable is not badly crimped. Either of these conditions can bind the cable, leading you to believe the problem is a shifter. Grip the cable near each derailleur and give a few tugs to make sure the cable is connected inside the unit. The repair manual for your bike describes how to replace a cable and secure a connection at a derailleur. If you find no damage to either derailleur, and the shift cables appear undamaged and securely connected, the problem is the shifter.
Addressing the Shifters
The shifter on your mountain bike has a lever or a control knob that operates the shift cable. Unless the shifter is clearly damaged, the problem lies within the unit. Loosen the circular adjuster where the cable enters the shifter by hand. Remove the small Allen bolts that attach the indicator to the shifter and remove the indicator to expose the shifter mechanism and cable connection. Loosen the small fixing bolt and pull the end of the shift cable out of the mechanism. Inspect the shifter for a broken component that tells you the shifter must be replaced.
If no damage is found, spray a few shots of lubricant in the mechanism and attempt to gently work the lever or knob. You can loosen the handlebar clamp, remove the shifter for inspection and make another attempt to free the mechanism.
The most common cause of nonworking components on your mountain bike gear system is damage, which you can identify during the inspection. Websites such as Utah Mountain Biking and Bicycle Tutor provide tutorials regarding shifters, cables and derailleurs, and the operation of the gear system on your bike. Lubricating the chain and shift cables regularly is recommended to ensure smooth operation. Replacement shifters and other parts are available through online sources and bicycle accessory outlets. You save time and the cost of a repair shop by becoming familiar with the parts and systems on your mountain bike and having bike tools and lubricants on hand.