The John Deere LX178 lawn tractor has a 48-inch cutting deck with a two-belt drive system. A primary belt runs from the electric PTO clutch under the engine and back to the top jack sheave on the cutting deck. The top jack sheave turns the secondary belt, which routes around the deck spindles and pulleys, turning the mower's blades. If either mower belt wears thin or breaks, you'll need to replace the mower belt.
Park the lawn tractor on a hard, level surface. Set the parking brake and turn the key to the OFF position. Pull the mower lift lever, located on the left side fender deck, all the way back. Turn the mower depth control knob to the blank mark.
Move the lift lever all the way forward, ensuring the lever locks in place. Locate the tensioner handle on the left side of the mower deck. Disconnect the handle from the bracket; pull the handle all the way back to relieve the tension on the belt. Slide the primary deck belt from the electric PTO clutch going back to the top jack sheave on the mower deck.
Pull out and push down the handle on the draft rod assembly at the front of the mower directly under the muffler. Remove the draft rod assembly. Pull out the J-pins on each side of the mower deck, in front of the rear tires. Turn the J-pins to the unlock position. Pull the lift lever all the way back until it is locked in the raised position. Slide the mower deck out of the right side of the tractor.
Remove the six 10mm nuts holding down the deck guards and remove both deck guards. Slide the belt off the pulleys by hand. Install the new belt and follow the diagram in this article's Resources section to ensure you route the new belt properly. Reinstall the deck guards and tighten down the six screws. Install the mower deck by following these directions in reverse.