The transmission belt on a John Deere L120 acts as an intermediate belt between the power take off, or PTO, clutch pulley, mower deck and transmission. Fortunately, the belt is attached to a spring-loaded tensioning system called an idler pulley. With the idler pulley, you can quickly release tension on the transmission belt and replace the belt within a few minutes. The only time you must replace the belt is when it starts to dry rot or you feel the transmission slipping when in gear.
Place the John Deere's lift lever in the lowest position to drop the mower deck.
Locate the drive belt that connects between the PTO pulley under the engine and the idler pulley for the belt on the mower deck. The idler pulley has a bracket connected to it that allows you to pivot the pulley. This makes it easier to locate.
Place a 15-mm close-ended wrench onto the nut on the idler pulley bracket and rotate the idler pulley away from the belt.
Lift the belt off the PTO clutch pulley, idler pulley and transmission pulley at the same time.
Install the new belt. Do not allow the idler pulley to move once you move it away from the original belt. You might need the help of a second person to remove the old belt and install the new belt while you hold the idler pulley in place.
Move the idler pulley back against the belt slowly. The idler pulley is under tension and will spring back quickly if allowed.