This video from Sears PartsDirect shows how to replace a transmission on a walk-behind lawn mower. The transmission rotates the axle to drive the wheels and move the mower forward. The drive cable pushes the transmission forward to engage the drive belt and spin the transmission pulley when you pull the drive lever up on the mower handle. Replace the transmission if it’s locked up or won’t drive the wheels when engaged.
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Disconnect the spark plug wire from the spark plug. Remove the fuel tank cap. Place a plastic sandwich bag over the fuel tank opening and reinstall the fuel tank cap to prevent gasoline from leaking out through the fuel cap vent. Tilt the mower back to lift the front wheels, then secure the mower handle to your work table to hold the mower steady as you remove the bottom mounting screws. Remove the bottom drive cover mounting screws. Release the mower handle from the work table and carefully lower the mower to the upright position. Remove the top drive cover mounting screws. Release the height adjuster lever and pull off the drive cover off. Pull off the transmission belt keeper. Roll the ground drive belt off the transmission pulley. Attach one end of a bungee cord to the drive belt and the other to the rear discharge chute door lip to keep tension on the drive belt while replacing the transmission. Release the drive cable from the drive control lever. Remove the drive cable end from the anchor post. Depress the release tabs and pull the drive cable out of the transmission lever. Pry off one of the front wheel hubcaps. While holding the front axle stationary, remove the wheel mounting nut. Pull off the wheel and the dust cover. Repeat this procedure on the other wheel. Attach locking pliers to the transmission axle to hold the axle stationary. Remove the screw that attaches the pinion gear to the axle. Remove the pinion gear and the transmission axle pin. Repeat this procedure to remove the pinion gear and axle pin on the other side. Remove the locking pliers from the transmission shaft. Remove the transmission axle clip. Pull the transmission up and disconnect the tension spring pin from the transmission. Pull the transmission support bearings off the ends of the transmission axle. Slide the transmission out of the wheel axle assembly. Slide the new transmission onto the wheel axle assembly. Install the support bearings on the ends of the transmission axle. Position the transmission above the front deck tray and connect it to the tension spring pin. Reinstall the transmission axle clip. Reinstall the axle pin and pinion gear on one side of the transmission axle. Connect the locking pliers to the transmission axle and hold the axle stationary. Reinstall the pinion gear screw and tighten it securely. Repeat the procedure to reinstall the axle pin and pinion gear on the other side. Remove the locking pliers from the transmission shaft. Reinstall the wheel dust cover. Push the wheel on the axle and reinstall the wheel mounting nut. Hold the front axle stationary and tighten the wheel mounting nut securely. Reinstall the hubcap. Repeat the procedure to reinstall the other wheel. Push the drive cable into the transmission lever and engage the locking tabs. Reconnect the drive cable end to the anchor post. Reconnect the drive cable to the drive control lever on the lawn mower handle. Roll the drive belt onto the transmission pulley. Reinstall the belt keeper. Put the drive cover in position on the mower and re-engage the height adjuster lever. Reinstall the drive cover’s top mounting screws. Tilt the mower back to lift the front wheels then secure the mower handle to your work table. Reinstall the drive cover’s bottom mounting screws. Look through the opening in the debris shield to make sure the drive belt is still connected to the engine pulley. If the drive belt worked its way off the engine pulley during the repair, follow the steps in this video for drive belt replacement to reconnect the drive belt to the engine pulley. Release the mower handle from the worktable and tilt the mower upright. Remove the plastic bag from the fuel tank opening and reinstall the fuel tank cap. Reconnect the spark plug wire to the spark plug.