March 1, 2016

How to replace a lawn mower transmission

By Lyle Weischwill
How to replace a lawn mower transmission

This DIY lawn mower repair guide gives step-by-step instructions for replacing a lawn mower transmission. The transmission rotates the axle to drive the mower wheels, moving the mower forward. Replace the transmission with a manufacturer-approved replacement part if the transmission is locked up or won't drive the wheels when engaged.

Follow this procedure to replace the front-wheel drive transmission on common Craftsman and MTD walk-behind lawn mowers.

A transmission problem is just one reason the wheels of a lawn mower won't drive forward. To learn how to diagnose a lawn mower drive problem, see Lawn Mower Won't Move Troubleshooting Video: Motion Drive Failure.

Quick links
Tools required
Nut driver set
Drill driver
Driver bit set
Locking pliers
Pliers
Socket wrench set
Wood block
bungee cords
Plastic bag
Work gloves
Repair difficulty
Time required
60 minutes or less
Repair difficulty
Time required
60 minutes or less
Replacing the transmission on a lawn mower

This video explains how to replace the transmission on a walk-behind lawn mower.

Instructions

  1. 01.

    Disconnect the spark plug wire

    Disconnect the spark plug wire from the spark plug.

    Disconnect the spark plug.

    Disconnect the spark plug.

  2. 02.

    Remove the drive cover

    Remove the fuel tank cap.

    Place a plastic bag over the fuel tank opening and reinstall the fuel tank cap to prevent gasoline from leaking out through the fuel tank 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 drive cover’s bottom-mounting screws.

    Release the mower handle from the work table and lower the mower to the upright position.

    Remove the drive cover’s top-mounting screws.

    Release the height adjuster lever and pull off the drive cover.

    Remove the drive cover.

    Remove the drive cover.

  3. 03.

    Release the ground drive belt

    Pull off the transmission belt keeper.

    Roll the ground drive belt off the transmission pulley.

    Keep tension on the drive belt while attaching one end of the bungee cord to the drive belt and the other end of the bungee cord to the rear discharge chute door lip to keep tension on the drive belt while replacing the transmission.

    Release the drive belt from the transmission pulley.

    Release the drive belt from the transmission pulley.

  4. 04.

    Release the drive control cable

    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.

    Release the drive cable from the anchor post.

    Release the drive cable from the anchor post.

  5. 05.

    Remove the front wheels

    Pry off the left front wheel hubcap.

    While holding the front axle stationary, remove the wheel mounting nut.

    Pull off the wheel and the dust cover.

    Repeat the procedure to remove the right front wheel and dust cover.

    Remove the front wheels.

    Remove the front wheels.

  6. 06.

    Remove the transmission

    Attach locking pliers to the left side of the transmission axle to hold the axle stationary while you remove the left drive pinion gear.

    While holding the transmission axle still, remove the screw that attaches the left pinion gear to the axle.

    Remove the pinion gear and the transmission axle pin.

    Repeat the procedure to remove the right side pinion gear and axle pin.

    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.

    Remove the pinion gear.

    Remove the pinion gear.

  7. 07.

    Install the new transmission

    Slide the new transmission into 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 the transmission to the tension spring pin.

    Reinstall the transmission axle clip.

    Reinstall the axle pin and pinion gear on the left side of the transmission axle.

    Connect the locking pliers to the transmission axle to hold the axle stationary while you reinstall the pinion gear screw.

    Reinstall the pinion gear screw and tighten it securely.

    Repeat the procedure to reinstall the axle pin and pinion gear on the right side of the transmission axle.

    Remove the locking pliers from the transmission shaft.

    Install the new transmission.

    Install the new transmission.

  8. 08.

    Reinstall the front wheels

    Reinstall the left wheel dust cover.

    Push the left wheel onto the axle and reinstall the wheel mounting nut.

    Hold the front axle stationary and tighten the wheel mounting nut securely.

    Reinstall the left front wheel hubcap.

    Repeat the procedure to reinstall the right front wheel.

    Reinstall the front wheels.

    Reinstall the front wheels.

  9. 09.

    Reconnect the drive cable

    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.

    Reinstall the drive cable.

    Reinstall the drive cable.

  10. 10.

    Reinstall the drive belt and drive cover

    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 the repair guide for drive belt replacement to reconnect the drive belt to the engine pulley.

    Release the mower handle from the work table and tilt the mower upright.

    Remove the plastic bag from the fuel tank opening and reinstall the fuel tank cap.

    Reinstall the drive belt.

    Reinstall the drive belt.

  11. 11.

    Reconnect the spark plug wire

    Push the spark plug wire back onto the spark plug.

Warning: Undertaking repairs or maintenance to appliances or power equipment can be hazardous. Should you choose to undertake repairs or maintenance, you are assuming the risk of injury to your person or property. In an effort to reduce the risk, use the proper tools and safety equipment noted in the applicable guide and follow all instructions. Do not proceed until you are confident that you understand all of the steps and are capable of completing the repair. Some repairs or maintenance, however, should only be performed by a qualified technician.

Symptoms for gas walk-behind mowers

Choose a symptom to see related walk-behind mower repairs.

Main causes: damaged cutting blade, loose cutting blade, damaged flywheel key, engine needs tune up
Main causes: dirty carburetor, bad spark plug, clogged air filter, engine choke problems, clogged gas cap vent
Main causes: uneven wheel height settings, damaged wheel, dull or damaged cutting blade
Main causes: engine needs tune up, dirty or clogged carburetor, damaged flywheel key
Main causes: drive control cable failure, worn or broken drive belt, bad transmission, broken drive wheel
Main causes: stale gas, engine needs tune up, bad spark plug, dead battery, bad recoil starter, faulty safety switch, bad ignition coil

Repair guides for gas walk-behind mowers

These step-by-step repair guides will help you safely fix what’s broken on your walk-behind lawn mower.

March 1, 2016
By Lyle Weischwill
How to replace a lawn mower carburetor

The carburetor mixes air with fuel to drive the piston. Replace the carburetor if it's clogged or damaged.

Repair difficulty
Time required
 30 minutes or less
How to replace a lawn mower flywheel key on a flathead engine

When you hit a hard object with your mower, the flywheel can shear to protect more expensive components. Learn how to replace this tiny but essential part.

Repair difficulty
Time required
 45 minutes or less
How to replace a lawn mower ignition coil on a flathead engine

If the spark plug doesn't fire when you try to start the mower, you could have a defective ignition coil. Follow these step-by-step instructions to replace it.

Repair difficulty
Time required
 30 minutes or less

Articles and videos common to all walk-behind mowers

Use the advice and tips in these articles and videos to get the most out of your walk-behind lawn mower.

October 1, 2019

Types of walk-behind mower blades video

By Sears PartsDirect staff

Learn about the different kinds of mower blades and how to maintain them.