John Deere equips all their mowers with a removable mower deck. This allows you to remove the deck and use the lawn mower as a garden tractor. Since the mower deck on your John Deere STX38 is removable, a serpentine drive belt connects the deck's pulleys to the transmission mounted under the motor. The deck has a built in tensioning system called an idler pulley. However, once the belt stretches too much, the idler pulley cannot compensate. When this happens, replace the belt to prevent deficiencies in the speed of your mower's blades.
Disengage the PTO or mower blade lever located on the rear fender of the John Deere STX38 lawn tractor to remove tension from the drive belt.
Position the mower deck to its lowest position or to the position closest to the ground so you can access the belt between the deck and the tractor.
Remove the belt guard mounted over the clutch pulley mounted below the engine. Use a wrench to remove the bolts from the guard and then place the guard off to the side.
Pull the old belt off the clutch pulley and then remove the remainder of the belt from the pulleys on the STX38 deck. If needed, make a note of the belt routing so you do not get confused when putting the belt back on.
Place the new belt onto the pulleys on the STX38 mower deck and then place the remaining end of the belt over the clutch pulley
Mount the belt guard back over the clutch pulley with both bolts and the socket set.